Motorola ASTRO XTSTM 4000 Detailed Service Manual Download

Page: 38 / 210

May 28, 2008

6871620L01-C

3-12

Theory of Operation:

Transceiver Section

3.1.4.2.3 Modulation

To support many voice, data, and signaling protocols, XTS 4000 radios must modulate the
transmitter carrier frequency over a wide audio frequency range, from less than 10 Hz up to more
than 6 kHz. The FracN supports audio frequencies down to zero Hz by using dual-port modulation.
The audio signal at pin 10 (MODIN) is internally divided into high- and low-frequency components,
which modify both the synthesizer dividers and the external VCOs through signal MODOUT (pin 41).
The IC is adjusted to achieve flat modulation frequency response during transmitter modulation
balance calibration using a built-in modulation attenuator.

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 V5A 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) – VHF only

The VCOBIC (U302) 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.

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. C315 acts as a DC block, and resistors
R314, R315, and R316 attenuate an output signal to an optimum level for the PA.

L312 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, Q310, together with the supporting components R310 and C330,
form the 3.3 Vdc-to-5 Vdc logic-level shifter between the AUX3 pin of the FracN IC, U202, and
VCOBIC, U302.