Motorola GM950E Service Manual page 60

Frequency Synthesis
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U5701-32), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry
(D5701-1-3, C5716, C5717). This voltage multiplier is basically a diode capacitor network driven by
two (1.05MHz) 180 degrees out of phase signals (U5701-9 and -10).
Output LOCK (U5701-2) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U5701 divides the 16.8 MHz reference frequency down
to 2.1 MHz and provides it at pin 11. This signal is used as clock signal by the controller.
The serial interface (SRL) is connected to the microprocessor via the data line SPI DATA (U5701-5),
clock line SPI CLK (U5701-6), and chip enable line FRACN CE (U5701-7).
10.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) is formed by the colpitts oscillator FET Q5741. Q5741
draws a drain current of 12 mA from the FRAC-N IC super Þlter output. The oscillator frequency is
half of the desired frequency and mainly determined by L5743, C5742, C5743, C5745 - C5748 and
varactor diodes D5741 / D5742. Diode D5743 controls the amplitude of the oscillator.
A balanced frequency doubler T5751, D5751 converts the oscillator fundamental to the desired UHF
frequency. With a steering voltage from 2.5V to 10.5V at the varactor diodes the full RX and TX
frequency range from 357.9 MHz to 470 MHz is covered.
After the doubler a 3-pole bandpass Þlter rejects unwanted harmonics at the Þrst and third oscillator
fundamental frequency and matches the output to the Common VCO Buffer Q5751. Q5751 draws a
collector current of 13 mA from the stabilized 5V (U5801) and drives the Pre-scaler Buffer Q5791,
the PA Buffer Q5781 (Pout = 13dBm) and Mixer Buffer Q5771 (Pout = 10dBm). Q5791 draws a
collector current of 8 mA from the stabilized 5V and Q5771, Q5781 both draw 17mA form the 9V3
source. The buffer stages Q5771, Q5781 and the feedback ampliÞer Q5791 provide the necessary
gain and isolation for the synthesizer loop.
Q5731 is controlled by output AUX3 of U7501 (pin 1) and enables the RX or TX buffer. In RX mode
AUX3 is nearly at ground level, in TX mode about 5V DC. In TX mode with R5732 pulled to ground
level by Q5731 the modulation signal coming from the FRAC-N synthesizer IC (U7501 pin28)
modulates the VCO via varactor diode D5731 while in RX mode the modulation circuit is disabled by
pulling R5732 to a higher level through R5772.
10.4 Synthesizer Operation
The complete synthesizer subsystem works as follows. The output signal of the VCO (Q5741) is
frequency doubled by doubler D5751 and, buffered by Common VCO Buffer Q5751. To close the
synthesizer loop, the collector of Q5791 is connected to the PREIN port of synthesizer U5701 (pin
20). The buffer output (Q5751) also provides signals for the Mixer Buffer Q5771 and the PA Buffer
(Q5781).
The pre-scaler in the synthesizer (U5701) is basically a dual modulus pre-scaler with selectable
divider ratios. This divider ratio of the pre-scaler is controlled by the loop divider, which in turn
receives its inputs via the SRL. 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 (Y5702).
4-22 Theory of Operation