Simple-vco

  
The output frequency of the VCO, U1, varies inversely with the input voltage. With a 1 `V input, the oscillator output frequency is about 1500 Hz; with a 5-V input, the output frequency drops to around 300 Hz. The output frequency range of U1 can be altered by varying the values of C1, R2, and R3. Increasing the value of any those three components will lower the oscillator frequency, and decreasing any of those values will raise the frequency. Output-waveform gymmetry suffers since the frequency varies from one extreme to the other.
Simple-vco - schematic

At the highest frequency, the waveform is almost equally divided. But when the frequency drops, the output of the circuit turns into a narrow pulse. If a gymmetrical waveform is required, add the second IC, U2, half of a 7473P dual TTL J-K flip-flop, to the oscillator circuit. The signal frequency output by U2 is "iz of the input.




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