Several questions were changed in this section. One AM question was deleted and two FM questions were deleted. There was only a net loss of one question, though. Questions about the modulation envelope (G8A11) and frequency-shift keying (G8A01) were added.
Modulation is the process by which we convey information over a radio link, and amplitude modulation is perhaps the most basic type of modulation. When we amplitude modulate a signal, what we’re doing is varying the amplitude of what we call the RF carrier signal in proportion to the amplitude of an audio signal.
Amplitude modulation is, therefore, the type of modulation that varies the instantaneous power level of the RF signal. (G8A05) The modulation envelope of an AM signal is the waveform created by connecting the peak values of the modulated signal. (G8A11) Figure 1 shows an amplitude modulated signal and the modulation envelope.
Single sideband, or SSB, is a type of amplitude modulation (AM). A conventional AM signal has three components, the carrier and two sidebands. Since the carrier signal carries no information and the information in both sidebands is the same, someone figured out that by eliminating the carrier and one of the sidebands you could still transfer the same amount of information.
Not only that, SSB voice communications has a couple of advantages of AM voice communications. One advantage of carrier suppression in a single-sideband phone transmission is that the available transmitter power can be used more effectively. (G8A06) The reason for this is that you’re putting all of the output power into the single sideband and not wasting it for the carrier and extraneous sideband.
Another advantage is that the phone emission that uses the narrowest frequency bandwidth is single sideband. (G8A07) An AM signal requires a bandwidth of 6 kHz or more; a single sideband signal takes up a bandwidth of only 3 kHz.
You must be careful when setting the audio level used to modulate a phone signal, no matter if it’s AM or SSB. If you set the level too high, the signal will be over-modulated, and this may cause “flat-topping.” Flat-topping of a single-sideband phone transmission is signal distortion caused by excessive drive. (G8A10) Another effect of over-modulation is excessive bandwidth (G8A08).
To set the appropriate audio level, you adjust the microphone gain control while watching the radio’s ALC meter. The transmit audio or microphone gain control is typically adjusted for proper ALC setting on an amateur single sideband transceiver. (G8A09)
Another type of modulation that we use in amateur radio is frequency modulation, or FM. Most VHF and UHF repeaters use frequency modulation. Frequency modulation is the name of the process which changes the frequency of an RF wave to convey information. (G8A03) In an FM signal, the carrier frequency changes proportionally to the instantaneous amplitude of the modulating signal.
Another way to produce a frequency-modulated signal is to use phase modulation. Phase modulation is the name of the process that changes the phase angle of an RF wave to convey information. (G8A02) Phase modulation is produced by a reactance modulator connected to an RF power amplifier. (G8A04)
Some modes, such as radio teletype (RTTY) use a modulation method called frequency-shift keying, or FSK. An FSK signal is generated by changing an oscillator’s frequency directly with a digital control signal. (G8A01)
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