74S124 High Frequency VCO with Varactor


Posted on Feb 5, 2014

The following schematic diagram shown a conventional which is modified to become a VCO (voltage-controlled oscillator) circuit. By replacing the conventional fixed capacitor with a variable capacitance diode (varactor), we can turn a 74S124 multivibrator into a wide-band VCO. High speed logic 74S series is used here to enable this circuit for high


74S124 High Frequency VCO with Varactor
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frequency operation. Here is the schematic diagram of the circuit: The need of a relatively high voltage source, the 30 V biasing voltage that the diode requires is the only disadvantage of this scheme. We can use lower voltage for this, but it means the VCO would be no more wide-range. Capacitors C1 and C2 couple the Philips BB909A variable capacitance diode to the 74S124. This capacitors can be selected to different values to accommodate different frequency range. Use C1 = C2 = 47pF for 6. 8 to 72 MHz, and use C1 = C2 = 22nF for 2. 2 to 65 MHz. R1 and R2 are large enough to isolate ground and control voltages from the timing capacitors. The potentiometer is used as a voltage divider for controlling the capacitance of varactor, thus controlling the frequency of this VCO circuit. This potentiometer can be omitted if we control this circuit using voltage signal from other devices, replacing the center tap of this potentiometer. In hybrid system, analog system with digital control, a voltage controlled things is very important since it can be interfaced by DAC from digital system. For radio tuning with digital control, a voltage controlled LC circuit is needed. If we Continue reading †’. VCO (voltage controlled oscillator) is an electronic signal generator which produce a signal that has variable frequency depends on a voltage level at its control input. Many VCO circuits are based on ramp generator to produce a variable...




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