Neon bar-graph VSWR/Power meter using the ΠΠ-13 (a.k.a "IN-13") "Nixie" - Part 1
Figure 1: Power/VSWR meter using ΠΠ-13 (a.k.a. "IN-13") neon bar-graph indicators. Click on the image for a larger version. |
- The ΠΠ-9 (a.k.a. "IN-9").Β This tube is 5.5" (140mm) long and 0.39" (10mm) diameter.Β It has two leads and the segments light up sequentially - starting from the end with the wires - as the current increases.
- The ΠΠ-13 (a.k.a "IN-13").Β This neon bar-graph tube is about 6.3" (160mm) long and 0.39" (10mm) diameter.Β Like the ΠΠ-9 its segments light up sequentially with increasing current but it has a third lead - the "auxiliary cathode" - that is tied to the negative supply lead via a 220k resistor that provides a "sustain" current to make it work more reliably at lower currents.
Figure 2: A pair of ΠΠ-13 neon indicator tubes.Β These tubes are slightly longer than than the ΠΠ-9 tubes and have three leads Click on the image for a larger version. |
This project would require two tubes:
- Forward power indicator.Β This would always indicate the forward RF power as that was that's something that is useful to know at any time during transmitting.
- Reverse power/VSWR.Β This second tube would switchable between reverse power, using the same scale as the forward power display, and VSWR - a measurement of the ratio between forward and reverse power and a useful indicator of the state of the match to the antenna/feedline.
The method used for this project and the aforementioned Zenith radio isΒ boost-type converter as depicted in Figure 3.Β The switching frequency must be pretty high -Β typically in the 5-30 kHz range if one wishes to keep the inductance and physical size of that inductor reasonably small.
As in the case of the Zenith Transoceanic project, I used the PWM output of the microcontroller - a PIC - to drive the voltage converter with a frequency in the range of 20-50 kHz.Β For our needs - generating about 140 volts at, say, 15 milliamps maximum, I knew (from experience) that a 220uH choke would be appropriate.Β Figure 4, below, shows the as-built boost circuit.
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.Β Β
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.Β 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.Β Figure 6 shows the diagram of this section.
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.Β 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.Β 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.Β 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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