AdvancedA-006-002-005
The BFO is off-set slightly (500 - 1 500 Hz) from the incoming signal to the detector. This is required:
C
Answer
Propagation and operating practice
Type
A
to protect the incoming signal from interference
B
to provide additional amplification
C
to beat with the incoming signal
D
to pass the signal without interruption
Answer Notes
A Beat Frequency Oscillator (BFO) is used in receivers to make unmodulated continuous wave (CW) or single sideband (SSB) signals audible. By offsetting the BFO frequency slightly (usually 500 to 1500 Hz) from the incoming intermediate frequency (IF), the two signals mix or 'beat' together in the product detector.
The mixing process creates audio frequency heterodyne outputs equal to the sum and difference of the two frequencies. The difference frequency falls exactly in the human audio range (500 - 1500 Hz), producing the audible tone you hear in the speaker. Distractors suggesting amplification or protection are incorrect because the BFO's sole purpose is signal mixing (demodulation), not gain or filtering.
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If the incoming signal to the mixer is 3 600 kHz and the first IF is 9 MHz, at which one of the following frequencies would the local oscillator (LO) operate?
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It is very important that the oscillators contained in a superheterodyne receiver are: