Radio Receivers
The frequency generated by each decade in a direct frequency synthesizer is much higher than the frequency shown; this is done to

Reduce the number of decades
Reduce the number of crystals required
Increase the frequency stability of the synthesizer
Reduce the spurious frequency problem

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Radio Receivers
In a ratio detector

stabilization against signal strength variations is provided
the linearity is worse than in phase discriminator
the circuit is the same as in a discriminator, except that the diodes are reversed
the output is twice that obtainable from a similar 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 permit easier tracking
To help the image frequency rejection
Because otherwise an intermediate frequency could not be produced
To allow adequate frequency coverage without switching

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