Stanford Research Systems SR865A Operation Manual page 63

Chapter 2 Basics 45
Advanced Filters
The SR865A also provides advanced filtering in place of the traditional 6dB/octave RC
filters. These filters can provide digital filtering that has no analog filter counterpart, and
can settle up to twice as quickly as corresponding RC filters. If the input signal is
changing rapidly, or if there is significant noise away from the reference frequency, then
the advanced filters often provide better signal to noise with faster output response. See
Appendix A for more information.
Floating Point Math in the SR865A
The output points of the digital PSD in the SR865A are converted into floating point
numbers. These numbers reflect the actual analog signal gain preceding the analog to
digital converter and are simply the signal input voltages at the input BNC's. All digital
filtering except for the Sync filter (described below) is performed using floating point
math.
The SR865A plots these floating point outputs in the strip chart displays in units of Volts
(or Amps) referred to the signal inputs. These values have no real limit in size (either too
small or too large) and do not overload.
Sensitivity
So how does the SR865A provide an analog output proportional to the signal when the
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result is a floating point value that can range between 10 to 10 , while the analog
output can only range between +10V and −10V?
The answer is that the user must set a Sensitivity which sets the output voltage
corresponding to full scale (10 V) at the output BNC. The Sensitivity also sets the scale
for the displayed bar graphs and numerical readouts of X, Y and R. Note that this is a
numerical output conversion. Output overloads do not affect the actual measurement
results. They only indicate that the output value exceeds 100% of the chosen Sensitivity
and the output BNC, the bar graph and the displayed numerical readout will be pinned at
their maximums. The results displayed on the strip charts or available over the computer
interfaces are the floating point outputs and are unaffected by output overloads.
The Sensitivity is chosen to conveniently and accurately display the measurement results
on the output BNC, the bar graph and the numerical readout. The Sensitivity, however,
must be chosen appropriately when using synchronous filters (see below).
Synchronous Filters
Even if the input signal has no noise, the PSD output always contains a component at 2f
(sum frequency of signal and reference) whose amplitude equals or exceeds the desired
dc output depending upon the phase. At low frequencies, the time constant required to
attenuate the 2f component can be quite long. For example, at 1 Hz, the 2f output is at 2
Hz and to attenuate the 2 Hz by 60 dB in two stages requires a time constant of 3
seconds.
A synchronous filter, on the other hand, operates totally differently. The PSD output is
averaged over a complete cycle of the reference frequency. The result is that all
components at multiples of the reference (2f included) are notched out completely. In the
case of a clean signal, almost no additional filtering would be required. This is
increasingly useful the lower the reference frequency. Imagine what time constant would
be needed at 0.001 Hz!
SR865A DSP Lock-in Amplifier