Motorola XTL 1500, Detailed Service Manual, Page: 97 / 452

June 15, 2005 6815854H01-A
3-48 Theory of Operation: Frequency Generation Unit (FGU)
3.13.1.6 Charge Pump Bias
External circuitry connected to pin 39 (Bias 2) and pin 40 (Bias 1) of U3751 determine the current
that is applied to the charge-pump circuitry. During receive mode, resistors R3778, R3763, and
R3758 set the current supplied to pin 40 (Bias 1). Transistor Q3755 and resistors R3779, R3756,
and capacitor C3808 form a circuit that momentarily increases the current to pin 40 (Bias 1) during
receiver programming of U3751. This circuit is activated by pin 46 (ADAPTSW) of U3751 during the
transition of programming U3751 to frequency and effectively decreases the length of time for the
synthesizer to lock on frequency. Similarly, during transmitter mode, resistors R3778, R3763, and
R3776 set the current supplied to pin 39 (Bias 2). Transistor Q3750 and resistors R3759, R3776,
and capacitor C3825 form a circuit that momentarily increases the current to pin 39 (Bias 2) during
transmitter programming of U3751.
3.13.1.7 Loop Filter
The loop filter operates in synchronization with the phase detector of U3751 in two modes, normal
and adapt. In normal mode, the loop filter forms a third-order loop filter consisting of components
R3765, R3773, R3770, C3833, C3810, C3834, C3855 to C3861 and C3881 to C3883.
Pin 43 (IOUT) of U3751 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 U3751 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.
3.13.1.8 Lock Detect
Lock status of the synthesizer loop is provided to the microprocessor by pin 4 (LOCK) of U3751. 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.
3.13.1.9 Transmitter Injection
The transmit (TX) injection string consists of three amplifier stages (Q3760, Q3759, and Q3758)
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 D3750, 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
R3840, R3833, and R3839. This output is coupled to the first-stage amplifier Q3760, further
attenuated 3 dB via resistors R3803, R3809, and R3803, and then coupled to the second-stage
amplifier Q3759. This output is tapped to supply the TX feedback signal to the synthesizer prescaler,
and the balance is further attenuated 5 dB via resistors R3818, R3823, and R3824. This output is
coupled to the third- stage amplifier Q3758, further attenuated 3 dB via resistors R3858, R3863, and
R3859, routed to the 7-pole Cow Chebychev low-pass filter C3917 through C3921, L3785, and
L3787 in order to attenuate harmonics. The output is, again, attenuated 3 dB via resistors R3842,
R3834, and R3841 and coupled to the input of the RF power amplifier section. The five sets of
resistive attenuators provide increased isolation between the TX VCO and RF power amplifier.