(BFO). This is a constant-amplitude signal whose frequency is set slightly higher
or lower than the IF. The output of the detector is the difference between the two
frequencies (a low audio frequency) in the form of the dots and dashes of code. CW
can only be heard if the BFO switch is in the BFO position.
e. FM Receiving System. Figure 2-17 compares the block diagrams of AM and FM
receivers.
In each receiver there is a stage of RF amplification, a local
oscillator, and a mixer stage, followed by one or more IF amplifiers. The stages
of both sets are similar in purpose up to this point. In the next two stages are
found the two main differences between the receivers.
The limiter of the FM set
may be considered as a special type of IF amplifier, while the discriminator is a
special type of detector.
Figure 2-17.
Comparison of AM and FM receivers.
(1) The IF signal applied to the limiter contains undesired amplitude
variations.
The limiter clips these amplitude variations from the
carrier, leaving only the carrier with the frequency variations from
the modulating signal.
The output from the limiter is sent to the
discriminator.
309 L2
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