Concepts and features
R&S
®
ZNB/ZNBT
141
User Manual 1173.9163.02 ─ 62
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 139). 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 can only be measured if more than 2 test
ports are available on the VNA and connected
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
