Extra Class question of the day: Modulation and demodulation

September 13, 2012 Dan KB6NU

Modulation is the process of adding some kind of information, including voice and digital information, to a carrier signal. The most common types of modulation that we use in amateur radio are amplitude modulation (AM) and frequency modulation (FM). Single-sideband, or SSB, is a form of amplitude modulation.

To frequency modulate a carrier, a transmitter will sometimes us a modulator that varies the phase of the signal. This is sometimes called phase modulation (PM). One way to generate FM phone emissions is to use a reactance modulator on the oscillator. (E7E01) The function of a reactance modulator is to produce PM signals by using an electrically variable inductance or capacitance. (E7E02) An analog phase modulator functions by varying the tuning of an amplifier tank circuit to produce PM signals. (E7E03)

To boost the higher audio frequencies, a pre-emphasis network is often added to an FM transmitter. (E7E05) For compatibility with transmitters using phase modulation, de-emphasis is commonly used in FM communications receivers. (E7E06)

Amplitude modulation and single-sideband signals are produced using mixer circuits. The carrier frequency and the baseband signals are input to the mixer circuit which produces an amplitude modulated output. The term baseband in radio communications refers to the frequency components present in the modulating signal. (E7E07) The principal frequencies that appear at the output of a mixer circuit are the two input frequencies along with their sum and difference frequencies. (E7E08)

When using a mixer, you must take care not to use too high of a signal at the inputs. Spurious mixer products are generated when an excessive amount of signal energy reaches a mixer circuit. (E7E09)

Single sideband is most often used for phone transmission on the HF bands and for weak-signal operation on the VHF and UHF bands. One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter.  (E7E04) A balanced modulator is a type of mixer.  The output of a balanced modulator, however, does not contain the carrier frequency, only the two sidebands.

Modern transceivers use digital signal processing to generate SSB signals. The quadrature method describes a common means of generating an SSB signal when using digital signal processing. (E7E13)

At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a diode detector circuit. A diode detector functions by rectification and filtering of RF signals. (E7E10)

For demodulating SSB signals, you want something a little more sophisticated. A product detector is a type of detector that is well suited for demodulating SSB signals. (E7E11) A product detector is actually a frequency mixer. It takes the product of the modulated signal and a local oscillator, hence the name. In an FM receiver, the circuit for detecting FM signals is a frequency discriminator. (E7E12)

Some modern receivers demodulate a signal entirely in software. These receivers are called software-defined receivers. When referring to a software defined receiver, direct conversion means incoming RF is mixed to “baseband” for analog-to-digital conversion and subsequent processing. (E7E14)

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