[KB6NU] 2016 Extra Class study guide: E7E - modulation and demodulation
 

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2016 Extra Class study guide: E7E - modulation and demodulation

Posted: 08 Feb 2016 11:40 AM PST
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The questions on digital signal processing (DSP) and software defined radio
(SDR) were removed from this section, and E7F, which does have questions on
these two topicsDan

E7E Modulation and demodulation: reactance, phase and balanced modulators;
detectors; mixer stages

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 use 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.

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)

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