Radio Receivers
The controlled oscillator synthesizer is sometimes preferred over the direct one because

Its frequency stability is better
It is relatively free of spurious frequency
It is a simpler piece of equipment
It does not require crystal oscillator

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Radio Receivers
In a ratio detector

the linearity is worse than in phase discriminator
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

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Radio Receivers
If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

Improving the selectivity of the receiver
Reducing the effect of noise at low modulation depths
Improving the effectiveness of the AGC
Reducing the effect of negative-peak clipping

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