Transmitters and Receivers

Revision part 3

Learning outcomes of this part

• a filter is a device that blocks some frequencies and passes others.

• Know the effects of low pass, band pass and high pass filters and know thier frequency/amplitude diagrams.

• low pass filter and a band pass filter can minimise the radiation of harmonics.

• Know that too fast a rise and fall time of the transmitted RF envelope of a CW transmitter may cause excessive bandwidth and that this can be minimised by suitable filters in the keying stage.

• Know the block diagrams of the crystal diode receiver, tuned radio frequency (TRF) or straight receiver and superhet receiver.

• Know the functions of the RF amplifier, mixer, local oscillator, IF amplifier, demodulator (detector), and audio amplifier.

• the intermediate frequency is the sum of or difference between the RF and local oscillator frequencies.

• Know that tuned circuits in RF and IF amplifiers select the wanted signal.

• Detectors

• Know how a diode detector will recover the audio from amplitude modulated signals.

• Know that to generate the audio for CW signals a beat frequency oscillator (BFO) is used;

• Know that for the recovery of single side band audio, a carrier insertion oscillator (CIO) is used;

• Know that for the recovery of FM audio, a discriminator is used.

• Know the waveforms produced in a diode AM detector.

• 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.

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