Explain how FM, SSB and mixing signals are produced and detected, and why pre-emphasis and de-emphasis are used.
Syllabus: Modulation and demodulation: reactance, phase, and balanced modulators; detectors; mixers
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
A reactance modulator lets the audio change a capacitance (often a varactor diode) in the oscillator's tuned circuit. The oscillator's frequency or phase follows the audio, giving FM or PM. You apply it to a local oscillator, not the final amplifier.
RememberReactance modulator: audio varies a capacitance, so the oscillator makes PM or FM.
A pre-emphasis network in the transmitter boosts the higher audio frequencies. A de-emphasis network in the receiver cuts them back, so the audio is flat again.
A phase modulator already produces that rising response, so receivers use de-emphasis for compatibility with transmitters using phase modulation.
RememberPre-emphasis boosts highs (transmit). De-emphasis cuts them (receive).
A balanced modulator multiplies audio by a carrier and cancels the carrier, leaving two sidebands (DSB). A sideband filter then removes one: single sideband. The audio before modulation is baseband: the frequency range of the message signal.
RememberBalanced modulator followed by a filter = SSB. Baseband = message range before modulation.
Two frequencies in, four principal frequencies out: each input, the sum, and the difference. A filter picks the one you want. Overdrive the inputs and the mixer's non-linearity creates extra spurious products at other frequencies.
RememberMixer out: f1, f2, f1 + f2, f1 − f2. Input too strong: spurious products.
| Signal | Detector |
|---|---|
| AM | Diode envelope detector: rectification and filtering of RF |
| SSB, CW | Product detector: a mixer with a local oscillator |
| FM | Frequency discriminator |
RememberDiscriminator = FM detector. Product detector = SSB detector. Envelope = diode rectify and filter.