Needed a new power meter as I blew up the sensor on my HP one months ago. I can now understand why there are so many meters without power sensors. Not willing to pay the price for a replacement, I looked at EMRFD and found the W7ZOI power meter.
This particular unit is tested by me from -72 to +13dBm (35uV to 1V RMS) and 1-180MHz and is flat to within 1dBm. This is somewhat limited due to my signal generation and attenuation capability being relatively restricted. If anyone has a Marconi 2019A they want to get rid of, please let me know
Schematics and article here:
http://www.qsl.net/sz1a/download/bui...er%20meter.pdf
I bought the AD8307's from Aliexpress from China for around £2.80 for 10. They are £10.15 each (!) on RS. Seem to work absolutely fine. Enclosure is a Pomona box from an ebay lot. I would use a die cast / eddystone box if there wasn't one on hand. Power is fed through one of the screw holes while I wait for some feed through capacitors to arrive.
Due to the linear response, if you have a DMM that can do math, you can hook this up and enter the M and C calibration constants (Y=MX+C). M is volts per decibel and C is the 0 dBm intersection voltage. This will then read dBm on the meter display!
Calibration is pretty simple. Stick a reference signal (I used 0dBm) on your generator and write down the voltage, then stick a 20dB attenuator inline with the PM and write down the voltage again. The M value is quotient of the difference of the two voltages and 20. The C value is derived by entering 0 to start with then applying 0dBm. The offset from zero is the C value. You can of course use a programmable calculator, graph or any other method to read the power with a normal DMM.
The buffer amplifier with LPF on the output is 100% necessary - the AD8307 output is quite sensitive so isolation is needed. The loop in the input resistor is actually a carefully (not) wound inductor which stops some UHF pick up. It was quite unstable without that little loop!
Construction is very ugly but it works.
Will be doing more SMD stuff. It's cheaper and not as hard as it looks. Tweezers and beer make it workable
