Operator's Manual

CIRCUIT DESCRIPTION
4-6
September 2001
Part No. 001-5100-001
The transmit VCO signal is amplified by an inter-
nal buffer, routed through a low-pass filter, and then
sent to the transmit power amplifier module. The refer-
ence oscillator supplies a 16.8 MHz clock to the synthe-
sizer where it is divided down to a 2.1 MHz clock. This
divided down clock is fed to the ABACUS IC.
4.3.2 ANTENNA SWITCH
A pair of diodes is used to electronically steer the
RF signal between the receiver and transmitter. In the
transmit mode, RF is routed through a transmit switch-
ing diode and sent to the antenna. In receive mode, RF
is received from the antenna, routed through a receive
switching diode and applied to the RF amplifier.
4.3.3 RECEIVER FRONT END
The RF signal from the antenna is sent through a
bandpass filter. The bandpass filter is electronically
tuned by the microcontroller via the D/A IC by
applying a control voltage to the varactor diodes in the
filter. The D/A output range is extended through the
use of a current mirror. Wideband operation of the
filter is achieved by retuning the bandpass filter across
the band.
The output of the bandpass filter is applied to a
wideband amplifier. After being amplified by the RF
amplifier, the RF signal is further filtered by a second
broadband, fixed tuned, bandpass filter to improve
spurious rejection.
The filtered RF signal is routed via a broadband
50 Ω transformer to the input of a broadband mixer/
buffer. The mixer uses GaAs FETs in a double-
balanced Gilbert Cell configuration. The RF signal is
mixed with a first LO signal of about –10 dBm
supplied by the FGU. Mixing of the RF and the first
LO results in an output signal which is the first IF
frequency according to Figure 4-1. The first IF signal
output is routed through a transformer and impedance
matching components and is then applied to a two-
pole crystal filter. The 2-pole crystal filter removes
unwanted mixer products.
4.3.4 RECEIVER BACK END
The output of the crystal filter is matched to the
input of the IF buffer amplifier transistor. The output
of the IF amplifier is applied to a second crystal filter
through a matching circuit. This filter supplies further
attenuation at the IF sidebands to increase radio
selectivity.
In the ABACUS IC the first IF frequency is
amplified and then downconverted to 450 KHz, the
second IF frequency. At this point, the analog signal is
converted into two digital bit streams via a sigma-delta
A/D converter. The bit streams are then digitally
filtered and mixed down to baseband and filtered
again. The differential output data stream is then sent
to the ADSIC on the Digital Board where it is
processed to produce the recovered audio.
The ABACUS IC is electronically program-
mable, and the amount of filtering, which is dependent
on the radio channel spacing and signal type, is
controlled by the microcontroller. Additional filtering,
which used to be provided externally by a conven-
tional ceramic filter, is replaced by internal digital
filters in the ABACUS IC. The ABACUS IC contains
a feedback 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 depen-
dent on sampling speed. A fractional-N synthesizer is
also incorporated on the ABACUS IC for 2nd LO
generation.
The 2nd LO/VCO is a Colpitts oscillator. The
VCO has a varactor diode to adjust the VCO
frequency. The control signal for the varactor is
derived from a loop filter.
4.3.5 TRANSMITTER
The transmitter consists of three major sections:
Harmonic Filter, RF Power Amplifier, and the ALC
(Automatic Level Control) circuit.
The RF signal from the PA module is routed
through a coupler, then through the harmonic filter,
then to the antenna switch. The RF power amplifier
module is a wide-band multi-stage amplifier. The
nominal input and output impedance of the power
amplifier is 50 Ω. The DC bias for the RF power
amplifier is controlled by a switching transistor. The
microcontroller uses the D/A IC to produce a ready
RF BOARD (CONT’D)