Frequency Generation Circuitry - Motorola Pro Series Elite Service Manual

VHF Transmitter
Resistors R3304 and R3305 are voltage dividers designed to turn on Q3301 at certain RSSI levels. In
order to turn on Q3301 the voltage across R3305 must be greater or equal to the voltage across
R3324, plus the base-emitter voltage (Vbe) present at Q3301. Capacitor C3209 is used to dampen
any instability while the AGC is turning on. The current flowing into the collector of Q3301, a high
current gain NPN transistor, will be drawn through the PIN diode to turn it on. Maximum current
flowing through the PIN is limited by the resistors R3316, R3313, R3306 and R3324. C3326 is a
feedback capacitor used to provide some stability to this high gain stage.
An additional gain control circuit is formed by Q3201 and its associated circuitry. Resistors R3206 and
R3207 are voltage dividers designed to turn on Q3201 at a significantly higher RSSI level than the
level required to turn on PIN diode control transistor Q3301. In order to turn on Q3201 the voltage
across R3207 must be greater or equal to the voltage across R3208, plus the base-emitter voltage
(Vbe) present at Q3201. As current starts flowing into the collector of Q3201, it reduces the bias
voltage at the base of IF amplifier transistor Q3200 and in turn, the gain of the IF amplifier. The gain
can be controlled in a range of -30dB up to +10dB.

2.5.10 Frequency Generation Circuitry

Voltage
Multiplier
Vmult1
Vmult2
16.8 MHz
Ref. USCG.
The Frequency Generation Circuitry, shown in Figure 2-7, is composed of two main ICs, the
Fractional-N synthesizer (U3701), and the VCO/Buffer IC (U3801). 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 5V and 3.3V which come from U3711 and U3201
respectively. The synthesizer in turn generates a superfiltered 4.5V which powers U3801.
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 3.3V dc signal from synthesizer lock detect line indicates to the microprocessor
that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to pin 10 of U3701. Internally the audio is digitized by
the Fractional-N 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.
VCP
Aux3
Synthesizer
U3701
Loop
Filter
MOD Out
Modulating
Signal
Figure 2-7. Frequency Generation Unit Block Diagram
TRB
VCOBIC
Rx VCO Circuit
U3801
Tx VCO
Circuit
Rx Out
To
Mixer
Tx Out
To
PA Driver
2-9