Transmitters and Receivers
Revision part 2
Learning outcomes of this part
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when two frequencies are mixed together, new frequencies are generated by the mixing process.
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The new frequencies are equal to the sum of and the difference between the original frequencies.
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when audio frequencies are mixed with radio frequencies, the new frequencies that are generated are called sidebands.
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the process of mixing audio frequencies with radio frequencies is called modulation.
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amplitude modulation contains two sidebands and the carrier.
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single sideband (SSB) is a form of amplitude modulation where one sideband and the carrier have been removed from the transmitted signal.
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SSB is more efficient because power is not used to transmit the carrier and one sideband.
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and second advantage is of the transmitted signal takes up a narrower bandwidth
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Know the block diagrams of the crystal diode receiver, tuned radio frequency (TRF) or straight receiver and superhet receiver.
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Know the functions of the RF amplifier, mixer, local oscillator, IF amplifier, demodulator (detector), and audio amplifier.
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the intermediate frequency is the sum of or difference between the RF and local oscillator frequencies.
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Know that tuned circuits in RF and IF amplifiers select the wanted signal.
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Detectors
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Know how a diode detector will recover the audio from amplitude modulated signals.
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Know that to generate the audio for CW signals a beat frequency oscillator (BFO) is used;
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Know that for the recovery of single side band audio, a carrier insertion oscillator (CIO) is used;
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Know that for the recovery of FM audio, a discriminator is used.
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Know the waveforms produced in a diode AM detector.
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Know that the automatic gain control (AGC) of a receiver operates by sensing the strength of the received signals to keep the audio output level fairly constant. takes up only half the bandwidth, e.g. 3 kHz not 6 kHz.
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CW occupies the least bandwidth and that FM occupies the most bandwidth.
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data transmissions commonly use two or more audio tones to modulate the carrier.
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a variable capacitance diode can be used in an oscillator to produce frequency modulation (FM).
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excessive audio amplitude or excessive audio bandwidth into a modulator can cause excessive bandwidth or excessive FM deviation which may result in interference to adjacent radio frequencies.
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oscillators, mixers and amplifiers can produce harmonics i.e. multiples of the fundamental frequency.
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harmonics can cause interference to other amateur bands and other radio users.
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Frequencies that are transmitted other than the wanted frequency could cause interference to other amateur bands users and other radio frequency users.