CDR-3250/80 TECHNICAL MANUAL
4-4
Issue 2.2
STATUS MSG
Receiver status information (current
signal strength, audio output level,
etc.) from the DSP Section.
STATUS RDY
External flag interrupt to inform the
Control Section that the DSP
Section's output register contains
the STATUS MSG for transfer.
ENABLE
Enables each receiver module or
circuit.
CLOCK
Clocks out the control data to all
modules.
DATA
Control information to each
module or circuit.
C.O.R.
Carrier operated relay signals to rear
panel.
Either a serial or IEEE-488 remote interface plug-in daughter
board is installed as the remote control option. If the
receiver is configured for serial bus operation refer to
paragraph 4-2.7. If the receiver is configured for IEEE-488
bus operation refer to paragraph 4-2.8.
4-2.6.2
DSP Section.
The digital signal processing (DSP)
section provides the following conventional receiver
functions: fourth LO, fourth mixer, IF filtering, AGC, IF gain
control, BFO, detection, demodulation, and receive
frequency tuning resolution in 1 Hz steps.
The DSP section contains an analog to digital converter,
digital signal processor, I & Q line driver, 128k x 8-bit flash
memory, 64k x 32-bit SRAM, IF reconstruction circuit, and
multiplexed digital to analog converter.
The DSP Section operates semi-independently from the
Control Section. All communication between the Control
Section and the DSP Section is through the I/O ports in the
Control Section. When the receiver operating parameters
are changed, the Control Section sends one or more
commands to the DSP Section which makes the necessary
changes to its operation, and then continues processing.
The analog-to-digital converter converts the 3RD IF analog
signal at 24 kHz to a digital signal for the digital signal
processor. The 3RD IF signal is applied to the A/D
converter where it is sampled at a 96 kHz rate. This
sampling rate is the DSP clock frequency (48 MHz) divided
by 500. The 48 MHz is generated by the VCO in the Third
LO section of the RF Analog module which also generates
the 3RD LO signal. This selection of frequencies allows all
internal signals to be phase locked to the reference
frequency standard.
The broadband digitized 3RD IF signal from the A/D
converter is applied to the digital signal processor where
filtering and demodulation takes place mathematically. DSP
functions are all performed by firmware within the DSP
microprocessor.
(See figure 4-1). The sampled IF input signal from the A/D
Converter at 96 ks/s is applied to a downconverter. This
function produces the "I" (In-Phase) and "Q" (Quadrature)
signals and reduces the sample rate to 24 ks/s. At this
point the tuned frequency is represented by an I&Q vector
rotating in either direction at up to 500 Hz (-500 to +500 Hz).
The I&Q signals are applied to a frequency translator
function. The digital FINE LO signal, representing the fine
tuning frequency, is also applied to the frequency
translator. The translator applies a rotation b500
Hz and -500 Hz to the vector which translates the tuned
frequency to 0 Hz (no rotation). This process provides fine
tuned I&Q signals with a resolution of 1 Hz.
The outputs of the frequency translator are independently
applied to two identical lowpass Finite Impulse Response
(FIR) filters. Fifty-one FIR filters can be selected ranging
from 100 Hz to 16 kHz. This function provides the overall IF
bandpass function in a conventional receiver.
The outputs of the FIR filters are applied to a complex
scaling function. Digital gain control is applied to the
complex scaling, effectively varying the signal gain. Either
automatic or manual gain is provided when selected. AGC
uses a combination of the values from the sampled IF and
AM detected digital signals to provide both digital and
front-end gain control. The front-end gain signal is used in
the RF Analog module. When selected, the MGC signal is
applied to the gain control function from operator input
through the Control Section.
AM demodulation is done mathematically using the signal
magnitude, which is calculated as the square root of the
sum of the squares of the I&Q outputs of the FIR filters.
The signal magnitude is also used to develop AGC when
selected. SSB and CW demodulation uses the digital
equivalent of a product detector. The signal is translated
from 0 kHz to approximately 1 kHz using a numerically
controlled oscillator (NCO). The NCO frequency is set
either positive or negative, depending on whether USB or
LSB reception is desired. In the ISB mode, the normal
channel functions provide USB and are duplicated in an
alternate channel to provide LSB. The functions include
input frequency translation, IF filtering, and the product
Courtesy of http://BlackRadios.terryo.org
