Question 6A379

Heising modulation, or constant current modulation, varies the RF amplifier's plate voltage using a common modulation choke shared with an audio modulator. To achieve 100% modulation, the RF amplifier's plate voltage must swing from nearly zero to twice its unmodulated DC value. A series resistor is placed in the DC plate supply for the RF amplifier, typically after the common modulation choke. This resistor causes a DC voltage drop, effectively lowering the RF amplifier's quiescent plate voltage. By operating at a slightly lower nominal voltage, the modulator can more easily drive the amplifier's plate voltage down closer to zero and up to double this reduced quiescent value, achieving the wide swing necessary for 100% modulation. Option B is incorrect because Heising systems often require this modification to achieve full modulation. Option C is incorrect; a capacitor in the DC grid supply would block essential DC bias, making the amplifier inoperable, and is not how plate modulation in Heising systems is accomplished. Option D is incorrect as C is an invalid solution.