Variable Frequency Crystal Controlled Adjustable Audio Oscillator Circuit

Figure 1. Block Diagram of Crystal Controlled Adjustable 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 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.

Figure 2. Schematic of Crystal Controlled Adjustable Audio Oscillator Circuit

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.

Figure 3. Schematic of SN7400 TTL NAND IC Internal Circuit

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 frequencies are adjusted with a small 50pF rotary capacitor. The oscillation is a high frequency that is adjustable by a small amount.

Figure 4. Input Output Waveforms on Mixer Stage

With the circuit as shown, with a five-volt power supply, the audio output frequency range is 10 Hz to a little over 2 kHz. The gates used are all belong to the first generation TTL group. It is very beneficial if the inside circuit is known to save many hours spent on discovery work. The inside circuit topology of the IC - Figure 3 - and the parts values are important to design a stable oscillator configuration.

Almost any currently available logic family is useable in this circuit with some minor modification of some component values. The LS version of these gates, or CMOS or other parts is also useable without much difficulty. All components used are easy to find and tolerances are not critical. These early TTL components were chosen, because they have good information on them. Figure 4, has all mixer waveforms before the low-pass filter. The filter section used here to attenuate the unwanted higher frequency harmonics.

                Parts and materials:

                    B1 - Battery, 5 Volt 
                    C1 - Capacitor, 50 pF, Rotary
                    C2 - Capacitor, 50 pF, Rotary
                    C3, C4 - Capacitor 0.1 uF, Disc
                    C5 - Capacitor 0.05 uF, Disc
                    IC1 - Integrated Circuit, TTL, SN7400, Quad NAND
                    IC2 - Integrated Circuit, TTL, SN7404, Hex Inverter
                    R1, R2 - Resistor Resistor, 220, 5%, 1/4W, CC
                    R3, R4 - Resistor Resistor, 120, 5%, 1/4W, CC
                    R5 - Resistor Resistor, 1k, 5%, 1/4W, CC