Concepts and features
R&S
®
ZNA
250
User Manual 1178.6462.02 ─ 20
change the differential and common mode reference impedances, if they differ from the
default settings. A subsequent source power calibration is recommended.
You can also use
Offset parameters and de-/embedding
to move the reference plane
where the analyzer provides an accurate differential or common mode signal.
4.7.5.2
Amplitude imbalance and phase imbalance sweep
In true differential mode, you can select a single balanced (logical) port L
I
to behave
"imbalanced", either w.r.t. magnitude or w.r.t. phase. During an amplitude or phase
imbalance sweep, the analyzer generates a balanced CW signal at L
I
, but sweeps the
power or phase of one of its constituent physical ports within a configurable range.
Since fimware version 2.40, amplitude and phase impalance sweeps are no longer
limited to true differential mode, but are also available in
Imbalance Compensation of a-Waves
Since the mixed-mode S-parameters of a linear balanced DUT depend only on the
DUT itself, they are independent of the
of the stimulus
signal. For example, S
dd21
of a differential amplifier in phase imbalance sweep does not
reveal the amplitude reduction of the differential output signal caused by unequal
lengths of the balanced input line conductors. This length asymmetry corresponds to a
phase imbalance increasing over frequency. To see the effect of such a phase or
amplitude imbalance, modified S-parameters are required. The modification is done in
such a way that, if for example the imbalance of port 1 is swept, the imbalance of the
a-wave of port 1 is compensated before the S-parameters are calculated.
The effect of the compensation is a constant amplitude of the differential or common
mode stimulus wave of port 1 over the imbalance sweep range. This reflects the situa-
tion of the user applying a stimulus signal of known nominal amplitude to the DUT, but
getting at the output only the amplified differential contents in this signal, which
depends on the imbalance. Usually, S-parameters of balanced devices measured with
active imbalance compensation will exhibit a maximum or minimum at zero imbalance.
The imbalance compensation is not only performed for mixed-mode S-parameters, but
also for the imbalance-swept a wave itself when it is selected as a measured quantity,
as well as for ratios including that wave.
4.7.5.3
True differential mode on frequency-converting DUTs
True differential mode can be combined with
frequency-converting measurement
. For this type of measurements, a consistent calibration is particularly impor-
tant.
Optional extensions and accessories