Transmitter; Vhf (136-174 Mhz) Band; 50-Watt Transmitter; Figure 3-22. 50-Watt Rf Power Amplifier (Rfpa) Gain Stages (Vhf) - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Transmitter
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
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
2mW
TX_INJ
rom
GU
R PA_CNTRL

Figure 3-22. 50-Watt RF Power Amplifier (RFPA) Gain Stages (VHF)

6815854H01-A
"Chapter 7. Schematics, Component Location Diagrams, and Parts
IRST
DRIVER
STAGE
STAGE
K9.1V
VGBIAS2
U3550
Q3530
C65
1518
Figure 3-22.
INAL
STAGE
VGBIAS1
A+
A+
63W
Q3550
1550
3-29
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R PA_OUT
To Antenna
Switch
MAEP -27887-O_VH
June 15, 2005