Classifying Crystals With An SDR Dongle

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

  1. 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.

  2. 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?

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