Dsp Logic Board - Stanford Research Systems SR810 Manual

DSP LOGIC BOARD

OVERVIEW
The DSP LOGIC BOARD takes a digital input from
the A/D Converter on the Analog Input Board and
performs all of the computations related to the
measurement before it is displayed on the screen.
This includes generating the digital reference sine
wave, demodulating the signal, low-pass filtering
the results, and offset and expanding the outputs.
The internal oscillator sine output and Aux D/A
outputs are generated on this board as well. The
reference phase lock loop controls the clock of this
board whenever the reference mode is external.
These functions are implemented within a system
comprised of five functional blocks: the Digital
Signal Processor (DSP), the DAC Outputs, the
Timing Signal Generator, the Reference Clock
Generator and the I/O Interface. Through the use
of highly efficient algorithms, the system is
capable of real-time lock-in operation to 100 kHz
with 24 dB/oct filtering on both X and Y as well as
providing a synthesized analog sine output.
DSP PROCESSOR
The SR810 utilizes a Motorola 24-bit DSP56001
digital signal processor (U501). The DSP is
configured without external memory. The lock-in
algorithms run entirely within the internal program
and data memory of the DSP itself. The Host
processor bus is connected to the main CPU
Board via the I/O Interface on the DSP Logic
Board. The 80C186 processor on the CPU Board
acts as the "host" processor to the DSP. DSP
firmware and commands are downloaded from the
CPU Board to invoke different operating modes.
The DSP also has two dedicated serial ports: one
for receiving, and one for transmitting.
REFERENCE CLOCK SOURCE
The clock to the DSP is derived from the timing
generator. U120, U121 and U122 are gates which
select the clock source for the entire digital board.
When the reference mode is internal, the
30.208 MHz crystal (U111) is used. The A/D
inputs and D/A outputs run with a 256 kHz cycle
and the DSP performs 59 instructions each cycle
(each instruction takes two clocks). The crystal
also sets the internal
accuracy.
reference frequency
Circuit Description
When the reference mode is external, the VCO
(voltage controlled oscillator, U110) is used as the
system clock. The VCO nominally runs at 30 MHz
as well. U105 is a phase comparator. The external
reference input, discriminated by U103 (or TTL
buffered through U104D) is one of the inputs to
the phase comparator. The other input is the
internal reference. The DSP always synthesizes a
sine wave at the reference frequency. This is the
Sine Output. This sine output is discriminated by
U209 into a TTL square wave (TTL Sync Out) and
is the other input to the phase comparator. The
phase lock loop then controls the VCO which is
the clock to the DSP. This in turn changes the sine
output frequency to maintain frequency lock with
the external reference. The DSP is constantly
getting external frequency information from the
host (based upon counter U622) which allows the
DSP to synthesize nearly the correct reference
frequency assuming a 30 MHz clock. This keeps
the VCO within range at all frequencies.
TIMING GENERATOR
All timing signals for the DSP and Analog boards
are derived from the system clock by PALs (U601-
604). These PALs generate the clocks for the
DACs and A/D converter, the multiplexing signals
for the Aux inputs and outputs, etc.
SERIAL CHANNELS
There are two serial data streams from the A/D
converter on the Analog Input board which need to
be received by the DSP. The digitized input signal
is received directly via the DSP's serial input port.
The Aux A/D input data is shifted into a pair of
serial-to-parallel registers (U502 and U503) and is
read via the DSP data bus. Each A/D input
channel provides a new sample every 4 µs.
There are two dual-channel D/A converters on this
board for a total of four D/A output channels. Each
output channel provides a new output every 4 µs.
This means that 4 output values must be written
by the DSP each 4 µs cycle. The DSP writes to
one channel of each D/A converter via its serial
transmit port each cycle. The transmit port
operates at twice the frequency of the receive
port. The DSP writes to the other channel of each
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