Transmitter - Motorola XTS 2500 Service Manual

4.1-4
The ABACUS IC (U401) is electronically programmable. The amount of filtering, which is dependent
on the radio channel spacing and signal type, is controlled by the microcomputer. Additional filtering,
which used to be provided externally by a conventional ceramic filter, is replaced by internal digital
filters in the ABACUS IC.
The ABACUS IC contains a feedback Automatic Gain Control (AGC) circuit to expand the dynamic
range of the sigma-delta converter. The differential output data contains the quadrature (I and Q)
information in 16-bit words, the AGC information in a 9-bit word, imbedded word sync information,
and fill bits. Two synthesizers are available on the chip for second LO and sampling clock (18 MHz)
generation.
For VHF, the second LO/VCO is a Colpitts oscillator built around transistor Q401. Varactor diode
(D402) in the VCO is used to adjust the VCO frequency. The control signal for the varactor is derived
from a loop filter consisting of C440, R417, C441, R418, and C480. The sampling clock is derived
from a negative resistance generator (on the chip) available at CLKP, CLKN, and an external tank
circuit (L403, C423, and D401). The loop filter for the clock is realized by C448, R408, and C425.

4.1.5 Transmitter

The VHF RF power amplifier (PA) consists of
• an RF driver (U101) and
• a Silicon N-Channel MOSFET type transistor specifically designed for VHF RF power amplifier
(Q101).
For VHF, an RF input drive level of approximately +3dBm is supplied from the VCO buffer IC (U250).
This input drive level is applied to pin 16 (RFIN) of U101. The dc power is applied to pins 6, 7, and 14
of U101 and to the drain of Q101 via a filtered RAW_B+. Power control is achieved through the
varying of the dc bias (and thus the gain) to pin 1 of U101 and the gate of Q101. The amplified RF
signal leaves the RF final PA (Q101) at the drain and is applied to the discrete directional coupler via
an impedance match. The RF signal passes through the coupler, a discrete antenna switch, and a
discrete harmonic filter before finally reaching the antenna launch connector.
A portion of the forward RF power is sampled by the directional coupler, applied to the diode (D172)
for rectification, and the resulting dc signal is fed back to pin 1 of the Power Control IC (PCIC, U102).
This dc signal is representative of the forward RF power being passed through the directional
coupler. The dc signal is used by the PCIC to regulate the transmitted RF power level.
The PCIC is the heart of the power control loop. The rectified feedback is internally compared to an
internal Digital-to-Analog Converter (DAC) output voltage in the PCIC to determine the amount of DC
bias voltage at pin 4. This voltage at pin 4 of the PCIC controls the gain (and thus the output power,
as explained above) of the RF driver (U101) and the RF power amplifier (Q101) via a fixed resistor
divider network.
February 14, 2012
VHF Detailed Theories of Operation: Transmitter
6816985H01-F