For homebrew crystal filters such as Cohn-Type or Ladder-Type filters it is often necessary to select crystals within 10 – 50 Hz of the target frequency. To make the selection process a little bit more pleasant, I designed this little test circuit.
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It is basically just a very simple oscillator utilizing a hand full of standard parts. None of the component values are critical. Pretty much any transistor will work in this circuit. I included a HC-18 socket on my PCB for a fast turnaround time while selecting crystals. The output of the oscillator can be connected to a frequency counter to determine the crystals exact frequency.
One word of advise. Be careful to not hold the crystal too long between your fingers before performing the test. The body heat will alter the resonance frequency of the crystal and falsify the result.
Simple oscillator circuit
Crystal tester assembled on a bit of perforated PC board.
Westerhold, S. (2012), "Simple oscillator as crystal tester". Baltic Lab High Frequency Projects Blog. ISSN (Online): 2751-8140., https://baltic-lab.com/2012/08/simple-oscillator-as-crystal-tester/, (accessed: January 21, 2025).
Sebastian is a self-taught independent researcher and tech influencer with a passion for radio frequency (RF) and digital signal processing (DSP). He's been active as a blogger, YouTuber and author for Elektor Magazine for over a decade.
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