Concepts and features
R&S
®
ZNA
138
User Manual 1178.6462.02 ─ 20
3. DUT with one balanced and one single-ended port.
4. DUT with two balanced ports or one balanced and two single-ended ports. Both
device types are fully characterized by 4x4 mixed mode S-matrices.
4.3.6.3
Imbalance and common mode rejection
An ideal unbalance-balance transformer (balun) converts an unbalanced signal into a
balanced one and vice versa. When it is driven with an unbalanced signal at its physi-
cal port 1 (= single-ended logical port 1), unbalanced signals with equal amplitude and
opposite phase appear at physical ports 2 and 4 (forming balanced logical port 2). This
means that the ratio –S
21
/S
41
of the physical transmission coefficients of an ideal balun
equals 1. This ratio is called
imbalance
; it is a measure for the deviation of the balun
from ideality. The general definition of the transmission imbalance between two differ-
ent ports (at least one of them balanced) is given below.
For a DUT with two balanced ports (e.g. an amplifier), the ratio between the (wanted)
differential mode power gain and the (unwanted) common mode power gain is called
common-mode rejection ratio
(CMRR). It can be calculated as |S
dd21
|/|S
cc21
| (see
Chapter 4.3.6.2, "Mixed-mode parameters"
on page 136). The general definition of the
complex CMRR between two ports (at least one of them balanced) is given below.
Imbalance and common-mode rejection ratio measurements can only be measured on
a 4-port R&S
ZNA.
General Definition
In general, imbalance and CMRR are quantities with two numeric indices, indicating
the logical output port and the logical input port of the DUT during the measurement
(Imb
<out><in>
, CMRR
<out><in>
).
●
DUT with single-ended logical port i and balanced logical port j:
Measurement results
