Motorola cm200 Commercial Series, Service Manual

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UHF (438-470 MHz) Frequency Synthesis 2-7
4.4 Synthesizer Operation
The synthesizer consists of a low voltage FRAC-N IC (LVFRAC-N), reference oscillator, charge
pump circuits, loop filter circuit, and DC supply. The output signal (PRESC_OUT) of the VCOBIC
(U201, pin 12) is fed to the PREIN, pin 32 of U200 via a low pass filter which attenuates harmonics
and provides a correct input level to the LVFRAC-N in order to close the synthesizer loop.
The pre-scaler in the synthesizer (U200) is a dual modulus pre-scaler with selectable divider ratios.
The divider ratio of the pre-scaler is controlled by the loop divider, which in turn receives its inputs
via the SPI. The output of the pre-scaler is applied to the loop divider. The output of the loop divider
is connected to the phase detector, which compares the loop divider’s output signal with the
reference signal. The reference signal is generated by dividing down the signal of the reference
oscillator (Y201).
The output signal of the phase detector is a pulsed dc signal that is routed to the charge pump. The
charge pump outputs a current from U200, pin 43 (IOUT). The loop filter (consisting of R224, R217,
R234, C2074, C2075, C2077, C2078, C2079, C2080, C2028, and L205) transforms this current into
a voltage that is applied the varactor diodes D203 and D204 for RX and TX respectively. The output
frequency is determined by this control voltage. The current can be set to a value fixed in the
LVFRAC-N or to a value determined by the currents flowing into BIAS 1 (U200, pin 40) or BIAS 2
(U200, pin 39). The currents are set by the value of R200 or R206 respectively. The selection of the
three different bias sources is done by software programming.
To modulate the synthesizer loop, a two-spot modulation method is utilized via the MODIN (U200,
pin 10) input of the LVFRAC-N. The audio signal is applied to both the A/D converter (low frequency
path) and the balance attenuator (high frequency path). The A/D converter converts the low
frequency analog modulating signal into a digital code which is applied to the loop divider, thereby
causing the carrier to deviate. The balance attenuator is used to adjust the VCO’s deviation
sensitivity to high frequency modulating signals. The output of the balance attenuator is presented
at the MODOUT port of the LVFRAC-N (U200,pin 41) and connected to the VCO modulation
varactor D205.