Radio Receivers
Three-point tracking is achieved with

Double conversion
The padder capacitor
Variable selectivity
Double spotting

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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 permit easier tracking
To allow adequate frequency coverage without switching

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Radio Receivers
In a ratio detector

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
stabilization against signal strength variations is provided

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