MW0LLO RF Milliwatt Meter

Posted on Feb 7, 2014

This project, based on an article published in the February 2008 edition of Radcom (the magazine of the Radio Society of Great Britain) is for a meter with a 50 ohm input impedance for measuring very small RF powers. Many RF power meters become inaccurate at low powers, but this meter is able to measure down to very low levels. From a theoretical

MW0LLO RF Milliwatt Meter
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standpoint, the meter is only able to accurately measure the power of steady, sine wave voltage RF - it would not be suitable for square wave signals for example. In practice, however, the most common requirement is to measure the power of sine wave signals and to compare power readings (when peaking up IF stages, for example). The reason that the meter is best suited to sine waves is that the circuit actually measures the peak-to-peak voltage in order to infer the power. The Radcom article explains how many RF `sniffer` type projects rely on detecting RF using a diode. This diode will require a certain amount of voltage to bring it into forward conduction, and this is the source of inaccuracy at low powers for more conventional designs. The original article gives details for a meter based on PNP transistors. I am more used to understanding circuits based on NPN transistors, so I first redrew the circuit diagram with NPN transistors and the diodes and PSU reversed. Once I had a good understanding, I realised that the design can be broken down into two sections; I call these the RF section and the DC voltmeter section. The RF part of the circuit is basically a dummy load with two detecting diodes to get the peak positive and peak negative voltages. A DC blocking capacitor is connected at the input, which acts as a short circuit at RF frequencies. What makes this design different is that the detecting diodes are driven into...

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