Concepts and features
R&S
®
ZNA
278User Manual 1178.6462.02 ─ 29
Normal (unbalanced)
mode
Measurement of wave quantities and S-parameters for unbalanced ports. Unbal-
anced stimulus signals.
Virtual differential
mode
Precondition: At least one balanced port is defined.
Measurement of unbalanced wave quantities using unbalanced stimulus signals.
Unbalanced system error correction. Conversion of unbalanced wave quantities
into balanced and mixed-mode S-parameters with possible renormalization of port
impedances.
True Differential
Mode
Precondition: At least one balanced port is defined, whose constituent ports are
driven by two independent sources.
Measurement of unbalanced wave quantities using balanced stimulus signals (dif-
ferential and common mode). System error correction and conversion of unbal-
anced into balanced wave quantities. Calculation of balanced and mixed-mode S-
parameters with possible renormalization of port impedances.
True differential mode relies on wave correction. A consistent system error correction is
essential for accurate balanced waves at the reference planes and accurate measure-
ment results. Perform a full n-port calibration (TOSM, UOSM or one of the Txx calibra-
tion types) of all physical ports involved in the true differential measurement and
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 source coherence mode.
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 amplitude or phase imbalance 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
Optional extensions and accessories
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