Voltage-controlled Oscillator (VCO)
The second input to the mixer comes from the VCO. It is a signal that varies linearly from 40 kHz to 60 kHz several times a second. The VCO used in this circuit is the ICL 8038 Function Generator IC. It delivers frequencies upto 90 kHz with negligeble distortion. It has been appropriately biased so as to work according to our specifications.
The VCO requires a sweep voltage at its input (Pin 8 w.r.t. Vcc) to generate a frequency that is proportional to the voltage. This biasing input to the VCO comes from the sawtooth generator which also controls the horizontal deflection of the CRO, thereby synchronizing the position of the electron beam with the selected frequency (20 Hz at extreme left; 20 kHz at extreme right).
The output is available from Pin 3. If RA = RB = R, then the selection of biasing resistors for the VCO to get the desired sweep frequency range is governed by the following formulae:
The sweep voltage comes from a ramp generator, which is discussed on the next page.