Stanford Research Systems DS360 Operating Manual And Programming Reference page 24

2-4 Basics
DDS with Advanced Signal Processing
A block diagram for the DS360 is shown in Figure 1. The DS360 utilizes direct digital
synthesis to generate its basic waveform. A Motorola DSP56002 advanced 24 bit digital
signal processor (DSP) acts as the phase accumulator and contains the internal waveform
RAM. The DSP chip gives the DS360 exceptional flexibility for generation of different
waveforms. A 32.333 MHz, 25 PPM crystal provides all clocking information for the
DS360, giving it exceptional frequency stability.
The DSP waveform RAM feeds an ultra low distortion 20 bit DAC, which is followed by
th
a 7 order Cauer anti-imaging filter to accurately reconstruct the sampled waveforms.
For sinewave generation, this is followed by a distortion reduction filter, that removes
nearly all of the remaining distortion components of the waveform. The output of this
filter passes through the fine amplitude control and to the low distortion balanced /
unbalanced power amplifier. The power amplifier is capable of generating a 40 Vpp
sinewave, with about -100dB of distortion in the unbalanced configuration and superior
performance at lower amplitudes. Finally the signal passes through output attenuators,
capable of 0, -20, -40 or -60 dB of attenuation.
Other waveforms follow slightly different paths. White noise skips the distortion
reduction filters, while pink noise adds the pink noise filter. Squarewaves and the
waveform sync signal are generated by discriminating the function with a high speed
comparitor. Burst signals are generated by passing any of the waveforms through the
burst DACs.
Main CPU
Display
Keys
Interfaces
56002
20 bit D/A
DSP Chip
DS360 Ultra Low Distortion Function Generator
Burst Control
Logic
Burst Level
Anti Distortion
Imaging Reduction
Filter Filters
Fine Amplitude
Control
Sine
Square
Comparator
Figure 2-1 DS360 Block Diagram
Power
Course
Amplifier
Attenuators
+
Attenuator
Burst
Normal
_
Attenuator
Source
Resistors
+
_
Sync