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

Theory of Operation: Frequency Generation Unit (FGU)

3.7.1.10 Receiver Injection

The receiver (RX) injection string is a three-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 3 dB via resistors R3825 through R3827 to increase isolation. This
buffered signal is amplified by the first-stage amplifier Q3761, which is supplied by the 8.2-V
superfilter for a gain of approximately 6 dB. Resistors R3790, R3798, R3800, and R3801 bias
Q3761. L3774 serves as a choke inductor; C3779 and C3868 are added for filtering. The output of
Q3761 is attenuated 9.5 dB via resistors R3854, R3865, and R3855. This output is coupled to
Q3769 which gain of approximately 3 dB. The output of Q3769 is split into two paths. The first path
feeds back to the synthesizer prescaler through blocking capacitor C3913. The second path, which
supplies the LO signal to the RX front-end mixer, is attenuated 3 dB via resistors R3857, R3864, and
R3856 to increase isolation. This buffered signal is amplified by the second-stage amplifier Q3762,
which is supplied by the 9.3-V regulator for a gain of approximately 15 dB. Resistors R3843, R3851,
and R3852 biases Q3762. L3783 serves as a choke inductor; C3873, C3870, and C3925 are added
for filtering. The output of Q3762 is passed through blocking capacitor C3759, then, routed to C3804,
L3781, C3786, and L3782 which form a low pass filter and attenuated 1 dB via resistors R3821,
R3794, and R3802 to increase isolation and supply approximately 20 dBm to the LO port of the
mixer.

3.7.1.11 Transmitter VCOs

Transmitter frequencies are generated from two external, discrete, Colpits VCOs, low band (136-
155 MHz) and high band (155-174 MHz), based on Q3753 and Q3754 respectively. The VCOs are
switched On and Off by transistors Q3765 and Q3763, which are controlled by the Frac-N outputs
AUX1 and AUX2 respectively when turned high.

3.7.1.12 Receiver VCOs

The receiver first local oscillator frequencies are generated from two external, discrete, Colpits
VCOs, low band (245.65-264.65) and high band (264.65-283.65), based on Q3751 and Q3752
respectively. The VCOs are switched On and Off by transistors Q3766 and Q3764, which are
controlled by the Frac-N outputs AUX3 and AUX4 respectively when turned high.

3.7.1.13 Prescaler Feedback

RF feedback for the synthesizer loop is provided by prescaler amplifier Q3757. Feedback from both
the transmitter and receiver injection strings are coupled to this amplifier through resistor networks
that both balance and attenuate the levels prior to amplification by Q3757. The output of Q3757 is
coupled to U3751 at pin 32 (PREIN), which is the prescaler input for the synthesizer.
3.7.2 UHF Range 1 (380–470 MHz) Band
The FGU (Figure 3-37 on page
frequency, receiver 1st local oscillator, and a modulated transmitter RF carrier that is further
amplified by the power amplifier section of the radio.
The FGU consists of the following:
• Reference oscillator (Y5750)
• Low-voltage Fractional-N (LV Frac-N) synthesizer (U5752)
• Three receiver voltage-controlled oscillators (VCOs)
• Two transmitter VCOs
• Three receiver LO amplifiers (Q5904, Q5902 and Q5906)
• Two transmitter injection amplifiers (Q5828, and Q5829)
6881096C74-B
3-50) provides the XTL 5000 radio with a 16.8 MHz reference
3-49
May 25, 2005