Sunday, December 1, 2013

Building a USB Powermeter


Although I can control the ANAN-10 remotely over the internet I cannot monitor the output power via the computer.  PowerSDR has a "forward power indication" but it isnt actually measuring the power at the ANAN-10 PA output! I could buy a minicircuits PWR-4GS USB power meter at $900 but I needed a cheaper option.

Searching the internet I came across a design A very simple to build, menu driven RF Power Meter, based on the AD8307 log amp by Loftur TF3LJ. It used a Teensy++  micro available at www.pjrc.com and a 16x2 LCD display. I already had a Teensy++ board and a 16x2 backlit LCD. All I needed was the AD8307. I while ago I had tried to build the W7IEQ SWR/Power meter published in QST Jan 2011 which used two AD8307, a PIC and an LCD. It was a disaster and I never got it working mainly due to software bugs which never got fixed in the code especially the version hosted on the ARRL website. I threw away the main PCB and box but kept the 30dB directional QRO coupler and the 4 PCB with AD8307 and BNC input connectors.


Modified AD8307 RF detector PCB
It did not take many minutes with a Scalpel to modify one detector PCB to match the TF3LJ detector circuit
The Teensy was mounted on a stripboard with a plug and socket , the whole unit was mounted in a 5x3x2" box.
Teensy++ Processor
The whole device is powered through the mini USB connector on the Teensy++. Programming was also done through this connector using the Arduino IDE v1.05 and the PJRC Teensy plugin. (note the plugin  wont even install with later Arduino IDE versions eg V1.5.2. Calibration was easy and requires 2 known signals 30 or 40dB apart. I used -12dBm and -42dBm available from my LPRO101 10MHz Rubidium.
Innards of completed meter

The meter also enumerates as a serial port on a computer and can be made to continually output a power value in Watts at 4800Baud using the $pcont command:-

0.000000000328
0.000000000311
0.000000000287
0.000000000272

more detailed information can be obtained on a 1 shot basis using the $ppoll command:-

Power (inst, pep, avg):
 286pW, 1.84nW, 1.04nW

This allows remote monitoring.
Total cost (with new parts) would be around $75

The meter measures from ~-70dBm to +17dBm thro 440MHz. It has provision for 3 calibrations (eg HF, VHF, UHF) to accomodate calibration issues over that range  I also found the -40dB tap flat through 500MHz shown on the TF3LJ site but originally published in the W7ZOI power meter addendum article. With this I can measure up to 200W and I can have the power meter read directly as I can enter the 40dB offset

I thought of building 2 units one for forward power one for reverse power. Then I spotted that Loftur now has a Power and SWR Meter with bargraphs, using 2x AD8307 design on his website using a 20x4LCD and a Teensy++. I can see one getting built, especially as I already have two AD8307 PCB!





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