6815854H01-A
June 15, 2005
Theory of Operation:
Transmitter
3-29
3.11.4.2.1 Second Local Oscillator (LO)
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is
75.6 MHz by default, or 71.1 MHz in special cases as necessary to avoid radio self-quieters. The
second LO signal mixes with IFIN to produce a 2.25 MHz final IF. The external VCO consists of
transistor Q6000, together with its bias and instability network and tank elements. Darlington
transistor Q6001 along with C6024 and C6025 form an active DC filter. The 2nd order loop filter is
comprised of C6056, C6057, and R6019.
3.11.4.2.2 Sampling Clock Oscillator
The ABACUS III IC sampling clock synthesizer, at Fclk=18 MHz (IF2=Fclk/8, where Fclk is the clock
rate), utilizes a negative-resistance core that is internal to the ABACUS III IC which, when used in
conjunction with an external LC tank (made up of L6003 and C6031) and a varactor (D6030), serves
as the VCO.
3.12 Transmitter
This section of the theory of operation provides a detailed circuit description of the transmitter, which
includes the RF power amplifier (RFPA), output network (ON), and power control.
When reading the theory of operation, refer to the appropriate schematic and component location
diagrams located in
“Chapter 7. Schematics, Component Location Diagrams, and Parts Lists”
. This
detailed theory of operation will help isolate the problem to a particular component. However, first
use the
ASTRO Digital XTL 1500 VHF/UHF Range 1/UHF Range 2/700–800 MHz Mobile Radio
Basic Service Manual
to troubleshoot the problem to a particular section.
3.12.1 VHF (136-174 MHz) Band
3.12.1.1 50-Watt Transmitter
The following text discusses the 50-W transmitter.
3.12.1.1.1 RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in
Figure 3-22. 50-Watt RF Power Amplifier (RFPA) Gain Stages (VHF)
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Содержание XTL 1500
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