Some RF Learnin'

Phil, Jon, and I got our ham radio licenses last weekend. Just 'cause. I donno.

This, and a grand plan to make an NMR spectrometer led us to do some experimenting with radio frequency electronics. We've learned a few things already.

First of all, you can't use breadboards, I guess. I think there were several problems with what we were doing at first, but it seems likely that this was one of them. As everyone seems to say, breadboards are problematic above a few MHz. Too much capacitance, or something. We've abolished them. Sad, I know. They're so nice.

Second, parasitic inductances and capacitances are everywhere. We tried to make a low-pass filter using a resistor and capacitor.


2000px-1st_Order_Lowpass_Filter_RC.svg

What could be simpler than that? Well, we got some confusing results:

Green - No Filter Blue - Low-Pass Filter
Green - No Filter
Blue - Low-Pass Filter

Phil has a white noise generator. The idea was that we'd run the white noise through the filter and see the noise power die off above the filter's cutoff frequency. The green curve shows the white noise with no filter, and the blue shows it with our low pass filter. This is not the signature of a low pass filter. We've concluded that the parasitic inductance of the capacitor effectively adds a small inductor in series with the capacitor and makes the vertical part of our filter into a resonator that shorts to ground at its resonant frequency.

Inspired by this, we found a way to measure the inductance of a 10 turn coil we made for our NMR experiment. We estimated its inductance at about 10μH and put it in series with a 0.47μF capacitor, aiming for a resonant frequency of about 100 kHz. We used the AD 9850BRS to apply a voltage at that frequency and measured the amplitude and the voltage across the coil. We varied the frequency until we saw the amplitude of the applied voltage fall to a minimum. This was at about 70 kHz.

Author | Ben Wiener

Currently a Ph.D. student in Physics at Brown University.