Tait TP9100 Service Manual

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58 Circuit Descriptions TP9100 Service Manual© Tait Electronics Limited May 2005ReferenceFrequencyThe 25kHz (approximate) reference is obtained by dividing the 13 MHz(approximate) output of the FCL. Any error in the FCL output frequencywill be multiplied by the synthesizer. Therefore, if the synthesizer is lockedbut not the FCL, then the synthesizer output frequency will be wrong.The FCL frequency division is performed by a digital counter inside thePLL IC. The divider setting is constant.VCO Frequency andOutput Power The output frequency from the synthesizer is generated by a VCO.The VCO frequency is tuned across the frequency range of the radio bymeans of a DC control voltage, typically between 2V and 12V. The VCOoutput power is amplified by a buffer amplifier. The power is low and variesfrom band to band. The buffer output power depends on which mode—receive or transmit—is used. In receive mode the output power should beabout 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 fortransmit. 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 onlyone output frequency to tune. A portion of the RF output from the VCOsis fed to the RF input of the PLL IC. The RF signal is divided by an integerthat 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 pumpis a current source that can sink or source current in proportion to thefrequency or phase error. The output is a series of 25kHz pulses with awidth that is dependent on the phase error. When the output frequency ofthe synthesizer is correct, there is no error and the charge pump output willbecome open circuit.Active Loop Filter The loop filter continuously integrates the current pulses from the chargepump and produces a steady DC output voltage that tunes the VCO(or VCOs). When the VCO frequency is correct, there is no frequencyerror and therefore no charge-pump output, and so the loop filter’s outputvoltage remains constant. If the frequency is too high or too low, the errorwill result in the output of charge-pump current pulses (negative or positivedepending on the sign of the error). The loop filter’s output voltage willchange accordingly, causing the VCO frequency to change in proportion.The synthesizer design is such that normally the VCO frequency will beautomatically corrected.Re-tuning of VCOFrequency 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 mainfactor. The loop filter is an integrator built around an operational amplifier.The resistors and capacitors of the filter affect both the switching time andthe stability of the synthesizer; the values of these components have beencarefully selected to give optimum control characteristics.