Synthesizer; Figure 7-8. 800 Mhz Frequency Generation Unit Block Diagram - Motorola HT Series Detailed Service Manual

7-12
• R315 and R318 provide a DC path for CR301 and also limit the current through CR301.
The blocking capacitor C317 prevents DC from the AGC stage from appearing at the input of the
filter FL301.
7.7.5 Frequency Generation Circuit
The frequency generation circuit is shown in
• Low Voltage Fractional-N Synthesizer, U201
• VCO/Buffer IC, U250
Voltage
Multiplier
VCP
Vmult1

Synthesizer

Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
Designed in conjunction to maximize compatibility, the two ICs provide many of the functions that
normally would require additional circuitry. The synthesizer block diagram illustrates the interconnect
and support circuitry used in the region. Refer to the relevant schematics for the reference
designators.
The synthesizer is powered by regulated 5 V and 3.3 V which come from U247 and U248
respectively. The synthesizer in turn generates a superfiltered 4.5 V which powers U250.
In addition to the VCO, the synthesizer must interface with the logic and ASFIC circuitry.
Programming for the synthesizer is accomplished through the data, clock and chip select lines from
the microprocessor. A 3.3 V dc signal from synthesizer lock detect line indicates to the
microprocessor that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to pin10 of U201. Internally the audio is digitized by
the Low Voltage Fractional-N synthesizer and applied to the loop divider to provide the low-port
modulation. The audio runs through an internal attenuator for modulation balancing purposes before
going out to the VCO.
7.8 Synthesizer
The Low Voltage Fractional-N (LV FracN) synthesizer (U201) shown in
a 16.8 MHz crystal (FL201) to provide a reference for the system. The LV FracN IC further divides
this to 2.1 MHz, 2.225 MHz, and 2.4 MHz as reference frequencies. Together with C235, C236,
C237, R211 and CR203, they comprise the reference oscillator which is capable of 2.5ppm stability
over temperatures of -30° to 85°C. It also provides 16.8 MHz at pin 19 of U201 to be used by ASFIC
and LVZIF.
Low Band, 800 MHz, PassPort & 900 MHz Theory of Operation: Synthesizer
Transistor
Aux3
Aux4
Loop
Filter
MOD Out

Figure 7-8. 800 MHz Frequency Generation Unit Block Diagram

Figure 7-8. The circuit is composed of the two main ICs:
Dual
TRB
Rx VCO
VCOBIC
Circuit
U250
Tx VCO
Circuit
Rx
Out
Injection
To Mixer
Amplifier
Tx
Out
Buffer
To PA Driver
Amplifier
Figure 7-9 on page 7-13 uses