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The SR600 series consists of three dual-channel instruments. Each
instrument channel has a low-noise preamplifier, a precision high
pass or low pass filter section, and an output amplifier. The filters
are 8-pole, 6-zero elliptic-type with 115 dB per octave rolloff,
0.1 dB of passband ripple, and 80 dB of stopband attenuation. Cutoff
frequencies can be set between 1 Hz and 100 kHz making the SR600
Series ideal for applications including anti-aliasing, audio analysis,
and frequency-domain signal conditioning.
Model - Configuration
SR640 - Dual Low Pass
SR645 - Dual High Pass
SR650 - High Pass/Low Pass (can be cascaded for Band Pass)
Inputs and Preamplifiers
Each filter channel has a floating, differential input with 1 MΩ
input impedance and 6 nV/√Hz of input noise. The analog ground
for each filter channel is available at a rear-panel BNC connector
to provide complete flexibility in grounding. The filter grounds
may be tied to the instrument's chassis ground or to any other point.
Both AC and DC input coupling can be selected, and an invert key
allows the phase of the signal to be shifted by 180° with a
single key press. Up to 60 dB of preamplifier gain can be specified
in 10 dB increments. A simple offset adjust screw lets you null
DC components in your signal.
Filter Characteristics
Filter characteristics for the SR600 series are shown below. The
exact filter transfer function is given in the specifications section.
(The graph shows the low pass response. The high pass response is
found by inverting the curve around f=1.) The large amount of excess
phase, typical of elliptical filters, makes the SR600 series filters
optimum in frequency-domain applications. Note that the 0.1 dB passband
ripple specification for the SR640 series filters is a measured
specification, not just the theoretical passband ripple of the transfer
function. Other filter characteristics include an attenuation slope
of -115 dB/octave and a stopband attenuation of greater than 80
dB. Worst-case phase match between channels is better than ±0.75°
from DC to 100 kHz, and improves to ±0.25° for cutoff
frequencies below 10 kHz.
Post-Filter Gain and Bypassing
After the filter, another amplifier section provides up to 20 dB
additional gain. The standard output drives loads of greater than
300 Ω at 10 Vpp, while the optional high-power output drives
10 Vpp into a 50 Ω load. Each filter may be independently
bypassed from the front panel or from the computer interface. When
bypassed, both the prefilter and postfilter gains remain active,
and the resulting amplifier has a bandwidth of 450 kHz.
Programming
A key feature of the SR600 series is its programmability. Both
GPIB and RS-232 interfaces are standard, and all instrument settings
can be queried and changed via the interfaces. Cutoff frequencies
can be changed in under 100 ms, making the SR600 series ideal for
applications involving frequency-agile filters. Careful attention
has been paid to minimizing noise resulting from the presence of
the microprocessor and GPIB and RS-232 interfaces. Both the interface
electronics and the instrument's microprocessor control circuitry
are optically isolated from the filter sections in order to reduce
digital-analog crosstalk. The CMOS microprocessor "sleeps"
while the interface and front panel are inactive in order to reduce
the amount of digital noise in the instrument.
Stored Settings
Up to nine complete instrument configurations, including amplifier
gains and cutoff frequencies for both channels, can be stored in
non-volatile memory for later recall. Settings can be recalled from
the front panel or through the computer interfaces.
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