Input Filter - Stanford Research Systems SR124 Operation And Service Manual

2.2 Instrument overview

2.2.3 Input Filter

SR124 Analog Lock-In Amplifier
path, and apply the minimum AC gain needed for the final sensi-
tivity. This configuration will have worse o set and drift behavior
compared with Low Noise, since the DC gain stages are providing
more of the overall total gain. However, larger interfering signals
can be tolerated without overloading the AC circuits, and the PSD
will then tend to greatly suppress that interference.
Between these two settings, a compromise Normal mode is also
available, which provides more DC gain than the Low Noise setting,
but less than the High Reserve setting.
See section 3.2 for more details about the exact gain allocations and
overload limits for these three modes.
The phase-sensitive detector is the primary feature for optimizing
recovery of small signals in the presence of noise. However, the
programmable input filter can be a helpful supplement in optimizing
the SR124's performance. In applications with significant noise or
other interference, the magnitude of the interfering signals can limit
the total amount of AC gain that can be used before the mixer. The
input filter is available to suppress those interfering signals, allowing
greater AC gain to be used for better low-level signal recovery.
The input filter is located in the AC signal path, between the pream-
plifier and the programmable AC gain stage. The input filter's func-
tion can be selected as Low Pass, High Pass, Band Pass, or Notch
filter. The filter can also be bypassed by selecting Flat. The filter
is realized as a two-pole state variable circuit, allowing fine control
of the filter tuning parameters. The input filter is typically used to
either selectively pass a frequency range that spans the input signal,
or to selectively reject one or more interfering signals at frequen-
cies removed from the signal; these two approaches are sometimes
indistinguishable.
The filter also has a user-configurable "Q" setting, which controls the
relative width of the filter's frequency response. In band pass and
notch settings, higher Q settings provide a narrower filter response,
allowing more selective frequency selection; lower Q settings wider
filters, with broader frequency selection. Note that for the low pass
and high pass settings, the filter gain is calibrated for unity gain at
the peak response—the response across the pass band far from the
peak response attenuates the signal by a factor of 1 Q. See section 3.3
for the detailed filter transfer functions of the SR124 input filter.
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