>>> FM219 VCO TUNING & FREQUENCY MODULATION PCA <<<
Shown in Figure 1 is the FM219 printed circuit Board Assembly (PCA) used to set the center frequency and frequency modulate the VCO2C, 1K, and VCO2K oscillator assemblies. The following is an explanation of the circuit operation. The FM modulation input of Figure 2 is coupled to U1A with a capacitor which blocks a DC level from the modulation source. If a DC level is not present on the modulation source the capacitor can be replaced with a wire to extend the frequency response down to DC. R7 of Figure 2 is a 20-Turn pot which provides a precise input voltage to U1B which sums this voltage with the modulation from U1A and drives the V-T terminal of the VCO assembly. The +/- 18 to +/-24 volt power supply input is regulated down to +/- 15 volts to provide a stable, noise-free voltage for center frequency adjustment with R7. The 12 volt regulator takes its input from the +15 volt regulator and supplies a stable, noise-free supply voltage to the VCO assembly.
Figure 1, VCO Tuning and Frequency Modulation Printed Circuit Board Assembly
Green Wire => +18 to +24 volt input
Black Wire => common ground
White Wire => modulation input
Yellow Wire => -18 to -24 volt input
Red Wire => +12 volt output
Blue Wire => CV output
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Figure 2, VCO Tuning and Modulation Circuit
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