Comment:

This article - while it centers about the investigation of a SolarEdge PV (PhotoVoltaic) system - the discussions of techniques and strategies should be generally useful when investigating interference from any make or model of PV system - or even interference from other sources.

* * *

Several months ago I got a call from a local amateur who was very concerned about a sudden rise in his noise floor across the HF spectrum (3-30 MHz).  This increase in noise seemed to be coincident with the installation and commission of a 5 kW PV (Photovoltaic, or "Solar") electrical system on the house of an adjacent neighbor.  I suggested that he talk to the manufacturer of the PV system to discuss the situation - and to request from them possible solutions.

A few weeks ago, he got back to me and he had, in fact, talked to the manufacturer and an online meeting was arranged in which they would remotely idle the neighbor's system while we were monitoring via the amateur's receive antenna.

Out of curiosity - and as sort of a practice run - I went over the weekend before the online meeting to get a better idea as to the spectral signature of this system - a SolarEdge series string system with optimizers - when it was operating normally.  The amateur had obtained permission from the neighbor to allow us to enter their yard to make very "close in" measurements (e.g. within a few inches/cm of the equipment, conductors) to obtain spectral "samples" of the system, thereby excluding external signals.

For these measurements, I used an amplified, shielded magnetic ("H") loop antenna (about 18"/50cm) in diameter as the "sense" antenna and an HP/Agilent/Keysight spectrum analyzer, recording the plots electronically - although a Tiny SA "Ultra" would likely have sufficed as well.  None of the readings were to represent "absolute" signal levels as all that we were really interested in were relative measurements, and as such all that we needed to do was keep our measurements consistent - that is, being able to precisely repeat the conditions between subsequent measurements.  These readings would allow us to understand the nature of the RF energy that it was creating - its "signature", if you will.

Note:  For information about the H-field loop used for this testing - and using an inexpensive spectrum analyzer such as the "Tiny SA" or, better yet, the "Tiny SA Ultra", see a previous blog entry "RFI Sleuthing with the Tiny SA" - Link.

The nature of this QRM:

The interference observed by this amateur - now known for certain to be signature of this model of SolarEdge PV system - was evident as two general types of signals:

• Moderate to strong clusters of carriers every "even" 200 kHz.  At 200 kHz intervals (e.g. 7.0, 7.2, 7.4 and 14.0, 14.2 and 14.4 MHz) from below 3 MHz through at least 30 MHz could be heard a melange of closely-spaced carriers within about 500 Hz of each other on the lower bands.  While these carriers sounded like mostly CW (unmodulated) signals, there was also evidence of low rate data signalling buried in the cacophony as well as additional lower-level carriers.
  

While the appearance of the above interference coincided with the activation of the neighbor's system, that fact that it disappeared at night further pointed to a PV system as the source of the QRM.

"Close-in" measurements

Placing the sense antenna right at the main inverter on the back of the neighbor's house, we wanted to take a snapshot of the spectrum at that location:

Figure 1:  0-30 MHz sampled right at the main inverter

With each horizontal division representing 3 MHz, this 0-30 MHz plot shows a high concentration of noise in the 3-9 MHz area from a location right at the inverter.

Because Figure 1 represents the spectrum at the inverter, we wondered what it would look like at one of the solar panels so we placed the sense antenna right against one of the solar panels:

Figure 2:  0-30 MHz sweep with sense antenna placed next to a solar panel

Figure 2 is in the same frequency and amplitude scale as Figure 1 - but with the "reference level" adjusted by 20 dB to move the trace "up" a bit - and we can see that the spectrum next to the panel looks quite different from that sampled right at the inverter.  This isn't unexpected as Figure 2 would likely represent more of the noise that is emitted from the DC (input) side of the optimizer whereas the spectrum represented in Figure 1 would be more likely to show that of the DC output of the optimizer plus whatever noise was riding on the conductors carrying the DC input and AC output of the main string inverter.

Although it is difficult to be sure, the 0-30 MHz plots taken from a greater distance (10 meters or more) had the general appearance of the noise spectra shown in Figure 2 more than that of Figure 1 leading me to believe that a significant portion of the QRM may be being radiated from the panels themselves rather than just the conductors going from the optimizers  to the main inverter - but certainly, both are likely involved.

Note:  For both of these plots, the RF energy from the PV system was many 10s of dB above the typical background noise floor - in this case, 40-50dB for Figure 1 and at least 30dB for Figure 2 in the area around 7 MHz.

As the 0-30 MHz sweep does not have enough resolution to visualize the narrower 200 kHz signals, the analyzer was readjusted as depicted in Figure 3 - again with the sense antenna next to the panel:

Figure 3:  From near the panel, a "zoomed in" spectral sweep showing narrowband birdies.

In this spectrum plot we can see not only the "white" noise on the floor of the sweep representing the "hummy hiss", but also the much stronger signals every 200 kHz - plus a number of weaker signals in between:  It is these signals that are the most obvious to the casual operator and appear to be unique to a SolarEdge system of this model/type.

On this same day we waited until after sunset - monitoring the groups of carriers at 7.2 MHz and hearing them "flicker" out of existence as it got dark and we re-did the "next to the panel" measurements - this time the spectrum was devoid of the 200 kHz-spaced carriers (they were no longer audible on the amateur receiver, either) and the 0-30 MHz plots were 10s of dB lower than in the daylight. 

Important:  The 2 MHz sweeps in Figures 3-7 use a resolution bandwidth of 10 kHz which is almost exactly 4 times wider than the typical SSB bandwidth of an amateur receiver of about 2.5 kHz making their apparent level above the background noise appear lower than it is actually is.

What this means is the coherent signals - such as the 200 kHz carriers - appear to be another 6 dB farther above the noise floor in an SSB bandwidth than what the analyzer plots show.

Plots from a distance

Having captured some "close-in" plots, we now had an idea as to what the signals emitted by the PV system looked like.

A few days after we made the above plots we were in a virtual meeting with the manufacturer of the PV system (SolarEdge) from the ham's shack.  Having reconfigured the feed to his main radio, we could quickly switch the feedline from the antenna feeding the radio and the spectrum analyzer.

At this time we also learned that there was a second SolarEdge system south of this amateur's QTH - about 150 feet (45 meters) away across the cul de sac - and that the neighboring system and the one across the street would but remotely shut down, in that order, to determine how much QRM was emanating from each.

While we captured 0-30 MHz plots of each stage of system shutdown, for the purposes of this article we'll show just the "narrow" plots in 2 MHz sweeps as depicted in Figure 3 as the presence of the 200 kHz signal are generally representative of the presence of the broadband noise as well and these signals were easily identifiable and now known to be indicators of QRM from this type of PV system.

First, here's the plot from the amateur's 40 meter inverted Vee antenna with both systems on:

Figure 4:  6-8 MHz plot from the 40 meter antenna showing the 200 kHz peaks - and a bit of broadband noise as well.

 

The next plot shows the effects when the neighboring system was turned off, but the one across the street still on:

Figure 5:  The neighboring system off - but the one across the street still on.

As can be seen, the broadband noise floor around the 40 meter band (approximately one horizontal division below and above the marker) has dropped visibly - around 3-4 dB - and the amplitude of the carrier at 7.2 MHz has dropped about 6 dB - and the 200 kHz signals have disappeared almost entirely below about 6.5 MHz.  The system across the street was then shut off and the only remaining signals were those that happened to be on the 40 meter band.  (No trace is available for this configuration, unfortunately.)

As the 40 meter inverted Vee is oriented to favor east-west signals it was not necessarily the best candidate to test the effects of the PV system across the street, so we switched to a 20 meter antenna which was responsive in that direction and this trace shows the plot between 13 and 15 MHz:

Figure 6:  This plot of the 20 meter band and surrounding frequencies shows only propagated signals, with no sign of PV system QRM.

As both systems were off, the trace was quite clear - only showing signals that actually were on or near the 20 meter band, propagated from elsewhere in the world.  The folks at SolarEdge then turned on the system across the street with the following result:

Figure 7:  Same as Figure 6, but with only the PV system across the street activated.

The effect is clear:  In the vicinity of the 20 meter band, the appearance of rather strong signals every 200 kHz is apparent - and there is an obvious 2-4 dB increase in the noise floor indicating that this system, too, is causing harmful interference.

Readings on the radio:

It would seem that the folks at SolarEdge had worked with more than one amateur radio operator on similar issues and I was pleasantly surprised when they asked for some "S-Meter" reading comparisons with the neighbor's system on and off.  Using a calibrated signal generator, I'd already determined the signal level (in dBm) that correlated with the S-meter readings for the Icom radio - and here are the results for 40 meters:

Both systems off:

S1 (<= -84 dBm) - no carrier groups every 200 kHz.

Neighbor system on:

S4 (-78 dBm) - white noise between 200 kHz carriers.

S9 (-67 dBm) - carriers at 7.2 MHz.

This shows that at 40 meters, the degradation to noise alone was on the order of 6 dB (most Japanese radios are calibrated for 3dB per S-unit) and that the cluster of carriers on 200 kHz intervals was far more destructive, rising a bit short of 20dB out of the noise floor.

As our time with the SolarEdge folks in the virtual meeting was limited, we were not able to do similar "S-meter" tests on 20 meters, but we can use the 40 meter results along with the relative strength of the 200 kHz-spacing carriers  and correlate them with the 40 and 20 meter spectrum analyzer traces and determine that the severity of QRM from the PV system on 20 meters on the receiver would have been roughly comparable to that on 40.

Analysis of these readings and implications:

As mentioned earlier, there are two types of interfering signals produced by these SolarEdge PV systems:

• Moderate to strong clusters of carriers every "even" 200 kHz.  These are very obvious, easy to identify, and quite strong compared to the noise with a few weaker signals in-between that were also clearly audible.
  

One of the (many) banes of the amateur radio operator's existence is often found at the end of an Ethernet cable - specifically a device that is being powered via "Ethernet":  It is often the case that interference - from HF through UHF - emanates from such devices.

Figure 1: POE camera with both snap-on ferrites installed - including one as close to the camera as possible - and other snap-on/toroids to suppress HF through VHF. Click on the image for a larger version.

Why this happens

Ethernet by itself is usually relatively quiet from an (HF) RF standpoint:  The base frequency of modern 100 Megabit and gigabit Ethernet is typically above much of HF and owing to the fact that the data lines are coupled via transformers making them inherently balanced and less prone to radiate.  Were this not the case, the integrity of the data itself would be strongly affected by the adjacent wires within the cable or even if the cable was routed near metallic objects as it would radiate a strong electromagnetic field - and any such coupling would surely affect the signal by causing reflections, attenuation, etc.

This is NOT the case with power that is run via the same (Ethernet) cable.  Typically, this power is sourced by a switching power supply - too often one that is not filtered well - and worse, the device at the far end of the cable (e.g. a camera or WiFi access point - to name two examples) is often built "down to a cost" and itself contains a switching voltage converter with rather poor filtering that is prone to radiation of RF energy over a wide spectrum.  Typically lacking effective common-mode filtering - particularly at HF frequencies (it would add expense and increase bulk) - the effect of RF radiating from the power-conducting wires in an Ethernet cable can be severe.

Even worse than this, Ethernet cables are typically long - often running in walls or ceilings - effectively making them long, wire antennas, capable of radiating (and intercepting) signals even at HF.  The "noisy" power supply at one or both ends of this cable can act as transmitters.

What to do

While some POE configurations convey the DC power on the "spare" conductors in an eight conductor cable (e.g. the blue and brown pairs), some versions use the data pairs themselves (often using center-tapped transformers in the Ethernet PHY) meaning that it may not be easy to filter just the DC power.

While it is theoretically possible to extract the power from the Ethernet cable, filter it and and reinsert it on the cable, the various (different) methods of doing this complicate the matters and doing so - particularly if the DC is carried on the data pairs - can degrade the data integrity by requiring the data to transit two transforms incurring potential signal attenuation, additional reflection and affecting frequency response - to name just a few.  Doing this is complicated by the fact that the method of power conveyance varies as you may not know which method is used by your device(s).

It is possible to subject the entire cable and its conductors to a common-mode inductance to help quash RFI - but this must be done carefully to maintain signal integrity.

Comment: 

Some POE cameras also have a coaxial power jack that permits it to be powered locally rather than needing to use POE.  I've observed that it is often the case that using this local power - which is often 12-24 volts DC (depending on the device) - will greatly reduce the noise/interference generated by the camera and conducted on the cable - provided, of course, that the power supply itself is not a noise source.  Even if a power supply is used near the camera, I would still suggest putting its DC power cable through ferrite devices as described below to further-reduce possible emissions.

There are some devices (such as those sold by DX Engineering) that are essentially back-to-back signal transformers that can reduce radiation of signals from Ethernet cable, but these typically do not permit the passage of power and are not candidates for use with POE devices.

Ferrite can be your friend

For VHF and UHF, simple snap-on ferrites can significantly attenuate the conduction of RF along, but these devices are unlikely to be effective at HF - particularly on the lower bands - as they simply cannot add enough impedance at lower frequencies.

To effectively reduce the conduction of RF energy at HF, one could wrap the Ethernet cable around a ferrite toroidal core, but this is often fraught with peril, particularly with cable carrying Gigabit Ethernet - as tight radius turns can distort the geometry of typical CAT-5/6 cable, affect the impedance and cause cross-coupling into other wire pairs.  If this happens, one often finds that the Ethernet cable doesn't work reliably at Gigabit speeds anymore (being stuck at 100 or even 10 Megabits/second) or starts to "flap" - switching between different speeds and/or slowing down due to retransmissions on the LAN.

One type of Ethernet cable that is quite resistant to geometric distortion caused by wrapping around a toroidal core is the flat Ethernet cable (sometimes erroneously referred to as "CAT6" or "CAT7").  This cable is available as short jumpers around 6 feet (2 meters) long and, with the aid of a female-female 8P8C (often called "RJ-45") coupler can be inserted into an existing Ethernet cable run - just be sure that it is from a reputable source and rated for "Gig-E" service.  As it is quite forgiving to being wrapped around ferrites, this flat cable can be pre-wound with such devices and inserted at the Ethernet switch end and/or the device end at a later time.  I have found that with reasonable quality cable and couplers that this does not seem to degrade the integrity of the data on the LAN cable - at least for moderate lengths (e.g. 50 feet/15 meters or less) - your mileage may vary with very long cable runs.

As the flat cable and female-female Ethernet coupler are to be inserted into the cable run, they must be of known, good quality so it is best to test the couplers and cable that you obtain prior to installation to be sure that their use doesn't cause a reduction in signal quality/speed.

Practical examples

Best attenuation across HF

Figure 2: Three toroids wound on "flat" Ethernet cable.  An FT114-43 is used on each end with an FT114-31 in the middle. Click on the image for a larger version.

One might be tempted to use the more-available FT-240 size of toroids (2.4", 60mm O.D.) but this is unnecessarily large, comparatively fragile and expensive:  While you can fit more turns on the larger toroid, one hits the "point of diminishing returns" (e.g. little improvement with additional turns) very quickly owing to the nature of the ferrite and coupling between turns.  Using the FT114 or FT140 sizes is the best balance as it may be much less expensive than a larger device, it can accept 6-8 turns with the cable's connector installed, and more than 6-8 turns is rapidly approaching the point of diminishing returns for a single ferrite device, anyway.

In bench testing with a fixture, it was found that three toroids on a piece of flat Ethernet cable provided the best, overall attenuation across HF and to 6 meters - significantly better than any combination of FT114, FT140 or FT240 toroids of either 43 or 31 mix alone:  Figure 2, above, shows what this looks like.  Two FT114-43 and one FT114-31 toroid were used - the #31 toroid being placed in the center, providing the majority of series impedance at low HF and a #43 at each end being more effective at higher HF through 6 meters.

To construct this, the flat Ethernet cable was then marked with a silver marker in the center and four turns were wound from each end, in turn, for a total of eight turns on the FT114-31.  Placing an FT114-43 at 12 inches (25cm) and winding seven turns puts the FT114-43 fairly close to each connector, allowing the installation of one or two snap-on ferrites very close  to the connector if it is determined that more suppression is required to suppress radiation at VHF frequencies.  Small zip-ties (not shown in Figure 2) are used to help keep the turns from bunching up too much and also to prevent the start and stop turns from getting too close to each other:  Do not cinch these ties up enough to distort the geometry of the Ethernet cable as that could impact speed - particularly when using Gig Ethernet.

It is important that, as much as possible, one NOT place a "noisy" cable in a bundle with other cables or to loop it back onto itself - both of which could cause inadvertent coupling of the RFI that you are trying to suppress into the other conductors - or to the far side of the cable you are installing.

Best attenuation at VHF and HF

If you are experiencing interference from HF through VHF, you will need to take a hybrid approach:  The use of appropriate snap-on and toroidal ferrite devices.  While snap-on ferrite devices are not particularly useful for HF - especially below about 20 MHz - they can be quite effective at VHF, which is to be expected as that is the purpose for which they are typically designed.  Similarly, a ferrite toroid such as that described above - particularly with type 43 or 31 material - will likely have little effect on VHF radiation - particularly in the near field.

Figure 3: A combination of a snap-on device with an extra turn looped through it and two ferrites to offer wide-band suppression from HF through VHF. Click on the image for a larger version.

Figure 3 shows such a hybrid approach with a snap-on device on the left and two toroids on the right to better-suppress a wider range of frequencies.  In this case it is important that the snap-on device be placed as close to the interference source as possible (typically the camera) as even short lead lengths can function as effective antennas at VHF/UHF.  You may also notice that the snap-on has two turns through its center as this greatly improves efficacy at medium/low VHF frequencies but may be counter-productive at high VHF/UHF frequencies owing to coupling between turns.

Doing this by itself is not likely to be as effective in reducing radiation at VHF/UHF from the cable itself, often requiring the placement of additional ferrite devices.  Figure 1 shows the installation of several snap-on devices placed as close to the POE camera as physically possible - mainly to reduce radiation at VHF and UHF as at those frequencies where even a few inches or centimeters of cable emerging from the noise-generating device can act as an effective antenna.

Determining efficacy

During the installation of these devices on my POE cameras I was interested in how much attenuation would be afforded at VHF:  Since I'd already used the "chokes on a flat cable" approach like that in Figures 2 and 3 I knew that this would likely be as effective as was practical at HF - but because the VHF/UHF noise could be radiated by comparatively short lengths of "noisy" cable - and that the 43 and 31 mix ferrites were probably not as effective at those frequencies - I needed to be able to quantify that what I did made a difference - or not.

Figure 4: The cable in Figure 3 installed, but not yet tucked into place as depicted in Figure 1. (This does not show the snap-on ferrites installed where the wire exits the camera housing.) The female-female RJ45/8P8C "splice" can be seen in the upper-left corner of the picture. Click on the image for a larger version.

For HF this was quite simple:  I simply tuned my HF receiver - connected to my main antenna - to a frequency where I knew that I could hear the noise from the cameras and compared S-meter readings with the system powered up and powered down.  This approach is best done at a time during which the frequency in question is "dead" or at least weak (e.g. poor propagation) - 80/40 meters during the midday and 15/10 meters at night is typical.

For VHF this required a bit more specialized equipment.  My "Go-To" device for finding VHF signals - including noise - is my VK3YNG DF sniffer which has extremely good sensitivity - but it also has an audible "S-meter" in terms of a tone that rises with increasing signal level:  This allowed an "eyes and hands off" approach in determining efficacy of the installation of a ferrite device simply by hearing the pitch of the tone..  Switching it to this mode and placing it and its antenna at a constant distance fairly close to the device being investigated allowed me to "hear" - in the form of a lower-pitched tone - whether or not the application of a ferrite device made a difference.

Slightly less exotic would be an all-mode receiver capable of tuning 2 meters such as the Yaesu FT-817, Icom IC-706, 703 or 705.  In this case the AM mode would be selected and the RF gain control advanced such that the noise amplitude audibly decreased:  This step is important as not doing this could mean that if the noise decreased, the AGC in the receiver would simply compensate and hiding the fact that the signal level changed.  By listening for a decrease in the noise level one can "hear" when installing a snap-on ferrite made a difference - or not.

One cannot use a receiver in FM mode for this as an FM detector is designed to produce the same amount of audio (including noise) at any signal level:  A strong noise source and a weak one will sound exactly the same.  It's also worth noting that the S-meter on a receiver in FM mode - or an FM-only receiver - are typically terrible in the sense that their indications typically start with a very low signal and "peg" the meter at a signal that isn't very strong at all which means that if you try to use one, you'll have to situate the receiver/antenna such that you get a reading that is neither full-scale or at the bottom of the scale to leave room for the indication of change.

Of course, a device like a "Tiny SA" (Spectrum Analyzer) could be used to provide a visual indication, using the "Display Line", markers and stored traces to allow a quick "before and after" determination.  As mentioned above, one would want to place the antenna and the receiving device (an actual receiver or spectrum analyzer) at a fairly close distance to the device being investigated - but keep it and its antenna in precisely the same location (or connected to a fixed-location antenna) during the entire time so that one can get meaningful "before and after" readings.

Conclusion

With the use of ferrites alone, one should not expect to be able to completely suppress radiation of RF noise from an Ethernet cable - the typical maximum to be reasonably expected is on the order of about 20dB (a bit over 3 "S" units) and this can vary wildly with frequency.  In a situation where the POE device is very close to the antenna, it may not be possible to knock the interference down to the point of inaudibility in which case relocation to place the two farther apart - or trying similar devices of different models/brands to try to find a combination to reduce it..

The most effective use will be for noise sources will be at some distance from the receive antenna - particularly if a long cable is used that may act as an antenna.  Additionally, these measures can be effective in situations where your transmitter causes problems with the device itself due to ingress of RF energy along the Ethernet cable.

Be prepared to install appropriate ferrite devices at both ends of the cable as it's often the case that not only does the POE device itself (camera, wireless device) radiates noise but also the POE switch itself:  No-name brand POE power supplies and Ethernet switches are, themselves often very noisy and the proper course of action would be to first swap out the supply or POE switch with a known quiet device before attaching ferrite.

As every interference situation is unique, your mileage may vary, and the best road to success is being able to quantify that changes you have made made things better or worse.

This page stolen from ka7oei.blogspot.com

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DQRM In The 3Y0J Pileups. In 51 years a Ham Radio operator, I've never heard what we just witnessed in almost every Bouvet pileup. We've always had DQRM but...

Figure 1:  Half of the array on my garage - the other half is on the west-facing aspect. There's a bit of shade in the morning around the end of June, but it detracts little during the peak solar production of the day - the hours on either side of "local" noon. Click on the image for a larger version

Personally, I thought that the article was a bit narrow in its scope, with an unsatisfying conclusion (e.g. "The QRM is still there after a lot of effort and expense, but I guess that it's OK") - but this impression is understandable owing to the constraints of the medium (magazine article) and the specific situation faced by the author.

Solar power need not cause QRM:

I can't help but wonder if others that read the article presumed that amateur radio and home solar were incompatible - but I know from personal experience that this is NOT necessarily the case:  There are configurations that will not produce detectable QRM on amateur bands from 160 meters and higher.

Before I continue, let me state a few things important to the context of this article:

Expertise in HF radio interference and home solar installations seems to mutually exclusive - which is to say that you will be hard-pressed to find anyone who is familiar with aspects of both.  This means that in the solar industry itself, you will not likely find anyone who can offer useful advice in putting together a system that will not contribute to the crescendo of electrical noise.

I have heard that many installers (at least in my area) will strongly pressure their potential customers to use microinverter-based systems - and this my experience as well:  From the very start of the process, I was adamant that the design of my system would be series string using SunnyBoy inverters which were known to me to be RF-quiet.  If your installer will not work with you toward your goals, consider a different company.

Designing an "RF-quiet" system as described here may incur a trade-off in available solar production as the use of microinverters can eke out additional efficiencies when faced with issues such as shading and complicated roofs that present a large number of aspects with respect to insolation (e.g. amount of light energy that can be converted to electricity).  Only in the analysis of proposed systems appropriate for your case can you reasonably predict the magnitude of this and whether or not you find it to be acceptable.

What is presented here is my own experience and that of other amateur radio operators with similar PV (PhotoVoltaic) system.  The scope of this experience is necessarily limited owing to the fact that when spending tens of thousands of dollars, one will understandably "play it safe" and pick a known-good configuration.

I will be the first to admit that there are likely other "safe" (low RF noise) combinations of PV equipment that can be demonstrated to be "clean" in terms of radio frequency interference.  I have anecdotally heard of other configurations and systems, but since I have not looked at them first-hand, I am not willing to make any recommendations that could result in the outlay of a large amount of money.  For this reason, please don't ask me a question like "What about inverter model 'X' - does it cause RFI?" as I simply cannot answer from direct experience.

An example system:

The system at my house consists of two series-string Sunny Boy grid-tie inverters:  I can unequivocally state that this system, which has both a SB 5000TL-US-22 (5 kW) and an SB3.8-1SP-US-40 (3.8kW) does not cause any detectable RF interference on any HF frequency or 160 meters - and I have yet to detect any interference on 6 meters, 2 meters or 70cm.  Near the LF and lower MF band (2200 and 630 meters, respectively) some emissions from these inverters can be detected - but none of the switching harmonics (about 16 kHz) land within either of these bands.  

Figure 2:  One of two inverters in the garage.  The Ethernet switch (upper right) produces more RF noise than the inverter! Click on the image for a larger version

Simple roof configuration can equal low noise:

The "simple" roof also has another advantage:  All panels on the faces are oriented the same and a larger number of panels may simply be wired in series.

This simple fact means that known-quiet series-string inverters may be used and known noise-generating components may be omitted from the system - namely, many models of "microinverters" and optimizers.  Both of these devices - despite being very different in their operation - are installed on a "per panel" basis and able to adjust the overall contribution of each panel to maximize the energy input of the entire solar power system.

Having each panel individually optimized for output power sounds like a good idea - and in most cases it is - but this nicety should be taken in context with the goals in mind - but considering that the panels themselves represent a rather small portion of the overall system cost, efficiency losses from not having optimizers can often be offset with the addition of more panels.  To be fair, it is not always possible to simply "add more panels" to make up for loss of production - but this must be carefully weighed against a major goal, which is to produce a "noise free" PV system.

The options have changed:

Since the 2016 article was written, the number of options for series-string inverters has significantly increased and the prices have gone down, allowing options to be considered now that may have been dismissed at that time.  Take the article as an example.

From the photographs accompanying the article, there appear to be two different aspects of panels:  A large array consisting of 30 panels, all seeming to face the same direction;  a smaller array of 8(?) panels:  There appears to be an array of 4 panels, but let us presume that this is an independent energy system.

Assuming that each panel is rated for 300 watts (likely higher than a circa-2016 panel) and that one would wish to limit the maximum open-circuit potential to about 450 volts, this implies the use of at least four MPPT circuits:  The 8 panel array and three arrays consisting of 10 series panels, each.  The maximum output of this system would theoretically be about 11.4 kW - but since one can optimistically expect to attain only about 80% of this value in a typical installation the use of an inverter system capable of 10 kW, as stated in the article, is quite reasonable.

Back in 2016, it would be reasonable to have a 10kW series string inverter with two MPPT inputs representing two separate inputs that could be independently optimized.  If such an inverter were used, this would mean that one input would have just 8 panels and the other would have all 30 panels on the main array - not particularly desirable in terms of balancing.  While all 30 of the panels in the larger array would ostensibly be producing the same output, snow, leaves and shading might cause the loss of efficiency should certain parts be thus impaired.

Having already ruled out the optimizing of each panel independently in the interest of having a "known-quiet" system, we might want to split things up a bit.  As an example, a single 10kW inverter with two MPPT inputs could be replaced with a pair of 5 kW inverters, each with 3 MPPT inputs and having a total of six independent DC inputs allowing the 8 panels of the isolated roof to be optimized together and the remaining 30 panels being divided into 5 arrays of about 6 panels, each.

The 2016 article did not mention the price the system, but a reasonable estimate for that time would be around US$35000 - and it was mentioned, in passing, that the cost of RFI mitigation might have been about 10% of the total system cost, implying about $3500 - about the cost of two Sunny Boy  SB5.0 5 kW series-string inverters, each with three MPPT inputs.

Replicating success:

At least two other local amateur radio operators used the same recipe for low-noise PV systems:  Series-string SunnyBoy grid-tie inverters - specifically the SB 3800TL, SB 5000TL and SB3.8s.  In none of these cases could RFI be detected that could be attributed to the inverter - and the only noise to be detected was with a portable shortwave receiver held within a few inches of the display.

What is known not to be quiet:

From personal experience I know for certain that microinverters such as the older Enphase M190 can be disastrous for HF, VHF and UHF reception.  As noted in the QST article, the Enphase power optimizers (model number not mentioned) also caused QRM.

Figure 3: The two Tesla Powerwalls, gateway and electrical sub- panels for the system located remotely on the east wall of the house. Click on the image for a larger version

Additionally, it has been observed that the Solaredge inverters - particularly coupled with optimizers - have caused tremendous radio frequency interference:  The aforementioned April, 2016 QST article about solar RFI deals with this very combination.

It probably won't work in all cases.

Compared to some installations that I have seen, my system - or the one in the 2016 article - are very simple cases - and there are a number of practical limitations, which include:

• A "minimum" array size limitation.  Taking the Sunny boy SB5.0 as an example, there is a 90 volt minimum input which means that one would (very conservatively) want at least four 60-cell panels on each circuit.  This limitation may affect what areas on a roof may be candidates for placement of solar panels, reducing the total system capacity as compared to what might be possible with individually-optimized panels.

• Systems with complicated shading.  If there are a number of trees - or even antennas and structures - portions of sub-strings may be shaded, causing reduction in output and compared to individually-optimized panels, series-strings are at a disadvantage, but careful selection of sub-string geometry can help.  For example, if a tower shades a series of panels during the period of highest production, placing all of those panels on one particular string can help isolate the degradation - but this sort of design consideration will require careful analysis of each situation.

Final words:

The design, configuration and layout of a home (or any) PV system is more complicated than depicted here and any system to be considered would have to take into account.  While I am certain that there are other ways to make an "RF Quiet" PV system, this article was intended to be limited to configurations and equipment with which I have direct experience.

Again, the likelihood of finding a "solar professional" who thoroughly understands RFI issues and knows which type of equipment is RF-quiet is unlikely, so it is up to you as the potential recipient of QRM to do the research.

Other articles at this blog on related topics:

• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link
  
• The Solar Saga Part 2:  Getting the system online - link
• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
• Reducing RFI from the Tesla Powerwall 2 - link 
• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 
  

This page stolen from ka7oei.blogspot.com

[End]

The SDRPlay RSP2pro (left) and RSP1a receivers (right)

This receiver shares a similar internal architecture of similar devices such as the RTL-SDR dongle and the AirSpy in that an analog frequency converter (mixer) precedes the analog-to-digital converter:  In the case of the SDRPlay, the frequency to which the receiver is tuned is (usually) converted to baseband I/Q signals, with the "center" frequency being at zero Hz (DC). ¹

Note:

For the purposes of this discussion, there is no difference between the RSP1a and some of the other receivers in the product lineup (e.g. RSPDuo, RSPdx and the discontinued RSP1, RSP2 and RSP2pro) in terms of harmonic response across the 2-30 MHz range as they all have about the same 12 MHz and 30 MHz cut-off frequencies on their input filtering - properties that would affect HF reception across the 2-30 MHz range in terms of harmonic response.

This issue was noted at least as far back as 2017 in the SDRPlay forum - See this thread:  https://www.sdrplay.com/community/viewtopic.php?t=2280

Imperfect mixers

By its nature, a frequency mixer is a non-linear device.  Ideally, the two frequencies applied to a mixer would yield just two more - the sum and difference.  For example, if we applied a 5 MHz signal and a 1 MHz signal to a mixer, it would output both the sum of 6 MHz and the difference of 4 MHz - and this is true, but there's more to the story.

In our example - with a real-world mixer, we will also get additional products - including those related to the harmonics of the local oscillator and the applied signal.  Because of this, we will see weaker signals at:

• 11 MHz (2 * 5 MHz + 1 MHz) 
• 9 MHz (2 * 5 MHz - 1 MHz) 
• 7 MHz (5 MHz + 2 * 1 MHz) 
• 3 MHz (5 MHz - 2 * 1 MHz) 
• And so on.

Typically, these "other" signals will be quite a bit weaker than the original - but they will still be present, possibly at a high enough level to cause issues such as spurious signals - a problem with both receivers and transmitters.  Typically, these are tamed by proper design of the mixer, proper selection of frequencies and careful filtering around the mixer to limit the energy of these "extra" signals.

Note:  There will be a response at 5x the center frequency as well, but it is suppressed better than the 3x response by the mixer and - for the 80 meter amateur band and higher - these responses are suppressed reasonably well by the filtering.

SDRPlay's poor harmonic response suppression on 80 meters and below.

ANY receiver will experience spurious responses related to mixing products.  Typically, filtering is employed to remove/minimize such responses, but for a wide-bandwidth receiver such an SDR, doing this is complicated by the fact that being able to cover wide swaths of bandwidth would ideally require a large number of overlapping filters.

An example of a radio where this is done - albeit of different architecture - is the Icom IC-7300 which has nine overlapping band-pass filters that cover 160 through 10 meters.  While the reasons for the '7300 having many filters has as much to do with its being a "direct sampling" ² type of SDR, good filtering on the signal path of any type of receiver - SDR or "HDR" (Hardware Defined Radio - or an "old school" analog type) is always a good idea

If this many filters had been implemented on the SDRPlay, there would be enough filtering to prevent a significant harmonic response.  In the case of the RSP1a, this was not done - partly to allow 5-8 MHz of continuous coverage without being significantly impacted by the filters in many cases, but more likely it was done due to practical reasons of economics ³ :  There are just three filters used for covering all of the "HF" amateur bands 160 through 10 meters:  One that covers up to 2 MHz, another that covers 2-12 MHz and third that covers 12-30 MHz:  This information is covered in the RSP1a technical information document ( https://www.sdrplay.com/wp-content/uploads/2018/01/RSP1A-Technical-Information-R1P1.pdf )

The sensitivity to harmonics was tested with the RSP1a's local oscillator (but not necessarily the virtual receiver) tuned to 3.7 MHz ⁴ .  For reasons likely related to circuit symmetry, it is odd harmonics that will elicit the strongest response which means that it will respond to signals around (3.7 MHz * 3) = 11.1 MHz.  "Because math", this spurious response will be inverted spectrally - which is to say that a signal that is 100 kHz above 11.1 MHz - at 11.2 MHz - will appear 100 kHz below 3.7 MHz at 3.6 MHz.  (It's likely that there are also weaker responses at frequencies around 5 times the local oscillator, but these are - for the most part - adequately suppressed by the filtering.)

In other words, the response to spurious signals follow this formula:

Apparent signal = Center frequency + ((Center frequency * 3) - spurious signal) )

Where:

• Center frequency = The frequency to which the local oscillator on the RSP is tuned.  In the example above, this is 3.7 MHz.
  
• Spurious signal = The frequency of spurious signal which is approximately 3x the center frequency.  In the example above, this is 11.2 MHz.
  
• Apparent signal = Lower frequency where signal shows up.   In the example above, this is 3.6 MHz.
  

While these spurious responses may not be too much of a problem for the casual user, it will be necessary to add additional filtering to allow the RSP1a to function on par with a modern, SDR receiver from one of the major manufacturers.

Unfortunately, the filtering in the RSP1a is not sufficient in the 80 meter case mentioned above as it doesn't have octave filters (or similar) - but what about 60 or 40 meters?

The table above answers this question.  In the case of 60 meters - with the receiver tuned to 5.3 MHz - our 3rd harmonic will land on 15.9 MHz.  Based on measurements of the receiver the response of signals around 15 MHz - which corresponds to the 19 meter Shortwave Broadcast Band - will be a bit more than 40 dB down from 40 meters with about 20 dB of this being due to the roll-off of the 2-12 MHz filter - but because this frequency range is inhabited by very strong shortwave broadcasters they are likely to still be quite audible around 60 meters.

The situation is a bit better for 40 meters where the 3rd harmonic is around the 15 meter band.  There, the 2-12 MHz filter knocks signals down by 50dB or more, putting them about 70dB below the 40 meter response - on par with about any respectable receiver.

What this means is that for amateur bands below 40 meters it is suggested that additional filtering be applied.

The best solution - and recommended for any software-defined radio (or even older "hardware-defined radios") is to have band-pass filter designed for the specific amateur band in question. This will not only significantly attenuate the harmonic response, but it will also reduce the total amount of RF energy entering the receiver, reducing the probability of overload.  The obvious down-side is that it will reduce the flexibility of the receiver in that unless you change/remove it, you won't be able to receive signals well outside the filter's design range.

Another possibility is to add a low-pass filter that is designed to cut off signals above the band of interest.  For example, if you have a filter that cuts off sharply above 8 MHz, you will be able to tune 80-40 meters and get reasonable attenuation of the 3rd harmonic response across this entire frequency range.

In the case of 160 meters the RSP1a will automatically select the 0-2 MHz low-pass filter and the 3rd harmonic response will be a respectable 50-ish dB down, depending on frequency.

On 20 meters - where the 3rd harmonic is around 42 MHz - the "12-30 MHz" filter will be selected, but the published response of this filter shows that at 42 MHz its attenuation will be quite limited.  Practically speaking, it is unlikely that there will be any signals in this frequency range so there being "only" 20-30dB of attenuation is unlikely to cause a problem in most cases, but one should be aware of this.

What can be done:

In short, none of the currently-made SDRPlay receivers - by themselves - will offer very good performance in terms of harmonic rejection between 2 and 5 MHz and it will be particularly bad on the 80 meter band where strong 25 meter SWBC signals can appear:  It is interesting that the ARRL review of the RSPdx (Link here) didn't catch this issue.

It is unfortunate that the designers of the SDRPlay receivers did not add at least one additional low-pass filter in the signal path to quash what is a rather strong response in the 2-6 MHz range - particularly on 80 meters, one of the most popular bands.  A low-pass filter with a cut-off frequency of 6 MHz (with attenuation becoming significant above 7 MHz) would ameliorate the harmonic response when tuning across this band.  This problem is made even worse by the fact that even antennas that aren't particularly resonant at their harmonic responses (e.g. the antenna for 80 meters) will likely do quite a decent job of receiving signals in the 11-12 MHz area.

The only real "fix" for this is to install additional filtering between the SDRPlay receiver and the antenna.  If single-band operation is all that is desired, the best choice will be a band-pass filter designed for the frequency range in question ⁵ - but unless you are dedicating the receiver just for that one band, this isn't really desirable unless you can easily switch/bypass the filter when tuning elsewhere.

A more flexible solution would be to use a low-pass filter.  As we noted above, the 12 MHz roll-off of the built-in (2-12 MHz) filter just doesn't do much to suppress signals from 20 meters, but if we had a filter that had a sharp cut off beginning, say, at 8 MHz, we could use it for 80, 60 and 40 meters - such a filter is depicted schematically, below:

8 MHz low-pass filter schematic, designed using ELSIE

 If one owns a receiver with a waterfall display, the increased cluttering of the 12 and 10 meter bands with weird "swooping" signals could not have gone unnoticed.  Take, for example, this recent snapshot of the lower portion of 10 meters from the waterfall of WebSDR #5 at the Northern Utah WebSDR (Link)

Figure 1: 10 Meters as seen on a beam antenna pointed toward Asia showing QRM from a large number of different sources - presumably dielectric heaters/welders/seamers.  These things radiate badly enough that they should have their own callsigns, right? Click on the image for a larger version.

In looking at this spectral plot - which comes from an antenna oriented to the Northwest (toward Asia and the Pacific) one could be forgiven for presuming that someone had somehow connected a can of "Silly String" to their coax and was squirting noodles into the ionosphere!

What, specifically, are we looking at?

Across the entire spectrum plot one can see these "curved" signals, some of them - like that near the bottom, just above the cursor at 28374 kHz - are quite strong while there are many, many others that are much weaker, cluttering the background.  These signals contrast with normal SSB and CW signals - the former being seen clustered around 28500 and the latter around 28100 kHz - which are more or less straight lines as these represent transmissions with stable frequencies.

What are these from?  The general consensus is that these are from "ISM" (Industrial, Scientific and Medial) devices that nominally operate around 26957 kHz to 27283 kHz.  Clearly, the waterfall plot shows many devices outside this frequency range.

What sort of devices are these?  Typically they are used for RF heating - most often for dielectric sealers of plastic items such as bags, blister packs - but they could also be used in the manufacture of items that require some sort of energetic plasma (e.g. sputtering metal, etching) in any number of industrial processes.

Where are they coming from?

The simple answer is "everywhere" - but in terms of sheer number of devices, it's more likely that much of the clutter on these bands originates in Asia.  Consider the above spectral plot from an antenna located in Utah pointed at Asia - but then consider the plot below, taken at about the same time from an antenna that is pointed east, across the continental U.S. and Canada - WebSDR #4 at the Northern Utah WebSDR (link):

Figure 2: 10 Meters on a beam pointed toward the U.S. Click on the image for a larger version.

To be absolutely fair, this was taken as the 10 meter band was starting to close across the U.S, but it shows the very dramatic difference between the two antenna's directionality, hinting at a geographical locus for many of these signals.

Further proof of the overseas origin of these signals can be seen in the following plot:

Figure 3: Spectrum from AM demodulation of some of the signals of Figure 1 showing 50/100 Hz mains energy. Click on the image for a larger version.

This plot was taken by setting the WebSDR to AM and setting for maximum bandwidth, tuning onto a frequency where several of these "swoops" seen in Figures 1 and 2 are recurring and then, using a virtual audio cable, feeding the result directly into the "Spectran" program (link).

As expected this plot shows a bit of energy at the mains harmonic frequencies of 120, 240 and 360 Hz owing to the fact that this antenna points into slightly-noisy power lines operating at the North American 60 Hz frequency - but on this plot you can also see energy at 50 and 100 Hz, indicative of a lightly-filtered power supply operating from 50 Hz power mains - something that is NOT present anywhere in North America.

Based on other reports (IARU "Intruder Watch", etc.) a lot of these devices seem to be located in Asia - namely China and surrounding countries where one is more likely to experience lax enforcement of spurious radiation of equipment that is manufactured/sold in those locales.

Why the "swoop", "curve" or "fishook" appearance seen in Figure 1?  If these devices were crystal controlled and confined to the nominal 26957 kHz to 27283 kHz ISM frequency range, we probably wouldn't see them in the 10 meter amateur band at all, but many of these devices - likely "built to cost" simply use free-running L/C oscillators that are accurate to within 10-15% or so:  As these oscillators - which are likely integral to the power amplifier itself (perhaps self-excited) - warm up, and as the industrial processes itself proceeds (e.g. plastic melts, material cures, glue dries) the loading on the RF output of this device will certainly change, and this results in an unstable frequency.

Why do they radiate?

Ideally, the RF would be contained to the working area and in the past, reputable manufacturers of such equipment would employ shielding of the equipment and filtering of power and control leads to confine the RF within.  But again, such equipment is often "built to cost" and such filtering and shielding - which is not necessary for the device to merely function is often omitted.

Can we find and fix these?

In this U.S. and parts of Europe such sources are occasionally tracked down and RF interference mitigated - either voluntarily or with "help" from the local regulator - but the simple fact is that the intermittent nature of these sources - and the fact that they radiate on frequencies that are prone to good propagation when the sun is favorable to such - makes them very difficult to localize.  If the signal source is coming from halfway around the world, there's likely nothing that you can do other than point your directional antenna the other way!

If it so-happens that you can hear such a signal at your location at all times of the day - regardless of propagation - you may be in luck:  There may be a device with a short distance (a few miles/km) of your location - and perhaps you can make a visit and help them solve the problem.

* * * * * * * 

Related article:

•  27 MHz ISM Fish Hook Swisher Interference RFI EMI RF HF Sealers Heaters Welders  - link - A thread on this topic found at "HF Underground".

This page stolen from ka7oei.blogspot.com

[End]

A few weeks ago I was on vacation in remote Eastern Utah - in Canyonlands National Park, to be precise and because we had some "down time" in the evenings, after hiking, after sunset, I was able to set up a portable HF station.  Using the homebrew end-fed halfwave antenna (EFHW) of Mike, K7DOU - one end of the rope tied around a rock laying on a shelf of slick rock some 40 feet above ground level and the other end tied to a bamboo pole attached to my Jeep - I connected my FT-100 through a manual tuner as the VSWR of the EFHW wasn't necessarily very low on some of the higher bands.

Figure 1: 150 Watt Samlex sine wave inverter, sitting on the workbench. Click on the image for a larger version.

For whatever reason, I had brought along my old lap top and sound-card interface so I could work some digital modes, specifically FT-8 - a mode that I was familiar with, but had personally never worked.  The battery in my laptop had discharged, so I needed an alternate source of power and I connected my 150 watt Samlex Sine Wave inverter (a PST-15S-12A) to the battery to power the computer's power supply.

The (expected!) result of this was a tremendous "hash" all across the HF spectrum - an obvious result of the various high-power converters contained within the inverter.  On some bands the interference wasn't too bad, but on others the result was unusable.  While the battery charged, I operated on the band (20 meters, IIRC) that wasn't as badly affected.

I left the inverter running and the laptop battery charging during the cooking and eating of dinner, and with a reasonable amount of power banked I could turn off the inverter and get a zero noise floor while operating.

Why so noisy?

Modern AC inverters first convert the DC input power to something around the peak voltage found on the AC output - typically around 155 volts for 120 volt mains.  This conversion is done using a switch-mode inverter with a transformer, typically operating in the 20-60 kHz range and this output is rather rich in harmonics.

For the less-expensive "modified sine wave" inverters, the DC output is chopped, typically using an "H" bridge switch using FETs (Field Effect Transistors) with the duty cycle being varied to provide the equivalent of a 120 volt sine wave - and this switching can also add a bit of extra RFI, most notably in the form of a "buzz" - but this action produces less energy at radio frequencies than the initial voltage conversion.

The "Sine Wave" inverters perform the same step of producing the high DC voltage, but will chop the output into much smaller bits.  The method that this is done can vary, but it's sometimes done by using a "buck" type switching converter to transform the higher voltage into a varying - usually lower - voltage to simulate a sine wave on the output.  This second conversion adds yet another source of RF interference atop what is likely already the significant source that already present in the high voltage converter.

Comment:  The power converter (wall wart) that I was using to charge my laptop is particularly quiet, so I did verify that the vast majority of noise was, in fact, from the AC inverter.

Figure 2: Various mains filtering components:  All of these are bifilar, common-mode chokes, except for that in the upper-left with is a combination filter and IEC power connector. Click on the image for a larger version.

Quieting the inverter:

Fortunately, the internal space of this inverter wasn't terribly cramped so there was just enough room to add the necessary components to suppress the RF "hash" that was being conveyed on both the DC and AC lines.  While the methods of doing this sort of RF quieting have been discussed in previous blog posts (see the references at the end of this article) I'll review them in detail here.

Snap-on chokes won't do!

It's worth noting (several times!) that simply winding the power cord (DC and/or AC) around a ferrite device (e.g. a clamp-on or even a large toroid) would likely NOT be enough to solve this problem.  While doing so may knock down RFI by, perhaps, 6-10 dB - maybe 20 dB if one is really lucky - this sort of noise egress must often be attenuated by several 10s of dB to effectively quash it.  In other words, knocking down the "grunge" by 1-2 S-units is nice enough, but there will still be a lot of hash left over to bury the weakest signals! 

Internally, this inverter did pass through some rather large ferrite cylinders the DC input and (separately) AC output connections, but this very small amount of inductance would have practically no effect at all at HF - likely having been added to make a dent in the noise at VHF so that it would pass muster when subjected to EMC compliance tests.

Filtering the AC output:

I presumed (but didn't actually measure) that the majority of the noise being radiated would be from the AC output as it is "closest" to the circuits most likely to generate a lot of noise, so I concentrated most of my effort there.

The most helpful component in filtering the mains voltage output is the bifilar choke - several varieties of these being displayed in Figure 2.  This component consists of two windings in parallel on the same ferrite core - typically both leads of the mains voltage.  For the low-frequency AC currents, the halves of the choke carry equal and opposite current so there is no DC component to magnetize the core and reduce its efficacy due to saturation, but because RF energy is likely not flowing in a differential manner as is the AC mains voltage, the inductance of the two parallel windings come into effect - the magnitude of this typically being in the 10s of microHenries to milliHenries range.

Where does one get these things?  They can be found at surplus outlets if you look around, but perhaps the easiest source is from defunct PC power supplies:  These devices, found in supplies made by reputable manufacturers, are typically the first things through which the AC mains voltage pass (after any fusing) before going to the rest of the circuitry.

Figure 3: Schematic of the output filter.  While it's likely that just one bifilar inductor would have sufficed, I decided that since there was room to do so, a second one would be added for even more filtering of the "grunge" that can emanate from such a noisy circuit. Click on the image for a larger version.

This much inductance has significant impedance to RF energy - but inductance alone will have only limited efficacy and intrinsic capacitance of the windings will also reduce the amount of attenuation that would otherwise happen - as would have winding the mains cord/cable on a ferrite toroidal core as noted previously - so capacitors are also required to be placed strategically to help shunt away some of the residue.

Figure 4: The AC output filter in the process of being installed.  L1 and C1-C4 are mounted to the outlet itself while the connection to L2 is made using the orange leads. Click on the image for a larger version.

The diagram in Figure 3 shows the as-installed filter.  As can be seen, two separate bifilar filters (both of them being the sort as seen as the second from the lower-right in Figure 2) were used to maximize attenuation.  In this circuit, C3 and C4 are used to force any RF on the two wires to be common-mode to maximize the efficacy of the bifilar chokes' attenuation and any residual RF - which will be at rather low level and high impedance - will then be shunted to the metal case of the inverter by capacitors C1 and C2.

Figure 4 shows the installation of the filtering components in the inverter.  C1 and C2 are the disk-shaped blue capacitors seen in the upper-left, mounted directly to the inverter's single AC outlet and capacitor C3 is just in "front" of the two round disks, also mounted directly to the socket.  The first inductor, L1, can be seen in the shadows, connected to the outlet with very short, flexible leads to the plug.

Earlier, I had removed this outlet from the body of the inverter and mounted C1, C2, C3 and L1 to it and with a bit of "tetris" action, was able to reinstall the outlet back in place with the components attached.  From that point I installed C4 (to the "other" side of L1) and the (orange) connecting wires from C4 to L2, which is shown floating in space.

You might ask why there isn't another capacitor (like C4) across the "inverter" side of L2 - or other capacitors to ground other than C1/C2:  There is already a degree of filtering on the AC output of the inverter, so there is little point in adding another capacitor like C4.  As for other capacitors to "ground" like C1/C2 elsewhere in the circuitry:  These were deemed unnecessary - and doing so, particularly at the "inverter" side of L4 would simply put relatively strong RF currents onto the ground lead (e.g. inverter's case) - and our cause won't be helped in making RF currents appear where don't need them to be.  

Figure 5: Noise filter on the DC input.  It looks suspiciously like the filter on the AC output - because it's the same type, although the current-carrying capacity of L1 is much higher and the values of the capacitors are orders of magnitude larger. Click on the image for a larger version.

Filtering the DC input:

While I would presume that most of the noise would be emitted via the AC output port, filtering the DC port must be considered as well.  With the inverter's rating being 150 watts, the maximum current on the AC output would be around 1.25 amps and rather light-gauge wire could be used in the inductors - but because this same power level represents 12.5 amps at 12 volts (likely more if the battery voltage is on the low side) the filtering inductance must be made using much larger wire.

Rummaging around in my box of toroids, I found a ferrite device that was about 1" (2.54cm) in outside diameter and wound as many turns of 14 AWG flexible wire onto it as would fit (about 6 bifilar turns) and measured it to have about 30 uH of inductance per winding.  This may not seem like much, but at 1 MHz, this represents about 180 ohms of reactance.   

In referring to Figure 5, above, you'll notice that it is pretty much identical to that of the output filter - except that there is only one section of filtering.  The capacitor values are different, too:  C1 and C2 are 0.1uF units that shunt residual RF getting through L1 to ground (the case) while C3 is a low-ESR electrolytic connected across the DC leads to help force any residual AC noise on the DC lead to common-mode.  Compared to the 180 ohms of reactance of the DC bifilar choke (at 1 MHz) a good-quality, monolithic ceramic capacitor like the 0.1uF units are likely to have well under an ohm of impedance and very little of the RF hash will remain after they do their job to bypass it to the chassis ground.

Figure 6: The DC input filter.  The capacitors (not visible) are mounted to the bottom side of the terminal strip, which serves as the RF "grounding" point to the case.  L1 is just visible. Click on the image for a larger version.

Because of the limited amount of room, only one inductor was used - although it would likely be possible to have crammed another in the limited space should the above filter have proved to be inadequate (it wasn't).

As can be seen in Figure 6, a small terminal strip is visible and to it is mounted C1-C3 (not visible as they are obscured by the strip itself).  The mounting point for this strip is the ground lug near the DC input cable and the center lug is the common point for C1 and C2.

An important point to mention is the fact that this inverter - like many - have their DC and AC lines isolated from the case - and that's also important here:  Because the DC has no connection to the inverter's metal case, ALL of the DC current passes through L1 of Figure 5 - but with both halves carrying the same current, the core is not magnetized:  Magnetizing the core would likely cause it to saturate and the result would be its effective inductance plummeting - possibly reducing its efficacy as an RF filter.  It is for this reason that a bifilar choke was used on the DC input as well.

As with the AC output, the "inverter" side of L1 of Figure 5 also lacks a common-mode capacitor, but this is well represented on the input of the inverter itself with its own, built-in capacitor.

Figure 7: The final arrangement of the added filtering components.  Liberal use of RTV (silicone adhesive) was used to stabilize the components as it works well, and can be removed should repairs/modifications be required.  On the left, a generous blob of RTV has been used to keep the terminal strip's lugs at the DC input from touching the inverter's bottom cover. Click on the image for a larger version.

Additional comments:

Figure 7 shows the final arrangement of the added components.  In the upper-left corner can be seen the components of the DC input filter with come clear RTV (silicone adhesive) added to the top of the terminal strip to insulate it and keep any metal parts of it from touching the bottom cover when it was reinstalled.

On the right side is the AC output filter and on the foreground can be seen L2, now with the "hot" terminals covered by heat-shrink tubing.  This choke was first attached "temporarily" to the inverter's end plate using instant (cyanoacrylate) glue - and then several large blobs of RTV were later added to permanently hold it in place.  Just above it can be seen the orange wires that connect L2 to L1 and these components were also stabilized with rather large blobs of RTV to keep them from "flapping in the breeze".  It's worth noticing that the original ferrite cylinder is still on the AC output connection (on the black and white wires) where it connects to L4 - mainly because there was still room for it, and its efficacy, such as it is, is likely only enhanced by the addition of the new filtering components. 

Did it work?

You might ask the question:  Did this filtering work?

The answer is yes.  Placing a portable shortwave radio next to either the DC or AC power leads from the inverter, one can't detect that it is running at all.  If the radio is placed right atop the inverter, some hash can be detected, but this is likely from direct radiation of magnetic fields from the inductors/transformers within, but detectable amounts do not appear to be emanating from DC and AC wires themselves - and that's the important part as they would otherwise be acting as antennas.

Perhaps the most important part of this modification is the fact that any bypass capacitors are placed on the "quiet" (not the inverter) side of the filtering inductances and that these bypass capacitors are connected, with short leads, to a large, common-point ground - namely the case of the inverter.  If any of the "ground" leads had been more than an inch or two long, it's likely that the impedance of it would have reduced the efficacy of the filtering - but the case, being a solid chunk of extruded aluminum, forms a nice, low-impedance tie point - effectively a single-point ground, preventing an RF current differential between the DC input and AC output leads.

* * *

Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:

• A high-current DC noise filter for UPS or RV use.  What if you have an inverter or UPS that is large - as in the multi-kilowatt class?  This article describes quieting one such device.