Tait TM8100 mobiles, Service Manual

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78 Circuit Descriptions TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
Re-tuning of VCO
Frequency
When the radio changes channels or switches between receive and transmit,
the VCO frequency must be changed. The rate at which the VCO is re-
tuned is dependent on many factors, of which the loop filter is the main
factor. The loop filter is an integrator built around an operational amplifier.
The resistors and capacitors of the filter affect both the switching time and
the stability of the synthesizer; the values of these components have been
carefully selected to give optimum control characteristics.
Figure 3.6 Block diagram of the frequency control loop circuitry—A4 band
Figure 3.7 Block diagram of the VCO—K5 band
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4
÷
4
Buffer
Amplifier
FCL
÷
4
÷
4
Buffer
Amplifier
FCL