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 A Basic RF Voltmeter  for 1MHz to 1GHz
By: Bertrand Zauhar, VE2ZAZ

One can build a basic RF voltmeter for a few dollars. The unit described below provides a meaningful reading from less than 1MHz to around 1GHz. The 1GHz frequency limitation comes from the parasitic reactances of the diode (lead inductance, junction capacitance). At around 1GHz, the diode loses it rectifying ability and the forward voltage dies rapidly as frequency is increased further.

With this RF voltmeter, I made reasonably accurate measurements from -15dBm to +23dBm. Accuracy was around +/-2dB. Below -15dBm, a relative response can be expected, but an absolute measurement cannot be made since the diode responds more to power variations than it does to voltage (a square-law response). Nevertheless, it still constitutes a valuable tool for peaking RF circuits.

The RF voltmeter is made of a SMA-female PCB-mount connector, a 1N5711 Schottky diode, a 10nF capacitor and two 100 Ohm 1206-size surface mount resistors, as shown in the drawing below. The unit is connected to a digital voltmeter that has an input resistance of at least 1 MegOhms. This RF voltmeter is of the terminated type, meaning that it must be placed as a termination at the end of a 50 Ohm circuit or transmission line.

A little bit of math is required to get  a reading in dBm. First, the user must understand that this circuit will charge the capacitor to the peak ampliture of the measured RF signal, minus some forward voltage drop through the diode (which we will neglect). The Digital VoltMeter (DVM) will read the DC voltage across the capacitor. Consequently, the following must be done to get a reading in dBm: For more information on expected accuracy and behavior, refer to the following links: When building this RF Voltmeter, keep the diode and capacitor leads as short as possible. As well, solder the two 100 Ohm resistors as close as possible to the SMA connector body. This will provide a stable 50 Ohm termination.

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