Charge Pump Bias; Loop Filter; Lock Detect; Transmitter Injection - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

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3.7.2.6 Charge Pump Bias

External circuitry connected to pin 39 (Bias 2) and pin 40 (Bias 1) of U5752 determine the current
that is applied to the charge-pump circuitry. During receive mode, resistors R5754, R5759, and
R5765 set the current supplied to pin 40 (Bias 1). Transistor Q5750 and resistors R5752, R5753,
and capacitor C5759 form a circuit that momentarily increases the current to pin 40 (Bias 1) during
receiver programming of U5752. This circuit is activated by pin 46 (ADAPTSW) of U5752 during the
transition of programming U5752 to frequency and effectively decreases the length of time for the
synthesizer to lock on frequency. Similarly, during transmitter mode, resistors R5764, R5759, and
R5753 set the current supplied to pin 39 (Bias 2). Transistor Q5752 and resistors R5767, R5764,
and capacitor C5762 form a circuit that momentarily increases the current to pin 39 (Bias 2) during
transmitter programming of U5752.

3.7.2.7 Loop Filter

The loop filter operates in synchronization with the phase detector of U5752 in two modes, normal
and adapt. In normal mode, the loop filter forms a third-order loop filter consisting of components
R5772, R5774, R5775, C5781 to C5787, C5790 to C5792, and C5809 to C5812.
Pin 43 (IOUT) of U5752 provides the charge-pump current for steering of the control voltage line to
the VCOs. During normal mode, pin 45 (IADAPT) is set to a high impedance and has no effect on the
loop filter. When U5752 is programmed to a new frequency, the IC is initially operated in adapt mode.
In this mode the loop filter is reconfigured for a wider bandwidth allowing the synthesizer to lock
faster. The charge-pump output is supplied through pin 45 (IADAPT) in this mode, and this
reconfigures the loop filter to behave like a second-order filter.

3.7.2.8 Lock Detect

Lock status of the synthesizer loop is provided to the microprocessor by pin 4 (LOCK) of U5752. A
high level (3.0 Vdc) indicates that the loop is stable. A low voltage indicates that the loop is not
locked and will result in a Fail 001 to be displayed on the control head display.

3.7.2.9 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.

3.7.2.10 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.
May 25, 2005
Theory of Operation: Frequency Generation Unit (FGU)
6881096C74-B