May 28, 2008
6871620L01-C
3-6
Theory of Operation:
Transceiver Section
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 3-4. Abacus III (AD9864) Functional Block Diagram (from data sheet)
Input signal RXIF is the 44.85 MHz (VHF) and 73.35 MHz (UHF) IF from crystal filter FL490 in the
receiver front end.
VHF:
Components L491 and C491 match the input impedance of the ABACUS III to the IF Filter.
UHF:
Components C584 and C491 match the input impedance of the ABACUS III to the IF Filter.
3.1.2.2.2 Second Local Oscillator
The second LO is controlled by the Abacus LO synthesizer, which mixes with IFIN to produce a
2.25 MHz final IF. The external VCO consists of Q550 and its bias network and frequency-
determining elements. Signal FREF is the 16.8 MHz reference from the FGU. Darlington transistor
Q551 with C583 and R552 form an active power-line filter.
The second LO frequency is 42.6 MHz (VHF) and 71.1 MHZ (UHF) by default, or 47.1 MHz (VHF)
and 75.6 MHz (UHF) in special cases as needed to avoid radio self-quieters. The loop filter is
composed of R551, C551, C552, and C553.
3.1.2.2.3 Sampling Clock Oscillator
The Abacus sampling clock synthesizer operates at 18 MHz = 8 x 2.25 MHz. The VCO uses an
internal transistor and external resonator. The resonator is composed of L570, C573, and D570.
The loop filter is composed of R571, C570, C571, and C572.
DAC
AGC
LNA
IFIN
FREF
LO
Synth.
Samp. Clock
Synthesizer
Voltage/
Current
Reference
Control Logic
SPI
Decimation
Filter
M
ADC
AD9864
-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-27412a-O
Summary of Contents for ASTRO XTSTM 4000
Page 1: ...COLOR CHORDS 7 ASTRO XTSTM 4000 Portable Radio Detailed Service Manual ...
Page 2: ......
Page 16: ...May 28 2008 6871620L01 C xiv Commercial Warranty Notes ...
Page 22: ...May 28 2008 6871620L01 C 1 6 Introduction Notes ...
Page 26: ...May 28 2008 6871620L01 C 2 4 Radio Power Notes ...
Page 56: ...May 28 2008 6871620L01 C 3 30 Theory of Operation Notes ...
Page 100: ...May 28 2008 6871620L01 C 6 8 Troubleshooting Waveforms Notes ...
Page 194: ...May 28 2008 6871620L01 C B 2 Motorola Service Centers Notes ...
Page 204: ...May 28 2008 6871620L01 C Glossary 10 Glossary Notes ...
Page 209: ......