Loop Filter; Vco Buffer Ic (Vcobic) U300 - Motorola ASTRO XTS 5000 Detailed Service Manual

4-10
4.2.5.2.4 Voltage Multiplier and Superfilter
Pins 12 (VMULT3) and 11 (VMULT4) together with diode arrays D201 and D202 and their associated
capacitors form the voltage multiplier. The voltage multiplier generates 11.5 Vdc to supply the phase
detector and charge-pump output stage at pin 47 (VCP).
The superfilter is an active filter that provides a low-noise supply for the VCOs and VCOBIC, U300.
The input is regulated 5 Vdc from Vdd5 at pin 30 (SFIN). The output is superfiltered voltage FSF at
pin 28 (SFOUT).
The output from pin 15 (VMULT1) is used as a clock for the SCF IC, FL200.

4.2.5.3 Loop Filter

The components connected to pins 43 (IOUT) and 45 (IADAPT) form a 3rd-order, RC low-pass filter.
Current from the charge-pump output, IOUT, is transformed to voltage VCTRL, which modulates the
VCOs. Extra current is supplied by IADAPT for rapid phase-lock acquisition during frequency
changes. The lock detector output pin 4 (LOCK) goes to a logic "1" to indicate when the phased-lock
loop is in lock.

4.2.5.4 VCO Buffer IC (VCOBIC) U300

The VCOBIC, U300, is an analog IC containing two bipolar npn transistors (BJT), active bias, buffer
amplifiers, and a switching circuitry. BJTs and an active bias are not being used in XTS 5000 700/
800 MHz radio. Buffered outputs TX_OUT (pin 10) and RX_OUT (pin 8) are the modulated
transmitter carrier and the receiver first LO; PRESC_OUT (pin 12) through the matching circuit is
connected to pin 32 (PREIN) of the FracN IC, U202. Transmit/receive control is a single 5 Vdc logic
input, TRB_IN (pin 19). When TRB_IN is low, the receiver buffer is active and the transmitter circuits
are disabled. The converse is also true. Two transistors, Q307 and Q310, form a 3.3 Vdc-to-5 Vdc
logic level shifter between the AUX3 pin of the FracN IC, U202, and VCOBIC, U300.
Transistor Q304 and its associated circuitry is an additional transmit buffer amplifier reducing the
transmit VCOs pulling. An additional buffer is switched On and Off by the transistor Q305.
The transistors Q301, Q303, and Q308 are active devices of the three Colpitts varactor-tuned VCOs.
VCOs are switched On and Off by the transistors Q302, Q306, and Q309 using AUX1, 2, and 4
outputs of the FracN IC.
The VCOBIC has three RF outputs: TX_OUT, RX_OUT, and PRESC_OUT. Bias to pin 10
(TX_OUT) is supplied through resistor R307. Components L307 and C208 form a low-pass bias
supply filter.
Components L309 and C325 form a low-pass bias supply filter for the RX_OUT. Coil L310
represents a one component match for the RX_OUT impedance. C327 is a dc block, and resistors
R321 and R323 attenuate an output signal to the level optimal for the mixer IC.
April 15, 2002
Detailed Theory of Operation: Transceiver Board
68P81094C31-O