Motorola ASTRO XTS 5000 Detailed Service Manual page 53

Theory of Operation: Transceiver Board
3.1.4.2.4 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. 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.
3.1.4.3 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.
3.1.4.4 VCO Buffer IC (VCOBIC)
The VCOBIC (U302 for VHF radios or U300 for UHF Range 1 and 700–800 MHz radios) is an
analog IC containing two NPN transistors for use as oscillators, an active-bias circuit, transmitter and
receiver buffer amplifiers, and switching circuitry. The VCOBIC has three RF outputs:
• TX_OUT (pin 10)—the modulated transmitter carrier
• RX_OUT (pin 8)—the receiver first LO
• PRESC_OUT (pin 12)—connected to FracN pin 32 (PREIN) through a matching circuit
Transmit/receive control is a single 5.0 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.
VHF: The VCOs in VHF radios use the VCOBIC internal transistors and implement the active bias
via resistors R304 and R305. Bias to TX_OUT is supplied through resistor R313. Components L309
and C316 form a matching circuit for the TX_OUT impedance. C319 acts as a DC block, and
resistors R314, R315, and R316 attenuate an output signal to an optimum level for the PA.
L312 and C320 form a low-pass bias supply filter for the RX_OUT. L310 and C317 are the RX_OUT
impedance-matching circuit. C322 is a DC block, and resistors R317 and R318 attenuate an output
signal to an optimum level for the mixer IC.
An NPN/PNP-packaged transistor, Q301, together with the supporting components R308, R309, and
R310, form the 3.3 Vdc-to-5 Vdc logic-level shifter between the AUX3 pin of the FracN IC, U202, and
VCOBIC, U302.
UHF Range 1: For these radios, four VCOs are used: two transmit and two receive. Control-logic
translation is done by level shifter Q310 and Q311, and wired-OR D320.
UHF Range 2: Two VCOs are used for receive and one for Transmit. The fractional N synthesizer
selects the appropriate VCO by toggling auxilary lines 2,3, or 4. The selection of the TX VCO with
auxilary line 4 also sets TRB_IN high.
700–800 MHz: These radios use three external, discrete, varactor-tuned, Colpitts VCOs based on
transistors Q301, Q303, and Q308. Bias current to the VCOs is switched on and off by the
transistors Q302, Q306, and Q309, which are controlled by FracN outputs AUX1, AUX2, and AUX4.
Transistor Q304 is an additional transmit buffer amplifier reducing frequency pulling on the transmit
VCO. An additional buffer is switched on and off by the transistor Q305. Bias to TX_OUT is supplied
through resistor R307. Components L307 and C208 form a low-pass bias supply filter. The RX_OUT
impedance is matched by a one-component match, coil L310.
6881094C31-D
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July 21, 2004