are Balanced Devices?
and Common Modes Model
Mixed Mode (Balanced) S-Parameters
the PNA makes Balanced Measurements
out the True Mode Stimulus Application
being offered at www.agilent.com/find/balanced.
Standard Single-ended devices generally have one input port
and one output port. Signals on the input and output ports are referenced
Balanced devices have two pins
on either the input, the output, or both. The signal of interest is the
difference and average of the two input or output lines, not referenced
On balanced devices, the signal of interest is the difference
and average of the two input or
output lines. In balanced device terminology, these signals are known
as the Differential and Common modes.
The following model shows how two signals (A and B) combine to create
Differential and Common mode signals:
A is fixed at 1V peak
B is selectable
is calculated as A
is calculated as the AVERAGE of
A and B
Click Signal B selections
to see various Differential and Common
Signal A =1V
(A - B)
(A + B) / 2
Signal B = SELECTABLE
- 0 = 1
+ 0)/2 = .5
180° Out of Phase.1V
- (-1) = 2
+ (-1))/2 = 0
180° Out of Phase 2V
- (-2) = 3
+ (-2))/2 = -.5
In Phase 1V
- 1 = 0
+ 1)/2 =
In Phase 2V
- 2 = -1
+ 2)/2 = 1.5
Even when Signal B is 0V, like a Single -ended
signal, there is still a unique Differential and Common mode representation
of the two individual signals.
The above model does not show a DUT. The difference
and average of two signals can be calculated for both the balanced INPUT
and balanced OUTPUT of a device.
Mixed mode S-parameters combine traditional S-parameter notation with
balanced measurement terminology.
Some balanced devices are designed to amplify the differential component
and reject the common component. This allows noise that is common to both
inputs to be virtually eliminated from the output. For example, a balanced
device may amplify the differential signal by a factor of 5, and attenuate
the common signal by a factor of 5. Using traditional S-parameter notation,
an S21 is a ratio measurement of the device Output
/ device Input. Mixing this with
balanced terminology, we could view the amplifier's Differential Output
signal / Differential Input signal. To
see this parameter on the PNA, we would select an Sdd21 measurement using
the following balanced notation:
device output mode
- device input mode
from the following for both a and b:)
- single ended
- device output "logical" port number
- device input "logical" port number
mapping with External Test Sets
Imbalance is a measure of how well two physical
ports that make up a balanced port are matched. With a perfectly balanced
port, the same amount of energy flows to both ports and the magnitude
of the ratio of these ports is 1.
The notation is similar to traditional S-parameters.
In the following diagrams, the letters a, b, c, and d are used because
any PNA port can be assigned to any logical port using the port
For example, in the following single-ended
- balanced formula, Sba indicates
the device output port is logical port b and the input port is logical
Imbalance parameter when measuring a single-ended
- balanced device.
Imbalance1 and Imbalance2 parameters when measuring a balanced
- balanced device.
Imbalance1 and Imbalance2 parameters when measuring a single-ended
- single-ended - balanced device.
CMRR is a ratio of the transmission characteristic
in differential mode over the transmission characteristic in the common
mode of the balanced port as the measurement parameter. A high value indicates
more rejection of common mode, which is desirable in a device that transmits
information in the differential portion of the signal. The table below
shows the CMRR parameter you can select when measuring each balanced device.
Single-ended - balanced device
Balanced - balanced device
Single-ended - single-ended - balanced device
As we have seen on balanced inputs and outputs, the signal of interest
is the difference or average of two BALANCED input or BALANCED output
lines. It is also possible to have single-ended ports AND balanced ports
on the same device. The two balanced input or output lines are referred
to as a single "logical" port.
When configuring a balanced measurement on the PNA, select a device
'topology'. Then map each PNA test port to the DUT ports. The PNA assigns
"logical ports". See
how to set device topology in the PNA.
The following device topologies can be measured by a 4-port PNA.
Balanced / Balanced
(2 logical ports - <4 actual ports>)
(2 logical ports - <3 actual ports>)
- Single-ended / Balanced
(3 logical ports - <4 actual ports>)
These topologies can be used in the reverse (<==)
direction to measure:
Balanced / Single-ended
Balanced / Single-ended
- Single-ended topology
For example, to measure a Balanced
/ Single-ended topology, measure the S12 (reverse direction) of
a Single-ended / Balanced topology.
The PNA does not provide true balanced measurements by stimulating both
balanced inputs together and measuring both outputs relative to one another.
Instead, the PNA makes only Single-ended measurements. On a Balanced/
Balanced device, it stimulates each input and measures each output individually.
From the output data, the PNA calculates the Differential and Common outputs
from the DUT using the same math formulas as the above model. However,
all measurements and calculations on the PNA are performed in frequency
domain using complex (magnitude and phase) data. The Balanced S-parameter
display data is then calculated from the Differential and Common inputs