Transmitter Injection; Receiver Injection; Transmitter Vcos - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Frequency Generation Unit (FGU)

3.13.4.9 Transmitter Injection

The transmit (TX) injection string consists of three amplifier stages (Q6764, Q6765, and Q6766)
whose main purpose is to maintain a constant output to drive the RF power amplifier 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 D6750, 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 TX VCO output is attenuated 3 dB via resistors
R6829 through R6831. This output is coupled to the first-stage amplifier Q6764, further attenuated
3 dB via resistors R6809 through R6811, and then coupled to the second-stage amplifier Q6765.
This output is tapped to supply the TX feedback signal to the synthesizer prescalar, and the balance
is further attenuated 3 dB via resistors R6816 through R6818. This output is coupled to the third-
stage amplifier Q6766, further attenuated 3 dB via resistors R6825 through R6827, and coupled to
the input of the RF power amplifier section. The four sets of resistive attenuators provide increased
isolation between the TX VCO and RF power amplifier.

3.13.4.10 Receiver Injection

The receiver (RX) injection string is a two-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. The
RX VCO output is attenuated 6 dB via resistors R6793 through R6795 to increase isolation. This
buffered signal is amplified by the first-stage amplifier Q6762, which is supplied by the 8.2-V
superfilter for a gain of approximately 10 dB. Resistors R6789, R6790, and R6796 through R6798
bias Q6762 to approximately 5 V and 35 mA. L6757 serves as a choke inductor; C6819 and C821
are added for filtering. The output of Q6762 is split into two paths. The first path feeds back to the
synthesizer prescaler through blocking capacitor C6822. The second path, which supplies the LO
signal to the RX front-end mixer, is attenuated 3 dB via resistors R6799, R6800, and R6824 to
increase isolation. This buffered signal is amplified by the second-stage amplifier Q6763, which is
supplied by the 9.3-V regulator for a gain of approximately 15 dB. Resistor R6801 biases Q6763 to
approximately 4.5 V (±1 V, due to possible part variations). L6758 serves as a choke inductor;
C6823, C6824, and C6826 are added for filtering. The output of Q6763 is passed through blocking
capacitor C6825 and attenuated 3 dB via resistors R6802 through R6804 to increase isolation and
supply approximately 15.5 dBm to the LO port of the mixer.

3.13.4.11 Transmitter VCOs

Transmitter frequencies are generated from two VCOs contained in U6754. U6754 is not serviceable
and should be replaced if it is determined to be non-functional. Transmitter frequencies in the range
of 764 to 776 MHz (repeater talkaround) and 794 to <806 MHz (trunking or repeater mode) can be
generated when Aux1 (pin 48) of U6751 is active high (3.0 Vdc). This 3.0 volts is applied to transistor
switch Q6753 allowing the superfiltered 8.2 Vdc supply to be connected to pin 2 (SWBPOSC1), bias
for the first VCO of U6754. Likewise, transmitter frequencies in the range of 806 to 825 MHz
(trunking or repeater mode) and 851 to 870 MHz (repeater talkaround) can be generated when Aux2
(pin 1) of U6751 is active high (3.0 Vdc). This 3.0 volts is applied to transistor switch Q6755 allowing
the superfiltered 8.2 Vdc supply to be connected to pin 17 (SWBPOSC2), bias for the second VCO
of U6754. Pin 20 (RFOUT) of U6754 is common to both oscillators and couples the oscillator's
output signal to the first stage of the transmitter injection string. Modulation and frequency steering is
accomplished through pin 8 (Cont_V) of U6754.
6815854H01-A
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June 15, 2005