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

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

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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
stabilization against signal strength variations is provided
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

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

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