Receiver Injection; Transmitter Vcos; Transmitter Injection - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Theory of Operation: Frequency Generation Unit (FGU)

3.7.3.9 Transmitter Injection

The transmit (TX) injection string consists of three amplifier stages (Q5828, Q5829, and Q5501)
whose main purpose is to maintain a constant output to drive the RF power amplifier chain and
supply the TX feedback signal to the FGU synthesizer loop. The first two stages are powered by the
superfiltered 8.2 Vdc, which is decreased by 0.7 Vdc via the dual diode D5833, resulting in a 7.5 Vdc
supply. The third stage is powered by the keyed 9.1 Vdc and the TX injection string is on only with
keyed 9.1 Vdc activated during transmit mode.
The output of the second stage amplifier Q5829 is tapped via capacitor C5863 to supply the TX
feedback signal to the synthesizer prescalar via the amplifier Q5755.

3.7.3.10 Receiver Injection

The receiver (RX) injection string is a four-stage amplifier that supplies the RX feedback signal to the
FGU synthesizer loop and supplies the first local oscillator (LO) signal to the RX front-end mixer.
Each RX VCO output is attenuated via resistive pads to increase isolation. The VCO signals are
buffered by the RX injection amplifier string Q5904, Q5902 and Q5906. The output of Q5906 is
tapped via C5957 and fed back to the synthesizer prescaler through amplifier Q5755. The main path
at the output of Q5906 is amplified by U5303 to a level of 24 dBm to provide the first LO signal to the
RX front end mixer in the receiver chain.

3.7.3.11 Transmitter VCOs

Transmitter frequencies are generated from two VCOs, Q5825 and Q5826.
• Q5826 supplies frequencies in the range 450 MHz up to (but not including) 485 MHz.
• Q5825 supplies frequencies in the range from 485 MHz to 520 MHz.
3.7.3.12 Receiver VCOs
Receiver first local-oscillator frequencies are generated from three VCOs, Q5901, Q5903 and
Q5905.
• Q5905 supplies frequencies in the range 559.65 MHz up to (but not including) 582.65 MHz.
• Q5903 supplies frequencies in the range 582.65 MHz up to (but not including) 605.65 MHz.
• Q5901 supplies frequencies in the range 605.65 MHz up to 629.65 MHz.
The RX VCOs operate at frequencies which are 109.65 MHz higher than the radio channel selected
frequency since the receiver is high side injected, and the first IF frequency is 109.65 MHz.
The five VCOs are selected by the following pattern of logic levels on the AUX pins from the
synthesizer chip U5752
6881096C74-B
(Table 3-12):
Table 3-12. VCO AUX Pin Logic UHF Range 2
VCO AUX1
RX VCO Q5905
RX VCO Q5903
RX VCO Q5901
TX VCO Q5826
TX VCO Q5825
AUX2 AUX3
0 0
1 1
0 1
1 0
0 0
1
0
0
0
0
May 25, 2005
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