Stanford Research Systems SR865 Operation Manual page 87

Chapter 3 Operation 69
Internal, External and Chop Reference
When the reference source is internal, external or chop, the synchronous filter period is
1/f regardless of the harmonic detect number. The output is averaged over N periods of
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the detect frequency where N is the harmonic detect number. Synchronous filtering is
only active when the reference frequency is 4.8 kHz or below. If the reference frequency
is above 4.8 kHz and synchronous filtering is on, then a Sync error is displayed with
displays of X, Y, R or θ indicating that the synchronous filter is not working as expected.
Dual Reference
When the reference source is dual, the synchronous filter period is a single period of the
actual detect frequency f = | N ×f – N ×f |, where N and N are the internal
dual 1 int 2 ext 1 2
harmonic and dual reference external harmonic detect numbers respectively.
Synchronous filtering is only active when the actual detect frequency is 4.8 kHz or
below. If the detect frequency is above 4.8 kHz and synchronous filtering is on, then a
Sync error is displayed with displays of X, Y, R or θ indicating that the synchronous filter
is not working as expected.
Settling Time
When the synchronous filter is active, the phase sensitive detectors (PSD's) are followed
by the specified low pass filtering (time constant filter) and then the synchronous filter.
This removes non-harmonic noise before the synchronous filter.
The settling time of the synchronous filter is one period of the filter (usually 1/f ). Since
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the synchronous filter follows the phase sensitive detectors, the time constant filters and
output scaling, any change in the signal amplitude, reference frequency, phase, time
constant, slope or sensitivity will cause the outputs to settle for one period of the filter.
These transients are because the synchronous filter provides a steady output only if its
input is repetitive from period to period.
The transient response also depends upon the time constants of the regular filters. Very
short time constants (<< period) have little effect on the transient response. Longer time
constants (< period) can magnify the amplitude of a transient. Much longer time
constants (≥ period) will increase the settling time far beyond a period.
[Sensitivity]
The Sensitivity setting of the SR865 determines what input signal corresponds to 10 V
full scale output for X, Y and R. For example, a Sensitivity of 100 mV means that a
signal at f of 100 mVrms will result in a 10 V output of R. X and Y will reach a
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maximum of 10 V depending upon the phase of the signal.
The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts.
Note that this is a numerical output conversion. Output overloads do not generally 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. Output overloads are indicated by a 'Scale' indicator in the X, Y or R
displays.
SR865 DSP Lock-in Amplifier