Vhf/Uhf Transceiver Board Detailed Theory Of Operation; Introduction; Frequency Generation Unit (Fgu) - Motorola ASTRO Digital Saber Service Manual

VHF/UHF Transceiver Board
Detailed Theory of Operation

Introduction

Frequency
Generation Unit
(FGU)
This section of the manual provides a detailed circuit description of an
ASTRO Digital SABER VHF and UHF Transceiver Board. When reading
the theory of operation, refer to your appropriate schematic and
component location diagrams located in the back section of this
manual. This detailed theory of operation will help isolate the
problem to a particular component. However, first use the ASTRO
Digital SABER Portable Radios Basic Service Manual to troubleshoot
the problem to a particular board.
The frequency generation unit (FGU) consists of three major sections:
the high stability reference oscillator (U203), the fractional-N
synthesizer (U204,) and the VCO buffer (U201). A 5V regulator (U202),
supplies power to the FGU. The synthesizer receives the 5V REG at
U204, and applies it to a filtering circuit within the module and
capacitor C253. The well-filtered 5-volt output at U204 pin 19 is
distributed to the TX and RX VCOs and the VCO buffer IC. The mixer
LO injection signal and transmit frequency are generated by the RX
VCO and TX VCO respectively. The RX VCO uses an external active
device (Q202), whereas the VHF TX VCO active device is a transistor
inside the VCO buffer. The UHF TX VCO uses two active devices, one
external (Q203) and the other internal to the VCO buffer. The base and
emitter connections of this internal transistor are pins 11 and 12 of
U201.
The RX VCO is a Colpitts-type oscillator, with capacitors C235 and
C236 providing feedback. The RX VCO transistor (Q202) is turned on
when pin 38 of U204 switches from high to low. The RX VCO signal is
received by the VCO buffer at U201 pin 9, where it is amplified by a
buffer inside the IC. The amplified signal at pin 2 is routed through a
low-pass filter (L201 and associated capacitors) and injected as the first
LO signal into the mixer (U2 pin 8). In the VCO buffer, the RX VCO
signal (or the TX VCO signal during transmit) is also routed to an
internal prescaler buffer. The buffered output at U201 pin 16 is applied
to a low-pass filter (L205 and associated capacitors). After filtering, the
signal is routed to a prescaler divider in the synthesizer at U204 pin 21.
The divide ratios for the prescaler circuits are determined from
information stored in a codeplug, which is part of the microcontrol
unit (U204 on the VOCON board). The microprocessor extracts data
for the division ratio as determined by the position of the channel-
select switch (S902), and busses the signal to a comparator in the
synthesizer. A 16.8MHz reference oscillator, U203, applies the
16.8MHz signal to the synthesizer at U204 pin 14. The oscillator signal
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