anritus - vna - superposition vs true balanced [11410-00659a]

8/13/2019 Anritus - VNA - Superposition vs True Balanced [11410-00659A]

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Superposition vs. True Balanced:Whats Required for Your SignalIntegrity ApplicationBy Jon Martens and Bob Buxton

Introduction

Signal Integrity applications commonly utilize balanced/differential transmissionlines which are typically characterized using vector network analyzers (VNA).There are two approaches to performing these measurements and the selectionof the best method depends on what you need to measure.

The Superposition technique relies on the inherent linear nature of a transmissionline and mathematically derives the differential and common-mode transmissionline characteristics through superposition while stimulating just one side of thebalanced transmission line at a time. The True Balanced/Differential technique

uses two sources to create actual differential and common-mode stimuli, hencethe shortened name true balanced. This white paper offers guidance to signalintegrity designers on the differences between these approaches and which onemay best fit their need.

Measurement Theory

The most complete description for balanced devices are the mixed modeS-parameters (Figure 1). Mixed mode S-parameters are comprised of four blocks offour parameters: differential parameters, common to differential mode parameters,differential to common mode parameters and common mode parameters. These 16

parameters are one way of providing a complete description of a 4-port device.The balanced transmission line does not need to have a ground plane or commonterminal. When no ground is physically present, a virtual ground is assumed.


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Figure 1.Mixed mode parameters include important mode conversion parameters for determining both the differential and

common mode characteristics.

bdm2

adm2

bcm2

acm2

adm1

bdm1

acm1

bcm1

PhysicalPort 1

PhysicalPort 2

Differtential-ModePorts

Common-ModePorts

Mixed-Mode

2-Port

bd1

bd2

bc1

bc2

Sd1d1

Sd2d1

Sc1d1

Sc2d1

Sd1d2

Sd2d2

Sc1d2

Sc2d2

Sd1c1

Sd2c1

Sc1c1

Sc2c1

Sd1c2

Sd2c2

Sc1c2

Sc2c2

ad1

ad2

ac1

ac2

=

(a)Mixed mode two port conceptual diagram. (b) Mixed mode s-parameter matrix.

Measurement Technique Overview

While both the superposition and true balanced techniques use VNAs, the difference betweenthem lies with the configured options, calibration stability and complexity of the instrument.

For the superposition technique, the mixed mode parameters are mathematically derived fromthe standard four port S-parameters of the device. This is achieved by measurement of thefour port s-parameters as a group of single ended parameters and the use of superpositionand relevant impedance transformations to calculate the end result. Superposition requiresthat the network is linear and time invariant. Due to this principle, the parameters resultingfrom the first port being stimulated can be combined with the result from the second port (ofthe differential/common-mode pair) being stimulated to result in the equivalent response froma true balanced or a true common-mode stimulus signal. The execution advantage is that onlya single source is required.


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Port 1 Port 2 Port 3 Port 4

The true balanced technique directly measures the differential and common-mode parameters.It requires the use of two sources that are phase coherent and uses active phase and amplitudecontrol of each of the two input sources. An example is shown in Figure 3. In addition, theVNA may have special features such as multipliers, amplifiers and ALC circuitry for precisely

controlling each source output. Controlling multiple sources with very precise magnitude andphase alignment also requires a more extensive calibration and can potentially limit the stabilityof the calibration over time. Further complicating this, the exact phase and amplitude relationshipof the signals delivered to the DUT can be a strong function of the DUT reflection characteristicsso the phase and amplitudes must be continuously adjusted resulting in further degradation instability and measurement time. Due to these issues, this technique is usually only employedwhen required by the device under test.

Figure 2.The superposition technique uses a standard

single source two port VNA with a switch matrix to measure

single ended s-parameters for deriving differential and

mixed mode parameters.

Port 1 Port 2 Port 3 Port 4

Time-coherent in

phase and amplitude

Figure 3.The true balanced technique uses a dual source

VNA with the sources synchronized with a 180 phase dif-

ference to directly measure the common, differential and

mixed mode parameters.


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What Do You Need to Measure?

Most signal integrity measurements are of passive components such as transmission lines, PCBs,cables and connectors. For passive devices, the superposition technique is widely accepted togenerate the required differential and common-mode responses from a balanced device. Evenactive devices, if kept in their linear region, can be accurately tested using the superpositiontechnique. For active devices that are driven into compression or saturation, the true balancedtechnique is required. Table 1provides a list of components and applicable measurement

techniques. Where both techniques are highlighted they will give identical results.

Device to be measured:Superposition

TechniqueTrue Balanced

Technique

Passive Balanced / Differential DUT

Transmission Lines X X

PCB X X

Lumped Components X X

Passive Filters X X

Unshielded and Shielded Twisted Pair, Quad Cables X X

Connectors / Interfaces X X

Linear Active Balanced / Differential DUT

Linear Amplifiers, Differential Amplifiers X X

Linear Active Filters X X

Input / Output Match ADC / DAC X X

Non Linear Active Balanced / Differential DUT

Devices in Compression / Saturation X

Log Amplifiers X

Table 1.Superposition offers identical results in most signal integrity applications.


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Understanding the Tradeoffs

Table 2highlights the tradeoffs between these two measurement solutions. Most of the tradeoffsare due to the added complexity of the true balanced measurement technique. The true balancedtechnique requires a VNA with several additional options which add to the overall cost of the testsolution. In addition, the modified system calibration also adds a complexity to the measurementprocess. The phase of the two sources is also susceptible to drift over time and so special andtime consuming measurement corrections are made prior to each measurement. Figure 4

highlights the challenges associated with source drift between the two techniques.

Superposition True Balanced

Type of VNA Single source VNA Dual source VNA

Method of obtaining Differential

and Mixed Mode ParametersCalculated Measured directly

Type of DUTs Passive and active linear Passive, active linear and

non-linear

Available Frequency Range 70 kHz to 110 GHz 10 MHz -67 GHz

Calibration Complexity Typical 4-portTypical 4-port plus calibration

of dual sources

Average Calibration TimeT (Time depends on number of

points, IF BW, skill of operator)Approx. 2T

Calibration stability

considerations

Normal measurement

calibration intervals

Calibrate more frequently due

to stability issues

Overall Solution Cost $ $$

Table 2.Comparison of Measurement Techniques.

-60

-55

-50

-45

-40

-35

-30

-25

-20

17 18 19 20 21 22 23 24 25 26

|Sd2d1|(dB)

Frequency (GHz)

drift: SE vs. TB on coupled lines

true balanced

single-ended

start value

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

12.3 2.5 2.7 2.9 3.1 3.3 3.5

anritus - vna - superposition vs true balanced [11410-00659a]

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