Voltage Controlled Oscillator (Vco); Synthesizer Operation - Motorola GM-950 Manual

Frequency Synthesis
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U2701-32), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry
(D2701-1-3, C2716, C2717). This voltage multiplier is basically a diode capacitor network driven by
two (1.05 MHz) 180 degrees out of phase signals (U2701-9 and -10).
Output LOCK (U2701-2) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U2701 divides the 16.8MHz reference frequency down
to 2.1MHz 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 (U2701-5),
clock line SPI CLK (U2701-6), and chip enable line FRACN CE (U2701-7).
10.3

Voltage Controlled Oscillator (VCO)

The Voltage Controlled Oscillator (VCO) uses 2 colpitts oscillators, FET Q2741 for transmit and FET
Q2751 for receive. The appropriate oscillator is switched on or off by FRAC-N IC output AUX3
(U2701-1) using transistors Q2742 and Q2752. In RX mode AUX3 is nearly at ground level and
Q2742 enables a current flow from the source of FET Q2751 while Q2752 is switched off. In TX
mode AUX3 is about 5V DC and Q2742 is switched off. Q2752 is switched on and enables a current
flow from the source of FET Q2741 while Q2751 is switched off. When switched on the FETs draw a
drain current of 8 mA from the FRAC-N IC super filter output. The frequency of the receive oscillator
is mainly determined by L2752, L2753, C2752 - C2756 and varactor diodes D2751 / D2752. Diode
D2754 controls the amplitude of the oscillator. The frequency of the transmit oscillator is mainly
determined by L2734, C2736 - C2740 and varactor diodes D2732 / D2733. Diode D2739 controls
the amplitude of the oscillator. With a steering voltage from 3V to 10V at the varactor diodes the RX
frequency range from 87.4MHz to 109.4MHz and the TX frequency range from 66MHz to 88MHz are
covered. In TX mode the modulation signal coming from the FRAC-N synthesizer IC (U2701 pin 28)
modulates the TX VCO via varactor diode D2731.
Both oscillator outputs are combined and buffered by the VCO Buffer Q2760. Q2760 draws a
collector current of 13 mA from the stabilized 5V (U2801) and drives the Mixer Buffer Q2770. Q2770
draws a collector current of 19 mA from the 9V3 voltage and drives the PA Buffer Q2780 (Pout =
13dBm) and the Pre-scaler Buffer Q2790. Q2790 draws a collector current of 8 mA from the
stabilized 5V (U2801) and drives the pre-scaler internal to the FRAC-N IC. In transmit mode Q2780
is switched on by the K9V1 signal and draws a collector current of 19 mA from the K9V1 voltage.
The injection signal VCO MIXER with a level of 10dBm feeds the mixer through R2774. The buffer
stages Q2760, Q2770, Q2780 and the feedback amplifier Q2790 provide the necessary gain and
isolation for the synthesizer loop.
10.4

Synthesizer Operation

The complete synthesizer subsystem works as follows. The combined output signal of the RX VCO
(Q2751) and TX VCO (Q2741) is buffered by VCO Buffer Q2760, Mixer Buffer Q2770 and Pre-scaler
Buffer Q2790. To close the synthesizer loop, the collector of Q2790 is connected to the PREIN port
of synthesizer U2701 (pin 20). The output of the VCO Buffer (Q2770) also provides signals for the
mixer (via VCO MIXER) and the PA Buffer (Q2780).
The pre-scaler in the synthesizer (U2701) 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 (Y2701 or Y2702).
5C.3-24 Theory of Operation