Concepts and features
R&S
®
ZNA
221
User Manual 1178.6462.02 ─ 20
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
ii
.
Select a transmission parameters 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.
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.
●
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 har-
monic. Due to the symmetry of the trace in the frequency domain, the time domain
result is harmonic.
See also
Chapter 4.7.2.4, "Harmonic grid"
Two different types of response are available in low pass mode; see below.
Optional extensions and accessories
