Stanford Research Systems SR785, Operating Manual And Programming Reference

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1-54 User Math Functions
SR785 Dynamic Signal Analyzer
Press <Function String> This key moves the marker to the function string at
the top of the screen. This allows you to delete
terms and insert new ones.
Use the knob to move the insertion point to the
beginning of the function (highlight FFT2). Move to the start of the string by highlighting the
first term (FFT2).
Press <Operations> ‘Ins’ above the function string indicates that new
terms will be inserted before the highlighted term.
‘Rep’ indicates that the new term replaces the
highlighted one. Use <Insert/Replace> to toggle
between insert and replace.
Use the knob to highlight GrpDly and press
[Enter]. Insert the Group Delay operator in front of the
transfer function.
Closing parentheses are not required (if they are at
the very end of the string).
Press <Enter Eq.> Enter the equation.
7. Press [Display Setup]
Select the Display Setup menu.
Press <Measurement> Change the measurement of the active display
(DisplayA).
Use the knob to select (FFTUsrFn2) and press
[Enter]. Choose FFT User Function2.
Press <View> Change the view. Group delay is the derivative of
the phase with respect to
ω
and is a real time.
Use the knob to select (Real Part) and press
[Enter]. Choose Real Part. The units automatically switch to
linear units.
User Functions are dimensionless quantities. You
can choose dB (logarithmic) or units (linear).
Press [Auto Scale A] Scale the display.
The group delay is the delay time caused by the
filter at different frequencies. The data values are
seconds of delay. Most points are in the
neighborhood of 10 to 600
µ
s.
At the 1 kHz notch, the group delay has a
singularity. Remember, the notch filter has a phase
discontinuity at the notch frequency.