Figure 1: Power/VSWR meter using ИН-13 (a.k.a. "IN-13") neon bar-graph indicators. Click on the image for a larger version.

Several years ago I bought some Soviet-era neon bar-graph displays - mainly because I thought that they looked cool, but I didn't have any ideas for a specific project.  

Figure 2: A pair of ИН-13 neon indicator tubes.  These tubes are slightly longer than than the ИН-9 tubes and have three leads Click on the image for a larger version.

For a device that is intended to indicate specific measurements, it's important that it is consistent, and for these neon indicators, that means that we want the bar graph to "deflect" the same amount anytime the same amount of current is applied to it.  In perusing the specifications of both the  ИН-9 and  ИН-13 it appeared that the  ИН-13 would be more suitable for our purposes.

Figure 3: Test circuit to determine the suitability of various inductors and transistors and to determine reasonable drive frequencies.  Diode "D" is a high-speed, high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and "Q" is a power FET with suitably high voltage ratings (>=200 Volts) and a gate turn-on threshold in the 2-3 volt range so that it is suitable to be driven by 5 volt logic.  V+ is from a DC power supply that is variable from at least 5 volts to 10 volts.  The square wave drive, from a function generator, was set to output a 0-5 volt waveform to make certain that the chosen FET could be properly driven by a 5 volt logic-level signal from the PIC as evidenced by it not getting perceptibly warm during operation.

Generating high voltage from a low is one of the aspects that I tackled in a previous project on this blog when I built a high voltage power supply for the Zenith Transoceanic:  You can read about that here - A microcontroller-based A/B Battery replacement for the Zenith TransOceanic H-500 radio, with filament regulation - link.

Figure 4: The voltage boost converter section showing the transistor/inductor, rectification/filtering and voltage divider circuitry.

Description:

 

Figure 5: The precision current sinks that drive the neon tubes precisely based on PWM-derived voltage. Click on the image for a larger version.

Figure 6: An exterior view of the tandem coupler module. Visible is the top shield and the three feedthrough capacitors used to pass voltage and block RF. Click on the image for a larger version.

RF sensing

For sensing forward and reflected power I decided to use an external "sensing head" that was connected inline with the radio, on the "tuner" side of the feedline.  

For sensing power in both directions I chose the so-called "Tandem" coupler which consists of a through-line sampler in which a short length of coaxial cable carrying the transmit power (T1 in the diagram of Figure 7) passes through a toroidal core - using some of the original cable's braid grounded at just one end as a Faraday shield.  An identical transformer (T2) is connected across the first (T1) for symmetry.

When carefully constructed this arrangement has quite good intrinsic directivity and a wide frequency range.  Figure 6 shows the diagram of this section.

Figure 7: Schematic diagram of the "Tadem" coupler.  A bidirectional coupler sends power to separate AD8307 logarithmic amplifiers - one for forward and the other for reverse. The outputs, expressed in "volts/dB" are sent to the microcontroller. Click on the image for a larger version.

The RF sensing outputs of the second tandem coupler (T2) then goes through resistive voltage dividers (R606/R607 for the reverse sample and R603/604 for the forward sample) to a pair of Analog Devices AD8307 logarithmic amplifiers - one for forward power and the other for reverse - to provide a DC voltage that is logarithmically proportional to the detected RF power.  This voltage is then coupled through series resistors (for both RF and DC protection) R605/R608 and to the outside world using feedthrough capacitors.

The use of a logarithmic amplifier precludes the need to have range switching on power meter as RF energy from well below a watt to well over 2000 watts can be represented with only a few volts swing.  Looking carefully at Figure 6 one can see a label that notes that the response of the AD8307 is about 25 millivolts per dB - and this applies across the entire power range of a few hundred milliwatts to 2000 watts.

All of this circuitry is mounted in a box constructed of circuit board material and connected to the display unit with an umbilical cable that conveys power and ground along with the voltages that indicates forward and reflected power.

Figure 8: An inside view of the Tandem Match (sense unit) showing the coupling lines, internal shielding and AD8307 boards. Click on the image for a larger version.

The AD8307 detectors themselves can be seen at the left and right edges of the lower half of the unit, built on small pieces of perfboard.  All signals - including the 12 volt power and the DC voltages of the output pass through 4000pF feedthrough capacitors to prevent both ingress and egress of RF energy which could find its way into the '8307 detectors and skew readings.

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In a future posting (Part 2) we'll talk about the final design and integration of this project.

This page stolen from ka7oei.blogspot.com

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Some years ago, I purchased and assembled an Oak Hills Research WM-2 QRP Wattmeter from Milestone Technologies. As far as QRP wattmeter kits go, it was something of a classic at the time, and as such, I wanted one. I’m glad I made this purchase, as they are no longer available – at least, in this form. Another company is offering a very similar kit, but without the decals on the case. I was told that they acquired the rights to it from Milestone Technologies, so this would be appear to be a direct clone of the WM-2. The WM-2 is a great little wattmeter, with 3 ranges representing 100mW, 1W, and 10W full-scale – an ideal selection of ranges for the QRPer. You can read both forward and reflected power. While direct readout of SWR is not offered, it can be calculated from the forward and reverse power readings. The WM-2 has an analog meter and can be left inline while operating. Apart from the satisfaction of being able to see the needle bob up and down when transmitting, this type of indicator is very useful when peaking circuits for maximum output. It can certainly be done with a digital readout, but an extra stage of “translation” needs to happen in the brain, converting the number on the readout to a “level”. This process when looking at an analog meter is more immediately intuitive.

That was so many photos of my WM-2, that you might be thinking, “Hang on – isn’t this a post about the QRPoMeter? Well, it is – and we’ll get to that very soon. I don’t think I ever blogged about the WM-2 when I built mine years ago, so felt it was time to give it some air time on my blog.

For my purposes, the WM-2 meets my needs. However, I don’t have any other instruments with which to check the accuracy of it’s readings. A Bird wattmeter would be nice, but the expenditure is hard to justify. Another option is to use an oscilloscope to measure the peak to peak voltage a transmitter develops across a 50 ohm dummy load, and use that to calculate power. This is a definite possibility in the future, as I do intend to add a digital storage oscilloscope to the shack at some point. In the meantime, it would be good just to have another wattmeter of similar accuracy, simply to increase my confidence in the readings I am getting from either one. For the kind of operating us QRPers do, absolute accuracy is not essential. 5% of full scale is good enough which, on a 10W scale, means ±0.5W. If I claim to be transmitting with 5W, then the difference between 4.5 and 5.5 W is unlikely to even be noticed at the receiving end.

The QRPoMeter, designed by Dave Cripe NM0S, has been on my radar for a very long time. Originally offered by the 4SQRP group, it is a very affordable instrument for measuring power and SWR. It has a built-in dummy load, to make measuring the power into a 50 ohm load an easy task. Also, when measuring SWR, it uses a resistive bridge, so that the maximum SWR your transmitter will see is 2:1. I’ve long wanted to assemble this kit. A few times, I’ve waited too long to purchase a kit, only to find that it was no longer available (the SST, once offered by Wilderness Radio, was one example). With that in mind, and also because the QRPoMeter is so reasonably priced, I went ahead and placed an order with NM0S Electronics.

A few days later it arrived, in a small flat rate Priority Mail box. I love getting radio parts and kits in the mail. It’s exciting! The PCB pieces that, as well as forming the circuit board, also comprise the case, were slipped in between pages of the assembly manual to protect them. There was also a little bag of goodies. I love little bags of radio parts!

The bag of parts, emptied out into a styrofoam tray –

Also included was a piece of thin 2″ x 3.5″ PCB material, etched and silkscreened on both sides. On one side was the business card of NM0S Electronics. On the other side were these handy little band plans –

The pieces that form the sides of the case have to be broken off from the larger pieces of PCB material, and given a very light filing to remove the rough edges. It was immediately obvious how smart the final product was going to look. It was raining very, very lightly when I took this photo. If you look carefully, you might see some very small raindrops on the panels –

For anyone who has assembled a few kits, construction is uncomplicated. Dave’s instructions are clear and straightforward, consisting mainly of a checklist for populating the board, and instructions for constructing the included PCB case. The switches and input/output BNC connectors are all mounted directly on the board. The only wiring required is a 4-conductor ribbon cable that is used for the connections between the board and the LCD panel meter. Other than wiring up this meter, and soldering the case pieces together, construction of the QRPoMeter consists of populating the single PCB with the parts. This picture is of the finished board before the two switches were installed –

I only discovered two very slight issues during assembly. Neither will present a problem for anyone with a little experience, but they might slightly confuse a beginner. These were due to a change in sourcing parts, and Dave said he will take care of them in future versions of the construction manual. These were –

1. U4, the TLC2272, had no dot or u-shaped indentation to denote the correct orientation. I used the printing of the device number on the top of the IC as a guide instead, and this turned out to be right. See picture below, with U4 circled in red –

2. When using the 4-conductor ribbon cable to connect the LCD panel meter to the board, because the instructions refer to an earlier version of the meter, a beginner might experience some uncertainty as to which holes on the meter board to connect to which holes on the main board. On the main board, there are two pads next to each other marked +Vin and -Vin. These are connected to the pads on the meter board that are marked “IN” and “COM” respectively. The other two connections are more obvious. The pads on the main board, next to the schematic symbol for a 9V battery, marked + and -, are connected to the pads on the back of the meter board, next to the “DC 9V” text, that are marked + and respectively.

A portion of the board in greater detail, showing the 8 large surface mount resistors that form the dummy load/resistive bridge (or, to be more accurate, 7 of them, and a small part of the remaining one) –

Calibration is straightforward, and requires a fairly accurate DMM. I used my Brymen BM235 (the EEV Blog version). The only other piece of equipment needed is any HF QRP transmitter with an output of between 2 and 10 watts. The output power doesn’t need to be known, as long as it falls within that range. When the unit is calibrated, you have a very handsome and useful piece of QRP kit!

The QRPoMeter seems to be accurate enough for my purposes. Power measurements are in line with the ones reported by my WM-2, taking into account the accuracies of both instruments. SWR measurements are similar at the lower readings. They differ by fairly large amounts at higher SWR’s. This doesn’t concern me though, as once the SWR goes much above 2 or 3:1, it’s exact value is of little interest to me. I just know that I want to get it back down below 2:1! A useful feature of the resistive bridge in the QRPoMeter that is used to measure the values of forward and reflected power, is that when SWR is being measured, the transmitter never sees an SWR higher than 2:1. This was verified with the SWR indicator in my Elecraft K2.

Thanks Dave. A good-looking and worthy little piece of QRP test gear! The QRPoMeter is available from NM0S Electronics.