Simple RF Power Reference For RF power meter calibration
You’ve just finished building a new RF power meter, like the ABPM from Down East Microwave (www.downeastmicrowave.com) – how do you calibrate it? Or perhaps you’ve acquired a surplus unit
– how do you check the calibration? You need some known reference point or a friend with a calibrated meter.
Some recent commercial power meters, like the HP 435, include a reference output to check and set the calibration of the meter. These fine meters are occasionally found as surplus at very reasonable prices, probably because the RF sensor heads command such exorbitant prices that the meter alone is unattractive. However, the meter is worth a modest amount for the reference output alone, to be used to check other power meters, either homebrew or older surplus.
With inexpensive RF power detector chips made for wireless networking, it is easy to homebrew a good microwave power meter like ABPM – so I started thinking about ways to make a reproducible reference source for RF power. The obvious starting point was the HP 435; I looked at the schematic, but it didn’t feel right.
One readily available, reproducible, component is a diode. If we use a pair of anti-parallel (back-to-back) diodes as a clipper, we get a pretty predictable voltage. Of course, the clipping action generates lots of harmonics – this is a good way to make a frequency multiplier or harmonic mixer – so a low-pass filter is needed to get a sine wave output. Many power detectors are sensitive to harmonics and give erroneous readings unless the signal is pretty clean. Finally, filters are only predictable with matched resistive loads, so an attenuator helps keep things matched.
For an oscillator with a reasonably predictable output, I chose a standard computer oscillator. Since the HP 435 output is at 50 MHz, high enough for microwave instruments but low enough for VHF and down, it seemed like a good idea. For diodes, the 1N914 or 1N4148 is available almost anywhere for pennies. The low-pass filter uses RF chokes for inductors and ordinary disk capacitors, and the attenuator is just three resistors. A small 3-terminal regulator provides constant voltage to keep things stable. The complete schematic is shown in Figure 1.
The real question is whether it is reproducible. Figure 1 was drawn in free Linear Technology LTSpice/SwitcherCAD III software (www.linear.com) to simulate the circuit. I fiddled with the values of R1 and R2 at different temperatures and oscillator outputs to find a combination where the final output was pretty constant as the temperature and oscillator output varied. The simulation file is power_ref50.asc.
Download RF Power Reference for power meter calibration