Motorola GM950E Service Manual page 67

Output LOCK (U3701-2) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U3701 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 (U3701-5),
clock line SPI CLK (U3701-6), and chip enable line FRACN CE (U3701-7).
13.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) uses 2 colpitts oscillators, FET Q3741 for transmit and FET
Q3751 for receive. The appropriate oscillator is switched on or off by FRAC-N IC output AUX3
(U3701-1) using transistors Q3742 and Q3752. In RX mode AUX3 is nearly at ground level and
Q3742 enables a current ßow from the source of FET Q3751 while Q3752 is switched off. In TX
mode AUX3 is about 5V DC and Q3742 is switched off. Q3752 is switched on and enables a current
ßow from the source of FET Q3741 while Q3751 is switched off. When switched on the FETs draw a
drain current of 8 mA from the FRAC-N IC super Þlter output. The frequency of the receive oscillator
is mainly determined by L3752, C3752, C3754 - C3756 and varactor diodes D3751 / D3752. Diode
D3754 controls the amplitude of the oscillator. The frequency of the transmit oscillator is mainly
determined by L3734, C3736 - C3740 and varactor diodes D3732 / D3733. Diode D3739 controls
the amplitude of the oscillator. With a steering voltage from 3V to 10V at the varactor diodes the RX
frequency range from 181.1 MHz to 219.1 MHz and the TX frequency range from 136 MHz to 174
MHz are covered. In TX mode the modulation signal coming from the FRAC-N synthesizer IC
(U3701 pin 28) modulates the TX VCO via varactor diode D3731.
Both oscillator outputs are combined and buffered by the VCO Buffer Q3760. Q3760 draws a
collector current of 13 mA from the stabilized 5V (U3801) and drives the Mixer Buffer Q3770. Q3770
draws a collector current of 17 mA from the 9V3 voltage and drives the PA Buffer Q3780 (Pout =
13dBm) and the Pre-scaler Buffer Q3790. Q3790 draws a collector current of 8 mA from the
stabilized 5V (U3801) and drives the pre-scaler internal to the FRAC-N IC. In transmit mode Q3780
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 R3774. The buffer
stages Q3760, Q3770, Q3780 and the feedback ampliÞer Q3790 provide the necessary gain and
isolation for the synthesizer loop.
13.4 Synthesizer Operation
The complete synthesizer subsystem works as follows. The combined output signal of the RX VCO
(Q3751) and TX VCO (Q3741) is buffered by VCO Buffer Q3760, Mixer Buffer Q3770 and Pre-scaler
Buffer Q3790. To close the synthesizer loop, the collector of Q3790 is connected to the PREIN port
of synthesizer U3701 (pin 20). The output of (Q3770) also provides signals for the mixer (via VCO
MIXER) and the PA Buffer (Q3780).
The pre-scaler in the synthesizer (U3701) 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 (Y3702).
Theory of Operation
Frequency Synthesis
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