When it comes to radio frequency oscillators, crystal controlled is the way to go when you want frequency precision. But not every slab of quartz in a tiny silver case is created equal, so crystals need to be characterized before using them. That’s generally a job for an oscilloscope, but if you’re clever, an SDR dongle can make a dandy crystal checker too.
The back story on [OM0ET]’s little hack is interesting, and one we hope to follow up on. The Slovakian ham is building what looks to be a pretty sophisticated homebrew single-sideband transceiver for the HF bands. Needed for such a rig are good intermediate frequency (IF) filters, which require matched sets of crystals. He wanted a quick and easy way to go through his collection of crystals and get a precise reading of the resonant frequency, so he turned to his cheap little RTL-SDR dongle. Plugged into a PC with SDRSharp running, the dongle’s antenna input is connected to the output of a simple one-transistor crystal oscillator. No schematics are given, but a look at the layout in the video below suggests it’s just a Colpitts oscillator. With the crystal under test plugged in, the oscillator produces a huge spike on the SDRSharp spectrum analyzer display, and [OM0ET] can quickly determine the center frequency. We’d suggest an attenuator to change the clipped plateau into a sharper peak, but other than that it worked like a charm, and he even found a few dud crystals with it.
Fascinated by the electromechanics of quartz crystals? We are too, which is why [Jenny]’s crystal oscillator primer is a good first stop for the curious.
4 thoughts on “Classifying Crystals With An SDR Dongle”
As per the Jenny List article: “One of the capacitors may sometimes have a small variable capacitor in parallel, allowing very small frequency adjustments to be made”
While a simple oscillator is great,I was expecting to also see a completed IF filter on the input of the SDR, along with a variable capacitor oscillator. That way, the frequency response of the whole filter could be tested.
You can use a noise source for that, much easier. You don’t need to tune noise, you just see what gets through.
I’ve never gotten my head around how the various overtone-mode crystals would work. In use, the circuit surrounding the crystal has additional frequency-dependent components, so it suppresses the fundamental, and the crystal can only resonate in the overtone. But how would you detect such a crystal with a rig like this? Wouldn’t it always start up at the fundamental?
Yes, it will, you will need to multiply the frequency. Most crystals can work at a harmonic, even if not specially made for it
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