Question 6A380

In a Heising (series) modulator, the plate choke plays a critical role in coupling the audio signal. Its high inductive reactance at audio frequencies prevents the audio energy from being shunted by the power supply. Instead, the varying audio current from the modulator tube causes a significant audio modulating voltage to "build up" across the choke. This developed audio voltage is then effectively connected in series with the DC plate supply of the RF power amplifier. By varying the RF amplifier's plate voltage at the audio rate, the choke feeds these audio modulating voltages to the RF amplifier, thereby producing classic plate modulation. Both functions—building up and feeding the audio—are essential for proper operation.