TM8100 Mobile Radio Service Manual Circuit Descriptions 69
May 2004 © Tait Electronics Limited
VCO Frequency and
Output Power
The output frequency from the synthesizer is generated by a VCO. The
VCO frequency is tuned across the frequency range of the radio by
means of a DC control voltage, typically between 2 V and 12 V. The
VCO output power is amplified by a buffer amplifier. The power is low
and varies from band to band. The buffer output power depends on
which mode — receive or transmit — is used. In receive mode the
output power should be about 7 dBm, whereas in transmit mode it
should be about 9 dBm.
Dual VCOs Some variants of the synthesizer use two VCOs: one for receive and one
for transmit. Synthesizers with two VCOs share the same tuning signal.
Only one VCO is switched on at a time, and so the PLL IC will see only
one output frequency to tune. A portion of the RF output from the
VCOs is fed to the RF input of the PLL IC. The RF signal is divided
by an integer that would give 25 kHz if the output frequency were
correct.
Phase-locked Loop The PLL IC compares the 25 kHz reference and the divided VCO
signal, and the error is used to control the internal charge pump. The
charge pump is a current source that can sink or source current in
proportion to the frequency or phase error. The output is a series of
25 kHz pulses with a width that is dependent on the phase error. When
the output frequency of the synthesizer is correct, there is no error and
the charge pump output will become open circuit.
Active Loop Filter The loop filter continuously integrates the current pulses from the charge
pump and produces a steady DC output voltage that tunes the VCO (or
VCOs). When the VCO frequency is correct, there is no frequency
error and therefore no charge-pump output, and so the loop filter’s
output voltage remains constant. If the frequency is too high or too low,
the error will result in the output of charge-pump current pulses
(negative or positive depending on the sign of the error). The loop filter’s
output voltage will change accordingly, causing the VCO frequency to
change in proportion. The synthesizer design is such that normally the
VCO frequency will be automatically corrected.
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.
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