Concepts and features
R&S
®
ZNA
248User Manual 1178.6462.02 ─ 29
nate unwanted responses using 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 1EP83 which is available on the R&S internet at http://
www.rohde-schwarz.com/appnotes/1EP83.
Trace selection
Select a reflection parameter S
ii
to perform a time domain reflectometry (TDR) mea-
surement, e.g. to detect cable faults. To measure the impedance of a network, you can
select a converted impedance parameter Z←S
ii
.
Select a transmission parameter S
ij
(i≠j) to perform a time domain transmission (TDT)
measurement, e.g. for measuring antennas.
●
For TDR traces and if distance is used as the x axis, the x value indicates the one-
way path to the reflection.
●
To export time domain traces, you have to use formatted values (ASCII, Matlab)
must be used. See Chapter 4.4.2.3, "Finding the best file format", on page 185.
4.7.2.1 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 computation time is increased by approx. a factor of 2. This increased com-
putation 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.
4.7.2.2 Band pass and low pass mode
The analyzer provides two 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 an
undetermined phase depending on the delay of the signal.
Optional extensions and accessories
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