Saturday, 23 August 2014

Power play

I've been meaning to make some kind of RF power meter for a while now. Well, today I made a start...


I was inspired (not for the first time) by Eamon "Ed" Skelton, ei9gq, whose monthly "Homebrew" feature in RadCom is - in my opinion - the best regularly published piece on practical radio engineering in the UK.

In the current (September) issue, Ed talks about "RF voltage and power measurement" and I drew heavily on his QRP load and compensated peak volt meter, to come up with a system, the important parts of which follow closely Ed's "words and music"...


As you see from the photo at the head of this post, I've hooked up the compensated detector to an Arduino, which makes a voltage measurement (rather than the moving coil meter / current measurement in Eamon's system). The Arduino runs some trivial code, the more important parts of which are reproduced here...


The "LCD_Display_Power()" function displays the measured peak voltage on the LCD (after doing some simple math to convert the number from the Analog to Digital converter to a voltage - and then to convert to dBm [assuming the measured peak voltage to have been associated with a sinusoidal waveform]), resulting in this sort of display...


I observed the response of the new meter as I applied different amplitude RF signals from my old Tektronix SG503, which sits on the shelf ready for little tasks like this...


I monitored the amplitude on a scope and plotted the RF peak voltage (as "eyeballed" from the 'scope) against the new meter's output...


All pretty encouraging for a first shot - there is a nice linear characteristic, indicating correct operation within the "operating range" of the instrument (if I might abuse that grand word). The bottom end of the dynamic range is pretty much where I expected (from a combination of an LTSpice simulation and consideration of the numerical resolution of the ADC), allowing useful measurements down to -6dBm and indication to below -12dBm. The top end of the measurement above ISN'T the top end of the dynamic range of my new meter - it's the top end of the available outputs from the SG503 source, which only goes up to 2V into 50 Ohms.

As you might understand, there are some "other refinements" in my power meter, not described in the "simple telling of the story" above. I'm working on these at the moment and will publish them when the time is right. Notwithstanding these other ideas, the basic architecture above will present a framework for some interesting test equipment - watch this space.

...-.- de m0xpd

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