Pulse Frequency Modulator


The pulse width of the
compact pulse cum frequency modulator can be varied by altering the
change-over point of comparator IC1 with a control voltage via resistor
R1. The hysteresis of the IC is determined by resistors R3 and R4. The
control voltage also causes the frequency of the present circuit to be
altered. When the input voltage is 0 V, the frequency is a maximum: in
the present design this is about 3.8 kHz. The level of the output
voltage is ±12–13 V. The more the change-over point has been shifted
with the control voltage, the longer it will take before the potential
across capacitor C1 has reached the level at which IC1 is enabled.

Pulse Frequency Modulator Circuit

Pulse Frequency Modulator Circuit Diagram

When the control voltage is larger than the zener voltage, the
oscillator ceases to work. The maximum period is 25 ms, which may be
adapted by altering the value of C1. This will, of course, also alter
the maximum frequency. The duty cycle is inversely proportional to the
control voltage. The minimum pulse width attainable at the lowest
frequency is about 6 µs. The modulator draws a current not exceeding 5
mA.


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