Radio Receivers
The local oscillator of a broadcast receiver is tuned to a frequency higher than the incoming frequency

Because otherwise an intermediate frequency could not be produced
To help the image frequency rejection
To allow adequate frequency coverage without switching
To permit easier tracking

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Radio Receivers
In a ratio detector

the linearity is worse than in phase discriminator
the output is twice that obtainable from a similar phase discriminator
the circuit is the same as in a discriminator, except that the diodes are reversed
stabilization against signal strength variations is provided

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Radio Receivers
In a broadcast superheterodyne receiver, the

Local oscillator operates below the signal frequency
Local oscillator frequency is normally double the IF
Mixer input must be tuned to the signal frequency
RF amplifier normally works at 455 kHz above the carrier frequency

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