Stanford Research Systems DS335, User Manual

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Circuitry 5-5
DS335 Synthesized Function Generator
Output Filters (DS335M5)
There are two output filter types: a Bessel filter and a Cauer filter. Both filters have a characteristic impedance
of 50 Ohms: they are driven by, and terminated into 50 Ohms. The filters are differential filters: they have
complementary inputs and outputs, and are closely phase matched between the inputs because they share
the same physical core.
The Bessel filter is a 5th degree filter with the -3 dB point set to 3.5 MHz. This filter is used when ramps,
triangles or noise functions are selected. The Bessel filter has excellent phase-linear response so that there
will be no overshoot or ringing of the output waveform.
The Cauer filter is a 7th degree filter with a passband to 3.4 MHz and a stopband attenuation of 86 dB. The
Cauer filter is used when sines or square wave outputs are selected. Cauer filters provide steep roll-offs and
flat passband characteristics.
A Sin(x)/x compensator precedes the Cauer filter. This circuit compensates for the reduced signal level of the
sampled waveform as the frequency of the output approaches the Nyquist limit. The circuit increases the
amplitude to compensate for the effect by increasing the termination impedance of the DAC output for higher
frequencies.
A pair of DPDT relays (U501 and U502, HS-212's) select between the Bessel and Cauer filters. These relays
are controlled by the LSB of the -RELAY_CTL latch (U800, a 74HC374) and are driven by an emitter-follower
(U801, a CA3082).
Pre-attenuator (DS335M5)
The output from the selected filter is terminated by a 0 to 14 dB resistive differential attenuator. This
attenuator must be used when an output offset other than zero is selected. (The peak ac amplitude plus offset
must not exceed 10 V at the output of the amplifier.) The attenuator will also be used for outputs of less that
0.3 Vpp.
A pair of 1:8 analog multiplexers (U600 and U604, 74HC4051's) select the attenuation factor in 2 dB steps.
The analog multiplexers are controlled by the -MISC_OUT latch (U109, a 74HC273). The multiplexers are
inhibited by SQ/-SINE if a square wave output is selected. This will reduce crosstalk in the function select
relay (U603, a HS-212).
SYNC Generator (DS335M6)
The un-attenuated signal from the filters is buffered by emitter followers (Q600 and Q602, 2N3904's). The
buffered differential signal drives a differential comparator (U601, an AD9696) to generate a SYNC signal.
The positive feedback provides lots of hysterisis, and additional resistive feedback to the emitter followers
cancels the kick-back from the comparator.
The comparator output is buffered by an octal driver (U602, a 74HC244) which has seven of its outputs wire
in parallel to drive the front panel SYNC output. The SYNC output is driven though a 47 Ohm resistor provide
reverse termination of reflected pulses.
The comparator also drives a pnp differential pair (Q603 and Q604, 2N3906's) to generate a differential
square wave at their collectors. The amplitude of the square wave is controlled by the analog voltage,
SW_AMPL, which controls the pnp constant current source (Q601, a 2N3906). The externally compensated
op-amp (U605, a CA3140) maintains a voltage across the emitter resistor which is proportional to (SW_AMPL
+ 5.00Vdc). As SW_AMPL varies from -5 to +5V, the constant current source varies from 0 to 11 mA,
generating from 0 to 1.1 Vpp square wave output.