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3-6
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.
AD9864
-16dB
IFIN
LNA
FREF
LO
Synth.
LO VCO and
Loop Filter
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.
May 28, 2008
DAC
AGC
Decimation
ADC
Filter
......=13-26MHz
Voltage/
Samp. Clock
Current
Synthesizer
Reference
CLK VCO and
Loop Filter
Theory of Operation: Transceiver Section
Formatting/SSI
Control Logic
SPI
MAEPF-27412a-O
DOUTA
DOUTB
FS
CLKOUT
6871620L01-C
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