Balanced Measurements


Other Measurement Setup Topics

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What are Balanced Devices?

Standard Single-ended devices generally have one input port and one output port. Signals on the input and output ports are referenced to ground.

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 to ground.

Differential and Common Modes Model

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:

Note: Click Signal B selections to see various Differential and Common signals.

Signal A =1V

 

 

Differential
(A - B)

 

 

Common (Avg)
(A + B) / 2

 

 

Signal B =  SELECTABLE

Calculations

Single-ended0V

1 - 0 = 1

(1 + 0)/2 = .5

180 Out of Phase.1V

1 - (-1) = 2

(1 + (-1))/2 = 0

180 Out of Phase 2V

1 - (-2) = 3

(1 + (-2))/2 = -.5

In Phase      1V

1 - 1 = 0

(1 + 1)/2 = 1

In Phase      2V

1 - 2 = -1

(1 + 2)/2 = 1.5

Notes:

Measuring Mixed Mode (Balanced) S-Parameters

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:

Sabxy -

Where

a - device output mode

b - device input mode

(choose from the following for both a and b:)

x - device output "logical" port number

y - device input "logical" port number

See Also

Logical port mapping

Port mapping with External Test Sets

Measuring Imbalance Parameters

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 mapping process.  

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 port a.

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.

Measuring CMRR (Common Mode Rejection Ratio)

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

Device Topology and Port Mapping

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.

These topologies can be used in the reverse (<==) direction to measure:

For example, to measure a Balanced / Single-ended topology, measure the S12 (reverse direction) of a Single-ended / Balanced topology.

How the PNA makes Balanced Measurements

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 and outputs.