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Experiment 1: AM Modulation and Demodulation

Trimester 2 2001/2002 Session

Group TE 1

Desmond Kho Teck Kiang	1001112325
Lai Koon Yen	1001112121

General Discussion about Amplitude Modulation

Amplitude modulation (AM) is the modulation method used in the AM radio broadcast band. In this system the intensity, or amplitude, of the carrier wave varies in accordance with the modulating signal. When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency.

If the modulated carrier is rectified and the carrier frequency filtered out, the modulating signal can be recovered. This form of modulation is not a very efficient way to send information; the power required is relatively large because the carrier, which contains no information, is sent along with the information.

In a variant of amplitude modulation, called single sideband modulation (SSB), the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.

Reference 1: Function of components in DSB/SSB AM Transmitter Board

The signal produced by the audio input signal select is applied to a balanced modulator, where the unwanted terms are removed by cancellation leaving only those frequencies which form the required modulation and either the upper or the lower sideband is selected by the appropriate ceramic bandpass filter.

The filtered signal is amplified before it’s applied to a balanced mixer, together with the local oscillator voltage, to shift the frequency of the signal to the allocated frequency band. The up-converted signal is amplified and then passed on to the final high-power amplifier to obtain the wanted transmitted signal.

Reference 2: Function of components in DSB/SSB AM Receiver Board

The wanted signal with many unwanted frequencies are passed by the Radio Frequency stage to the mixer. The main RF is not to select the wanted signal but to prevent certain particularly troublesome frequencies reaching the mixer stage.

The function of the mixer stage is to convert the wanted signal frequency of the receiver. This process is carried out by mixing the signal frequency with the output of the local oscillator and select the resultant difference frequency.

The purpose of the IF amplifier is to provide most of the gain and the selectivity of the receiver. The high first intermediate (IF Amplifier I) pushes the image frequency farther away from the signal frequency and permits much better attenuation of it. The low second intermediate (IF Amplifier II), for low fixed operating frequency, particular sharp selectivity and hence good adjacent-channel rejection.

AGC (Automatic Gain Control) detector keeps the IF signal in the output stages relatively constant. The purpose of the diode detector (envelope detector) stage is to recover the information modulated onto the carrier wave appearing at the output of the IF amplifier (Demodulated).

The function of the audio frequency stage is to develop sufficient audio frequency power to operate the loudspeaker or microphone. The audio frequency will include a volume control.

References:

- Kennedy and Davis, “ Electronic Communication Systems”, 4th Edition, McGRAW-HILL, 1993

- Paul H. Young, “Electronic Communication Techniques”, 3rd Edition, Macmillan Publishing Company, 1990

- D C Green, “Radio Systems for Technicians”, 2nd Edition, Longman Group Limited, 1995

- John Pearson, “ Basic Communication Theory”, Prentice Hall, 1992

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Last updated: 15 October 2001