|
|||||||||||||
|
|
||||||||||||
|
|||||||||||||
Syllabus Sections:- Demodulation 3K1 28 Understand the operation of basic AM, CW, SSB and FM demodulator circuits and the function of the limiter for FM. Please have in front of you Page 60 fig 9.7 and refer to it for most of this section of text. AM demodulation circuit In the Intermediate course notes your were shown how a diode detector recovers an AM signal (check back here). The diode allows all the positive signal to pass but rejects all of the negative part of the signal. The circuit diagram above shows a parallel tuned circuit, which can be tuned to the desired radio station (frequency) by the variable capacitor. At the resonant frequency, the parallel tuned circuit has a high impedance (or opposition to AC / RF currents), the effect of this is to generate a maximum AC / RF potential difference across the tuned circuit. The potential difference is rectified by the diode and DC is smoothed out by the capacitor C. The end result is that the recovered audio is only the overall shape of the signal which is applied to the ear piece / headphones which would have been the same shaped audio signal used to modulate the transmitter hence you hear what was transmitted.
Now it is possible for the AM
simple detector to demodulate a CW signal but you will need a
Beat Frequency Oscillator (BFO). This BFO (which is about 700Hz
offset from the the IF) mixes with the IF signal which results
an audible tone which will be at about 700Hz each time the RF is
present which represents the morse key pressed down by the
transmitting operator. |
|||||||||||||
|
|||||||||||||
|
|||||||||||||
|