Here are a few oscillators, beginning with AF and ending with RF that can be used as local oscillators for homebrew receivers. There are litterally millions of different oscillator configurations so I will restrct this to a few blocks.
I will not go into ICs and logic gates as there is loads of information regarding logic gate oscillators available on the net. Here I will show a cheap'n chearfull multivibrator oscillator.
Typical values for 1KHz:
| Component | Value | Component | Value |
|---|---|---|---|
| R1 | 100K | R2 | 100K |
| R3 | 4K7 | C1 | 33n |
| C2 | 33n | +ve | 10v |
This simple AF oscillator is quite stable and if polystyrene capacitors are used can be accurate enough for a repeater access tone-burst generator. Two such oscillators could be used to to generate the two-tone signal required to test SSB transmitters.
The 10n and 10K resistors govern the frequency of the oscillator. The values shown place the frequency of oscillation somewhere in the center of the communications AF spectrum.
This oscillator is ideal for use up to about 15MHz when a variable frequency source is required. This could be required for variable frequency receiver local oscillator circuits in receivers. To improve stability at higher frequencies it is adviseable to wind L1 on a ferrite ring.
The tuned circuit may be replaced by a crystal and the circuit will oscillate at the fundamental frequency only. If used as a VFO then the mechanical construction of the oscillator should be very solid. Use laquer or even wax on the coil to prevent microphony.
This circuit is ideal when a crystal must be made to resonate on its fundamental or any specific odd harmonic. It is therefore very usefull for VHF receivers and crystal markers. Crystals above about become 20MHz too fragile so higher frequency crystals will normally use harmonics. Choose the correct harmonic by selecting the tuned circuit.
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I have also shown the same oscillator with a frequency multiplier. A typical crystal would be cut to 15MHz but designed and marked for operation on its 3rd harmonic - 45MHz. The multiplier stage multiplies the frequency to 135MHz to create a local oscillator for 145.700MHz. In the second drawing above I have shown how a 3rd harmonic 24.1667MHz crystal may be used to generate a signal on 145.000MHz.