Voltage Controlled Oscillator (Vco); Synthesizer Operation - Motorola GM1200E Service Manual

Frequency Synthesis
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 filter 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 filter rejects unwanted harmonics at the first 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 amplifier Q5791 provide the necessary
gain and isolation for the synthesizer loop.
Q5731 is controlled by output AUX3 of U5701 (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 (U5701 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 or Y5701).
The output signal of the phase detector is a pulsed DC signal which is routed to the charge pump.
The charge pump outputs a current at pin 29 (I OUT of U5701). The loop filter (which consists of
R5715-R5717, C5723-C5725, C5727, R5741, C5741) transforms this current into a voltage that is
applied to the varactor diodes D5741, D5742 and alters the output frequency of the VCO (Q5741).
The current can be set to a value fixed in the FRAC-N IC or to a value determined by the currents
flowing into CPBIAS 1 (U5701-27) or CPBIAS 2 (U5701-26). The currents are set by the value of
R5724 or R5726 respectively. The selection of the three different bias sources is done by software
programming.
3.1-24 Introduction/Theory of Operation