Basic RF Voltmeter for 1MHz
By: Bertrand Zauhar, VE2ZAZ
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.
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.
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
type, meaning that it must be placed as a termination at the end of a
50 Ohm circuit or transmission line.
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
more information on expected accuracy and behavior, refer to the
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
- Multiply the DVM
reading by 0.707 to get the RMS RF voltage VRMS,
- Use this equation to
derive the power value in dBm: dBm = 20*Log(VRMS / 0.224), or
Consult a Voltage to power
table such as the one
provided on this website.