The ADF modulates the received RF at what frequency?

The ADF modulates the received RF at what frequency?

• A. 50 Hz
• B. 100 Hz
• C. 155 Hz
• D. 200 Hz

Answer: (C)

The ADF uses an audio modulation on the received NDB signal so the bearing information can be extracted reliably by the receiver’s demodulation and indicator circuitry. The chosen modulation frequency sits in the audio range and is designed to be well away from mains interference and RF carrier noise, providing a stable reference for the bearing display. Among common options, 155 Hz is the standard fixed tone used for this purpose, whereas 50 Hz or 100 Hz would be more susceptible to power-line hum and 200 Hz is not the conventional tone used in ADF systems. So, modulating at 155 Hz gives a clean, stable audio signal for accurate direction finding.

The ADF uses an audio modulation on the received NDB signal so the bearing information can be extracted reliably by the receiver’s demodulation and indicator circuitry. The chosen modulation frequency sits in the audio range and is designed to be well away from mains interference and RF carrier noise, providing a stable reference for the bearing display. Among common options, 155 Hz is the standard fixed tone used for this purpose, whereas 50 Hz or 100 Hz would be more susceptible to power-line hum and 200 Hz is not the conventional tone used in ADF systems. So, modulating at 155 Hz gives a clean, stable audio signal for accurate direction finding.