Crystal audio oscillator circuit

Generating variable audio frequencies with crystal precision is not often done because it isn`t easy to make. Low frequency quartz crystals are difficult to find, and each would only provide a singular frequency. Some small tweaking of the crystal oscillator`s frequency with a trimmer capacitor is common, but the variation is almost insignificant. Most, but real expensive oscillator designs usually produce low audio
Crystal audio oscillator circuit - schematic

frequency waves with time proven, dependable, stable, adjustable Twin-T and Wien-bridge sine-wave oscillators. Many of the better ones of these use large size inductors and good quality large, high-grade capacitors. If square waves, triangle waves or pulses are needed then the sine wave is chopped up, processed and amplified as necessary. For less precise requirements some home made 555 circuit will usually do even if most of those - because of the uncertainty of the IC`s trigger point - has noticeably high jitter. This circuit is a working example that combining two frequencies produces a new useable audio product. Block diagram of this AF oscillator is depicted on Figure 1. This unique oscillator circuit uses two regular, TTL ICs and two of the most often used, low cost, color TV crystals in an unusual way. Using one quad NAND IC for both oscillators worked reliably but it had a tendency of the oscillators of locking together. That problem arises because both of the oscillators were sharing the same silicon chip, and thus they had too much coupling and influence on each other. The easiest way to prevent the lock-up is to make one oscillator out of one IC - the 7400 quad NAND gate package. Use the 7404 hex inverter for the other oscillator. The nominal quartz frequency for a color television crystal is 3, 579, 545 kHz and one is used for each oscillator. As seen on the schematics - Figure 2, the crystal oscillator...

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