Lambda SR1, Operation Manual, Page: 152 / 317

SR1 Operation Manual 152
© 2014 Stanford Research Systems
Sam e Spectrum w ith Phase Unw rapping On
The FFT2 analyzer uses the coherence as a threshold value for calculating phase. Coherence is a
measure of the phase stability between the input and output channels, thus regions of high coherence
will yield stable phase measurements and regions of low coherence will likely yield noisy phase values.
Setting a threshold value instructs the FFT2 analyzer to only calculate phase for frequency bins where
the coherence exceeds the threshold value. Setting a small threshold can often clean up the phase
response spectrum considerably.
DC Correction
Small amounts of DC in the FFT time record can be removed using the DC Correction control. Selecting
Average will subtract the average value of each time record from the time record before taking the FFT.
1/2 Pk-Pk will subtract the average of the maximum and minimum values found in each time record.
Effect of DC Correction on Spectrum
An example of the us of DC correction is shown above. The original spectrum has some DC offset which
has "leaked" into adjacent low-frequency bins because of windowing. The red trace shows the same
signal with "Average" DC correction applied.
Spectrum Weighting
The spectral outputs of the FFT2 analyzer can have weighting curves applied to them. Weighting curves
are represented by
EQ files . The standard EQ files supplied with SR1 include: