A low frequency xtal oscillator: Austrian generosity, gold, and crystals

A while ago, an Austrian fellow contacted me for some collectibles, long-range telephone line filters (from carrier multiplex phone lines). Many decades ago, phone lines were used at some 50-100 kHz frequencies, to transmit several (!) calls per wire pair. This required good filter, quartz filters were commonly used.

These are 4-electrode filters that are held only by 4 wires soldered to it. Probably oscillating in some flexing mode.

The electrodes are normally connected diagonally, and with a few resistors and an amplifier, I got the part to oscillate nicely. Be aware that you can’t feed a lot of power to these crystals, so it needs a rather high impedance oscillator circuit.

Resonance is at about 50 kHz.

Also connected the specimen to a HP 3562A analyzer, in swept frequency mode, and good nice response plots. There is another dip at 100 kHz!

The schematic, pretty simple, using a 74HCU04 unbuffered inverter, it is a very handy circuit, and years ago I got several tubes of these… you may use any other type of amplifier, gate, or even transistor circuit to get any such xtal oscillating.

Also did some some study on the temperature effect – heated to 100 degC, the frequency dropped by 200 Hz!

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