6881094C31-E
November 16, 2006
Theory of Operation:
Transceiver Board
3-7
3.1.2.2 Receiver Back End
The receiver back end, which consists of the Abacus III (AD9874 IF digitizing subsystem) IC and its
associated circuitry, processes the down-converted IF signal to produce digital data for final
processing by the VOCON DSP.
NOTE:
Table 8-1, “List of Transceiver Schematics and Board Overlays,” on page 8-1
for a
listing of receiver back end schematics.
3.1.2.2.1 Abacus III IC U500
The AD9874 (
) is a general-purpose, IF subsystem that digitizes a low-level 10–300 MHz
IF input with a bandwidth up to 270 kHz. The output of the Abacus III IC is SSI data to the VOCON.
The signal chain of the AD9874 consists of a low-noise amplifier, a mixer, a bandpass sigma-delta
A/D converter, and a decimation filter with programmable decimation factor. An automatic gain
control (AGC) circuit provides the AD9874 with 12 dB of continuous gain adjustment. The high
dynamic range and inherent anti-aliasing provided by the bandpass sigma-delta converter allow the
AD9874 to cope with blocking signals 80 dB stronger than the desired signal.
Auxiliary blocks include frequency synthesizers for the second LO and sampling clock LO, as well as
an SPI port. The second LO uses a discrete external loop filter and VCO. The clock oscillator has an
external loop filter and resonator.
Figure 3-5. Abacus III (AD9874) Functional Block Diagram (from data sheet)
VHF:
Input signal RXIF is the 44.8 MHz IF from crystal filter FL451 in the receiver front end.
Components C547 and L542 match the input impedance from 50 ohms at the RXIF to the Abacus
input IFIN.
DAC
AGC
LNA
IFIN
FREF
LO
Synth.
Samp. Clock
Synthesizer
Voltage/
Current
Reference
Control Logic
SPI
Decimation
Filter
M
ADC
AD9874
-16dB
......=13-26MHz
DOUTA
DOUTB
FS
CLKOUT
LO VCO and
Loop Filter
CLK VCO and
Loop Filter
IOUTL
LOP
LON
IOUTC
CLKP
CLKN
VREFP
RREF
VREFN
PC
PD
PE
SYNCB
MXOP
MXON
IF2P
IF2N
GCP
GCN
Formatting/SSI
MAEPF-27412-O
Summary of Contents for ASTRO XTS-5000
Page 7: ...vi Table of Contents November 16 2006 6881094C31 E Notes ...
Page 11: ...x List of Figures November 16 2006 6881094C31 E Notes ...
Page 17: ...November 16 2006 6881094C31 E xvi CommercialWarranty Notes ...
Page 31: ...November 16 2006 6881094C31 E xxx Portable Radio Model Numbering System Notes ...
Page 83: ...November 16 2006 6881094C31 E 3 44 Theory of Operation Encryption Module ...
Page 135: ...November 16 2006 6881094C31 E 5 44 Troubleshooting Charts Secure Hardware Failure Notes ...
Page 175: ...November 16 2006 6881094C31 E 7 32 Troubleshooting Tables List of Board and IC Signals Notes ...
Page 367: ...November 16 2006 6881094C31 E Glossary 10 Notes ...
Page 373: ...Index 6 November 16 2006 6881094C31 E Notes ...