Rohde & Schwarz R&S UPP, User Manual

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Instrument Functions
R&S
®
UPP
452 User Manual 1411.1055.32 ─ 10
"Freq & Grp
Del" Only available for two-channel measuring mode or in a multichannel
analyzer.
Frequency measurement is performed on the reference channel; the
measurement windows of the other channels contain the group delay
of the respective channel referenced to the reference channel.
Note: This measuring mode should only be selected if the frequency
on the reference channel is identical with the frequency on at least
one other measurement channel. The group delay measurement
should only be used in the case of frequency changes (e.g. with fre-
quency sweeps); a group delay is not defined in the case of constant
frequency.
The group delay is measured by differentiation of phase with respect
to frequency, i.e. by evaluating the difference quotient.
 
f
s t
  




2
2
where
mes mes 1 2
  
  
The following points must be taken into consideration when selecting
a frequency sweep for measuring the group delay:
●
The frequency step size and the number of sweep points must be
selected such that no phase shifts of more than 180° occur
between two consecutive sweep points.
●
The spacing should be linear; with logarithmic spacing the fre-
quency increments at the bottom could become too small and the
phase shifts at the top could become too large.
●
In the case of group delay measurement over frequency (e.g. by
means of a generator frequency sweep), the first sweep point is
undefined because a reference point for calculating the delta val-
ues of frequency and phase is only available as of the 2nd fre-
quency value.
Note: For signals with a poor S/N ratio, the quality of the group delay
measurement can be considerably improved by increasing the "FFT
size" or by "FFT" averaging (Avg Normal). "To this effect, the mea-
surement function is to be set to FFT."
Frequency, Phase and Group Delay Measurement