E7E: Modulation and Demodulation

Modulation puts information onto a carrier wave; demodulation extracts it at the receiver. The specific circuits used depend on the modulation mode — FM and PM require reactance modulators, SSB requires balanced modulators, and the corresponding detectors must match the transmitted signal type.

This lesson covers reactance modulators for FM/PM generation, balanced modulators for SSB, frequency discriminators, product detectors, envelope detectors, mixers, pre-emphasis and de-emphasis, and the baseband concept.

Reactance Modulators and FM/PM

A reactance modulator is a circuit that varies a reactance (capacitance or inductance) in response to an audio signal, producing frequency or phase modulation. When a reactance modulator is coupled to an oscillator circuit, it shifts the oscillator's frequency in proportion to the modulating signal — this is FM.

FM phone signals are generated by reactance modulation of a local oscillator. The function of the reactance modulator is specifically to produce PM or FM signals by varying a capacitance. Applying the audio to a voltage-controlled capacitance (such as a varactor) in the oscillator tank circuit produces the frequency deviation.

Balanced Modulators and SSB

A balanced modulator is a circuit that multiplies the carrier and audio signals together in a balanced configuration. This multiplication produces both upper and lower sidebands but suppresses the carrier — the output is a double-sideband suppressed carrier (DSB-SC) signal. To produce SSB, a filter is added after the balanced modulator to remove one sideband.

The standard method to produce an SSB signal is: use a balanced modulator followed by a filter. The balanced modulator generates both sidebands with the carrier suppressed; the filter passes one sideband and rejects the other.

FM Detection and Discriminators

A frequency discriminator is a circuit that detects FM signals by converting frequency variations into amplitude variations that can be amplified and listened to. The discriminator output voltage is proportional to the instantaneous frequency deviation of the input signal.

SSB Demodulation: Product Detector

Demodulating an SSB signal requires a product detector. A product detector multiplies the received SSB signal by a locally generated carrier (BFO — beat frequency oscillator) at the original suppressed carrier frequency. This multiplication recovers the original audio. The product detector is essential for SSB because the carrier was removed at the transmitter and must be reinserted at the receiver.

AM Detection: Envelope Detector

A diode envelope detector demodulates AM signals by rectification and filtering of RF signals. The diode rectifies the AM carrier, and the RC filter smooths the output to follow the envelope — the amplitude variations that contain the audio information. This is the simplest detector circuit and works well for AM.

Mixers

A mixer is a nonlinear circuit that multiplies two input signals, producing new frequencies at their sum and difference. The principal frequencies that appear at the output of a mixer are the two input frequencies along with their sum and difference frequencies. Additional products (harmonics and their intermodulation) also appear but at lower levels.

When input signal levels to a mixer are too high, the nonlinearity becomes excessive and spurious mixer products are generated — unwanted signals that can appear as interference in the IF passband. Proper input level control (attenuators, gain control) is essential for clean mixer operation.

Pre-Emphasis and De-Emphasis

FM transmitters use a pre-emphasis network to boost higher audio frequencies before modulation. This compensates for the higher noise level at higher frequencies in FM reception and improves the signal-to-noise ratio for high-frequency audio content.

FM receivers use a de-emphasis network to restore the original flat frequency response after demodulation. De-emphasis is used for compatibility with transmitters using phase modulation — PM naturally pre-emphasizes higher frequencies, so receivers must de-emphasize to produce flat audio. The pre-emphasis time constant in amateur FM is typically 75 μs in North America.

Baseband

The term baseband in radio communications refers to the frequency range occupied by a message signal prior to modulation. For a voice signal, the baseband is typically 300 Hz to 3 kHz or 300 Hz to 3.4 kHz — the audio range before it is mixed with a carrier. After modulation, the baseband signal is shifted up to the RF carrier frequency.

E7E Practice Questions