Astro Spectra Plus Rx Signal Path; Figure 3-16. Astro Spectra Plus Rx Signal Path - Motorola ASTRO Digital Spectra Plus Service Manual

Theory of Operation: ASTRO Spectra Plus VOCON Board
3.4.4

ASTRO Spectra Plus RX Signal Path

The vocoder processes all received signals digitally. This requires a unique back end from a
standard analog radio. This unique functionality is provided by the ABACUS IC with the KRSIC
(U200) acting as the interface to the DSP. The ABACUS IC located on the transceiver board
provides a digital back-end for the receiver section. It provides a digital output of I (In phase) and Q
(Quadrature) data words at a 20 kHz sampling rate (refer to the Receiver Back-End section for more
details on ABACUS operation). This data is passed to the DSP through an interface with the KRSIC
(U200) for appropriate processing. The KRSIC interface to the ABACUS is comprised of the four
signals SBI, DIN, DIN*, and ODC (refer to
PATRIOT
U300
DSP 56600
SAP
SCKA
STDA
SC2A
512 kHz
Data
8 kHz
NOTE: An asterisk symbol (*) next to a signal name indicates a negative or NOT logic true signal.
ODC is a clock ABACUS provides to the KRSIC. Most internal KRSIC functions are clocked by this
ODC signal at a rate of 2.4 MHz and is available as soon as power is supplied to the circuitry. This
signal may initially be 2.4 or 4.8 MHz after power-up. It is programmed by the KRSIC through the SBI
signal to 2.4 MHz when the KRSIC is initialized by the MCU (in the Patriot IC) through GPIO. SBI is
a programming data line for the ABACUS. This line is used to configure the operation of the
ABACUS and is driven by the KRSIC. The MCU programs many of the KRSIC operational features
through the GPIO interface. When the KRSIC is programmed properly by the MCU, the KRSIC in
turn sends this data to the ABACUS through the SBI.
DIN and DIN* are the data lines on which the I and Q data words are transferred from the ABACUS.
These signals make up a differentially encoded current loop. Instead of sending TTL type voltage
signals, the data is transferred by flowing current one way or the other through the loop. This helps to
reduce internally generated spurious emissions on the RF board. There are single-ended driver
circuits between the ABACUS and the KRSIC, which are used to convert the differential current
driven by the ABACUS. After the driver circuits, the I and Q samples are detected and transferred to
a serial transmitter.
6881076C25-D
Figure
GPIO
BBP
800 KHz
SC0B
Serial Receive Data
SRDB
20 kHz
SC1B
CODEC
MCLK
U402
SDO
DR
RO_NEG
FSR

Figure 3-16. ASTRO Spectra Plus RX Signal Path

3-16).
KRSIC
U200
D0-D7,
RS0-RS4
RXSBI
ABA_CLK RXData_HI
ABA_RXD RXData_LO
ABA_FSYNC RXODC
Command
Board
J501-40
3-41
ABACUS II
Interface
SBI
J501-6
Data In
J501-2
Data In*
J501-1
ODC
J501-7
October 28, 2002