Modulation is the process of adding information, such as voice or 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 process is called phase modulation (PM), and the type of modulator used to phase modulate a signal is called a reactance modulator. It uses an electrically variable inductance or capacitance to produce PM or FM signals.
QUESTION: Which of the following can be used to generate FM phone emissions? (E7E01)
ANSWER: A reactance modulator on the oscillator
QUESTION: What is the function of a reactance modulator? (E7E02)
ANSWER: To produce PM or FM signals by using an electrically variable inductance or capacitance
When generating FM signals, a pre-emphasis network is often added to an FM transmitter to boost the higher audio frequencies. Conversely, de-emphasis is commonly used in FM communications receivers to maintain compatibility with transmitters using phase modulation.
QUESTION: What circuit is added to an FM transmitter to boost the higher audio frequencies? (E7E05)
ANSWER: A pre-emphasis network
QUESTION: Why is de-emphasis commonly used in FM communications receivers? (E7E06)
ANSWER: For compatibility with transmitters using phase modulation
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. In other words, the term baseband refers to the frequency range of a modulating signal prior to it being mixed with a carrier signal. A mixer combines the carrier and baseband signals and produces four different output frequencies: the two input frequencies along and the sum and difference of the two input frequencies. When using a mixer, you must take care not to use too high of a signal level at the mixer inputs, or it may generate spurious products.
QUESTION: What is meant by the term “baseband” in radio communications? (E7E07)
ANSWER: The frequency range occupied by a message signal prior to modulation
QUESTION: What are the principal frequencies that appear at the output of a mixer circuit? (E7E08)
ANSWER: The two input frequencies along with their sum and difference frequencies
QUESTION: What occurs when an excessive amount of signal energy reaches a mixer circuit? (E7E09)
ANSWER: Spurious mixer products are generated
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. A balanced modulator is a type of mixer, but its output contains only the two sidebands, not the carrier or baseband signal. A filter follows the balanced modulator to eliminate the undesired sideband.
QUESTION: What is one way a single-sideband phone signal can be generated? (E7E04)
ANSWER: By using a balanced modulator followed by a filter
At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a simple diode detector circuit. A diode detector rectifies and filters a modulated signal, thereby producing an audio signal at its output. For demodulating SSB signals, you want something a little more sophisticated, like a product detector. A product detector is actually a frequency mixer, followed by a filter . Its output is the product of the modulated signal and a beat frequency oscillator, hence the name.
QUESTION: How does a diode envelope detector function? (E7E10)
ANSWER: By rectification and filtering of RF signals
QUESTION: Which type of detector circuit is used for demodulating SSB signals? (E7E11)
ANSWER: Product detector
FM receivers use a circuit called a discriminator to detect an FM signal. A discriminator converts a signal that changes in frequency to one that changes in amplitude.
QUESTION: What is a frequency discriminator stage in a FM receiver? (E7E03)
ANSWER: A circuit for detecting FM signals
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