7.12 Frequency Generation Circuitry; 7.13 900 Mhz Synthesizer; Figure 7-14. Frequency Generation Unit Block Diagram - Motorola HT Series Detailed Service Manual

7-24 Low Band, 800 MHz, PassPort & 900 MHz Theory of Operation: Frequency Generation Circuitry

7.12 Frequency Generation Circuitry

Voltage
Multiplier
VCP
Vmult1
Synthesizer
Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
The Frequency Generation circuitry is comprised of two main ICs, the Low Voltage Fractional-N
(LV FracN) synthesizer (U201), and the VCO/Buffer IC (U250). 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 LV FracN IC 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.

7.13 900 MHz Synthesizer

The Low Voltage Fractional-N (LV FracN) synthesizer (U201) uses a 16.8 MHz packaged 1.5 ppm
reference oscillator (Y200) to provide a reference for the system. The LV FracN IC further divides the
16.8 MHz to 2.1 MHz, 2.225 MHz, and 2.4 MHz. Y200, together with C238, C239, C241, R212,
R213, and R214 comprise the reference oscillator which is capable of 1.5 ppm 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.
The loop filter which consists of C801, C802, C803, C804, C805, C225, C226, R204, R209, and
R210 provides the necessary dc steering voltage for the VCO and provides filtering of noise and
spurs from U201.
In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current
at pin 45 of U201 to put the synthesizer within the lock range. The required frequency is then locked
by the normal mode charge pump at pin 43.
Dual
Transistor
Aux3
Aux4
Loop
Filter
MOD Out

Figure 7-14. Frequency Generation Unit Block Diagram

TRB
Rx VCO
VCOBIC
Circuit
U250
Tx VCO
Circuit
Rx
Out
Injection
To Mixer
Amplifier
Tx
Out Buffer
To PA Driver
Amplifier