Motorola M11URD6CB1_N Service Manual page 45

GTX LTR/Privacy Plus 800 MHz Mobile Service Manual
quency and a higher control voltage produces a higher
frequency respectively.
The RX/TX VCO (806 - 825 MHz band) provides the
first LO injection frequency which is 45.1 MHz below
the carrier frequency and TX injection for conventional
mode. The RX/TX VCO is selected by pulling U5702-1
low.
The Talk-Around (TA) VCO (851 - 870 MHz band) pro-
vides the transmit frequency in TA mode. The TA VCO
is selected by pulling U5702-1 high.
The buffer stage U5701 and feedback amplifier Q5703
provide the necessary gain and isolation for the syn-
thesizer loop.
Fractional-N Synthesizer IC U5702 consists of:
• Prescaler,
• Programmable loop divider,
• Control divider logic,
• Phase detector,
• Charge pump,
• A/D Converter for low frequency digital mod-
ulation,
• Balance attenuator to balance the high and low
frequency analog modulation,
• 13-V positive voltage multiplier,
• Serial interface for control,
• Super filter for the regulated 9.3 V DC
C5712 is the super filter capacitor. The output voltage
of the super filter U5702-18, drops from 9.3 VDC to
about 8.5 VDC. This filtered 8.5 VDC supplies voltage
for the VCOs, the VCO switching units U5704 & U5705,
feedback amplifier Q5703, and synthesizer charge
pump resistor network R5707- R5709.
The synthesizer supply voltage is provided by the 5
VDC regulator U5703. The 2.1 MHz reference signal at
U5702-11 is generated by dividing down the signal of
reference oscillator U5700 after it is applied to U5702-
14.
In order to generate a high voltage that supplies the
charge pump output stage at VCP (U5702-32), 13 V DC
is generated at pin 3 of CR5700 by the positive voltage
multiplier circuitry. This voltage multiplier is a diode
capacitor network driven by two 1.05 MHz, 180-degree
out-of-phase signals from U5702-9 &10.
The serial interface (SPI_DATA) is connected to the
microprocessor via the data line (U5702-5), clock line
(U5702-6), and chip enable line (U5702-5). Proper
June, 2000
enabling of these lines allows the microprocessor to
program the synthesizer.
The output of the VCO is fed to the buffer input port of
U5701-1. The output of the buffer U5701-5 is applied to
the input of the feedback amplifier Q5703 through an
attenuator network consisting of R5735, R5733 and
R5732. To close the synthesizer loop, the output of
Q5703 is connected to PREIN (U5702-20) of the synthe-
sizer. The buffer output U5701-5 also provides the sig-
nal for receiver LO injection and transmit injection line-
up. The charge pump current is present at U5702-31.
The loop filter (which consists of R5716, R5717, R5734,
C5750, C5744, C5745, C5736, C5746, C5747, C5794,
C5795, C5796, C5797, C5798, C5799) will then trans-
form this current into a voltage that will, in turn, be
applied to Vcontrol of the VCOs and steer the output
frequency.
In order to modulate the PLL a two-point modulation
method is utilized. The audio signal is applied to both
the A/D converter (low frequency path) as well as the
balance attenuator (high frequency path) via U5702-8.
The A/D converter will convert the low-frequency,
analog modulating signal into a digital code that will
in turn be applied to the loop divider. This will cause
the carrier to deviate. The balance attenuator is used to
adjust the VCO deviation sensitivity to high frequency
modulating signals. The output of the balance attenua-
tor is present at the MODOUT port U5702-28.
The transmit injection line-up consists of two amplifier
stages, Q5707 & Q5705, used to provide a constant out-
put to drive the RF transmitter and provide isolation.
The Q5705 stage is actively biased through Q5701, and
Q5707 has passive biasing. The TX injection line-up is
on only during the transmit mode (K 9.1V line is at
9.1 V).
The first VCO is used for receive LO_INJ and TX_INJ
in the conventional 806-825 MHz transmit band. The
second VCO is used for TX_ INJ in the talk-around 851-
870 MHz transmit band.
The VCOs get their control voltage through L5701 and
L5702. Q5704 and Q5706 are the oscillation transistors.
C5793, C5792 and C5785, C5784 are the oscillator feed-
back capacitors. U5706 and U5707 are micro strip reso-
nators. CR5701 and CR5702 are the varactors. C5790
and C5782 determine the VCO KV. C5703 and C5704
determine the VCO operation frequency. C5791 and
C5783 are the coupling capacitors between the tank
and the oscillation transistor.
The two VCOs are coupled to buffer amplifier Q5702
through C5789 and C5786. The buffer amplifier output
signal is fed to U5701-pin 1 through C5770 & R5736.
6880905Z99-O
Theory of Operation
Synthesizer Detailed Functional Description
8-5