Stanford Research Systems DS345, Manual

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Introduction
2-3
The reconstruction filter is key to accurately reproducing a waveform in a
sampled data system. The DS345 contains two separate filters. For sine
w ave generation the output of the DAC goes through a 9
th
order Cauer filter,
while ramps, triangles, and arbitrary waveforms pass instead through a
10 MHz 7
th
order Bessel filter. The Cauer filter has a cutoff frequency of 16.5
MHz and a stopband attenuation of 85dB, and also includes a peaking circuit
to correct for the sine(x)/x amplitude response characteristic of a sampled
system. This filter eliminates any alias frequencies from the waveform output
and allows generation of extremely pure sine waves. The output of the Cauer
filter is then frequency doubled by an analog multiplier. This multiplies the
DAC's 0 - 15 MHz output frequency range to the final 0 - 30 MHz range. How-
ever, the Cauer filter has very poor time response and is only useful for CW
waveforms. Therefore, the Bessel filter was chosen for its ideal time re-
sponse, eliminating rings and overshoots from stepped waveform outputs.
This filter limits the frequency of arbitrary waveforms to 10 MHz and rise
times to 35 ns.
The output of the filters pass to an analog multiplier that controls the ampli-
tude of the waveform. This multiplier controls the waveform amplitude with an
AM signal that may come from either the ASIC or the external AM input. This
allows both internally and externally controlled amplitude modulation. The
amplitude control is followed with a wide bandwidth power amplifier that out-
puts 10 V peak-to-peak into a 50 ohm load with a rise time of less than 15 ns.
The output of the power amplifier passes through a series of three step atten-
uators (6, 12, and 24 dB) that set the DS345's final output amplitude. The
post amplifier attenuators allow internal signal levels to remain as large as
possible, minimizing output noise and signal degradation.
Square waves and waveform sync signals are generated by discriminating
the function waveform with a high-speed comparator. The output of the com-
parator passes to the SYNC OUTPUT and, in the case of square waves, to
the amplitude control multiplier input. Generating square waves by discrimi-
nating the sine wave signal produces a square wave output with rise and fall
times much faster than allowed by either of the signal filters.