TB7100 Service Manual Circuit Descriptions 49
© Tait Electronics Limited October 2005
Reference
Frequency
The 25kHz (approximate) reference is obtained by dividing the 13MHz
(approximate) output of the FCL. Any error in the FCL output frequency
will be multiplied by the synthesizer. Therefore, if the synthesizer is locked
but not the FCL, then the synthesizer output frequency will be wrong.
The FCL frequency division is performed by a digital counter inside the
PLL IC. The divider setting is constant.
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 base station
by means of a DC control voltage, typically between 2V and 12V.
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 7dBm, whereas in transmit mode it should be about 9dBm.
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 25kHz if the output frequency were correct.
Phase-locked Loop The PLL IC compares the 25kHz 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 25kHz 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 base station 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.
To Next Page
Loading...
