Modulators, Mixers, and Detectors

Generating each mode

What is the function of a reactance modulator?

Produce PM or FM signals by varying a capacitance.

A varactor diode’s capacitance changes with the voltage across it, so audio applied to a varactor in an oscillator’s tank circuit shifts the oscillator’s frequency. That is FM, generated directly.

What is one way to produce a single-sideband phone signal?

Use a balanced modulator followed by a filter.

The General track’s two stages: the balanced modulator cancels the carrier, the filter removes one sideband.

Recovering each mode

What is a frequency discriminator?

A circuit for detecting FM signals.

It converts frequency deviation into an amplitude that varies the same way — which is also why an off-frequency FM signal sounds distorted, from the Technician controls lesson: the discriminator is operating on the wrong part of its curve.

How does a diode envelope detector function?

By rectification and filtering of RF signals.

The simplest detector there is: rectify the RF and filter off what remains, leaving the envelope. It works for AM because AM’s information is the envelope, and it fails for SSB because SSB has no envelope in that sense.

Which type of detector is used for demodulating SSB signals?

Product detector.

Because SSB needs the carrier reinserted, which is a multiplication rather than a rectification.

Mixers

What are the principal frequencies that appear at the output of a mixer?

The two input frequencies along with their sum and difference frequencies.

Note the “principal”: both inputs appear at the output as well as the sum and difference. This is why a mixer needs a filter after it, and why mixer balance — how well it suppresses the inputs — is a real specification.

What occurs when the input signal levels to a mixer are too high?

Spurious mixer products are generated.

Overdrive it and it stops behaving as a clean multiplier: higher-order products appear, and they land where nothing should be. This is the mechanism behind the receiver-overload story from the Technician track, seen from inside the mixer.

Pre-emphasis and de-emphasis

What is added to an FM speech channel to boost the higher audio frequencies?

A pre-emphasis network.

Why is de-emphasis used in FM communications receivers?

For compatibility with transmitters using phase modulation.

The pair works like this. In FM, noise increases with audio frequency, so the high notes suffer most. Pre-emphasis boosts the highs before transmission; de-emphasis cuts them by the same amount at the receiver. The audio comes out flat and the noise, which was never boosted, comes out reduced.

The pool’s answer names the other reason: FM and PM produce inherently different frequency responses — PM’s deviation rises with audio frequency where FM’s does not — and the emphasis pair reconciles them, so an FM receiver correctly demodulates a phase-modulated transmitter.

Baseband

What is meant by the term “baseband” in radio communications?

The frequency range occupied by a message signal prior to modulation.

The signal before it is put on a carrier — speech at 300 to 3000 Hz, or a data stream at its native rate. Modulation moves baseband up to the carrier; demodulation brings it back down.

Check yourself

1. You need to demodulate a CW signal. Which detector, and why not an envelope detector?
2. Your mixer output is full of unexpected signals. First thing to check?
3. What does pre-emphasis do, and where is it undone?