Radio Receivers
One of the main functions of the RF amplifier in a superheterodyne receiver is to

Provide improved tracking
Increase the tuning range of the receiver
Improve the rejection of the image frequency
Permit better adjacent-channel rejection

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Radio Receivers
In a ratio detector

stabilization against signal strength variations is provided
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
the linearity is worse than in phase discriminator

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

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

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