6881076C25-E September 5, 2008
Theory of Operation: RF Board 3-7
3.1.3.2 ABACUS II IC
Once in the ABACUS II IC (U301), the first IF frequency is amplified and then down converted to
450 kHz, the second IF frequency. At this point, the analog signal is converted into two digital bit
streams by 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 VOCON board
where it is decoded to produce the recovered audio.
The ABACUS II IC is electronically programmable, and 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 II IC.
The ABACUS II 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 dependent on
sampling speed. A fractional-n synthesizer is also incorporated in the ABACUS II IC for the 2nd LO
generation.
The second LO/VCO is a Colpitts oscillator (see Section 3.1.2.7, "Second VCO," on page 3-6). Its
output feeds into the ABACUS II IC on pin 35, providing injection to the second mixer for converting
the IF frequency to 450 kHz.
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