Concepts and Features
R&S
®
ZNC
98
User Manual 1173.9557.02 ─ 13
3.7.1 Time Domain (R&S ZNC-K2)
The network analyzer measures and displays complex S-parameters and other quantities
as a function of the frequency. The measurement results can be filtered and mathemat-
ically transformed in order to obtain the time domain representation, which often gives a
clearer insight into the characteristics of the DUT.
Time domain transforms can be calculated in band pass or low pass mode. For the latter
the analyzer offers the impulse and step response as two alternative transformation
types. A wide selection of windows can be used to optimize the time domain response
and suppress sidelobes due to the finite sweep range. Moreover, it is possible to eliminate
unwanted responses by means of a time gate and transform the gated result back into
the frequency domain.
For a detailed discussion of the time domain transformation including many examples
refer to the application note 1EZ44_OE which is posted on the R&S internet.
3.7.1.1
Chirp z-Transformation
The Chirp z-transformation that the analyzer uses to compute the time domain response
is an extension of the (inverse) Fast Fourier Transform (FFT). Compared to the FFT, the
number of sweep points is arbitrary (not necessarily an integer power of 2), but the com-
putation time is increased by approx. a factor of 2. This increased computation time is
usually negligible compared to the sweep times of the analyzer.
The following properties of the Chirp z-transformation are relevant for the analyzer set-
tings:
●
The frequency points must be equidistant.
●
The time domain response is repeated after a time interval which is equal to Δt = 1/
Δf, where Δf is the spacing between two consecutive sweep points in the frequency
domain. For a sweep span of 4 GHz and 201 equidistant sweep points, Δf = 4 GHz/
200 = 2 * 10
7
Hz, so that Δt = 50 ns. Δt is termed measurement range (in time domain)
or unambiguous range.
Additional constraints apply if the selected Chirp z-transformation is a lowpass transfor-
mation.
3.7.1.2
Band Pass and Low Pass Mode
The analyzer provides two essentially different types of time domain transforms:
●
Band pass mode : The time domain transform is based on the measurement results
obtained in the sweep range between any set of positive start and stop values. The
sweep points must be equidistant. No assumption is made about the measurement
point at zero frequency (DC value). The time domain result is complex with a generally
undetermined phase depending on the delay of the signal.
●
Low pass mode : The measurement results are continued towards f = 0 (DC value)
and mirrored at the frequency origin so that the effective sweep range (and thus the
response resolution) is more than doubled. Together with the DC value, the condition
of equidistant sweep points implies that the frequency grid must be harmonic. Due
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