Measurements, Testing, and Calibration Advanced Measurement Topics Informational

How do I perform a mixed-mode S-parameter measurement of a differential device?

Q: What does mode conversion tell me?

Mode conversion (SCD and SDC) indicates how much of the differential signal is converted to common mode (and vice versa) by the device. In a perfectly symmetric differential device: mode conversion is zero. In practice: any asymmetry (unequal trace lengths, asymmetric component placement, impedance imbalance) creates mode conversion. Mode conversion degrades: CMRR (common-mode rejection ratio), EMI performance (common-mode energy radiates more than differential-mode), and signal integrity (mode conversion causes differential signal loss). Target: SCD, SDC < -25 to -30 dB.

Q: Do I need a 4-port VNA?

For complete mixed-mode characterization: yes. A 4-port VNA measures all 16 single-ended S-parameters simultaneously, enabling accurate computation of all mixed-mode parameters including mode conversion. With a 2-port VNA: you can measure the differential ports separately using baluns, but: the common-mode behavior is hidden, mode conversion is not measurable, and the balun introduces errors. For high-speed digital applications: a 4-port VNA is strongly recommended.

Q: How do I handle the ground reference for differential measurements?

Differential signals do not require a ground reference in theory, but the VNA measurement does. Each VNA port has a ground (shield) connection. For differential device measurement: connect the signal (center conductor) of each VNA port to one terminal of the differential pair, and connect the grounds of all VNA ports together and to the device's ground reference (if any). For PCB differential pair measurement: use differential probes (GSG-SGS or GSSG) that contact both the signal traces and the ground reference simultaneously.

A mixed-mode S-parameter measurement of a differential device characterizes the device's response in terms of its differential-mode and common-mode behavior, rather than the individual single-ended port responses. A differential device has two physical ports that form one differential port (the signal is the difference between the two conductors). The mixed-mode S-parameters describe: differential-to-differential (Sdd): the S-parameters for the differential signal path (the desired signal; SDD21 is the differential gain/insertion loss, SDD11 is the differential input match), common-to-common (Scc): the S-parameters for the common-mode signal path (the undesired signal; SCC21 is the common-mode gain/rejection), differential-to-common (Sdc): mode conversion from differential to common mode (a measure of device asymmetry; ideally zero), and common-to-differential (Scd): mode conversion from common to differential mode (also ideally zero). The measurement is performed by: measuring the full 4-port single-ended S-parameters of the device (a 2-port differential device has 4 single-ended terminals: port 1+, port 1-, port 2+, port 2-; measure the complete 4x4 S-parameter matrix using a 4-port VNA), and applying the mathematical transformation to convert single-ended S-parameters to mixed-mode S-parameters: Smm = M x Sse x M^(-1), where M is the mode transformation matrix that combines pairs of single-ended ports into differential and common-mode port pairs.

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Mixed-Mode S-Parameter Measurement

Mixed-mode S-parameters are essential for characterizing differential components used in high-speed digital interfaces (USB, HDMI, PCIe, Ethernet), balanced RF circuits (differential amplifiers, filters, mixers), and transmission line structures (differential pairs on PCBs).

Measurement Procedure

4-port VNA measurement: Connect all four terminals of the differential device to four calibrated VNA ports. Perform a full 4-port SOLT or SOLR calibration at the DUT reference planes. Measure the complete 4x4 single-ended S-parameter matrix. The VNA software automatically computes the mixed-mode parameters from the single-ended data
2-port VNA with balun: If a 4-port VNA is not available: use a balun (balanced-to-unbalanced transformer) to convert each differential port to a single-ended port. The VNA then measures a 2x2 matrix (differential-mode only). This is simpler but: the balun limits bandwidth and accuracy, common-mode parameters cannot be measured, and mode conversion is not captured
True-mode stimulus: Some modern VNAs (Keysight PNA-X, R&S ZNB) can directly generate differential and common-mode stimuli by driving pairs of ports with appropriate phase relationships (0/180 degrees for differential, 0/0 degrees for common mode). This measures the mixed-mode S-parameters directly without mathematical conversion

Mixed-Mode S-Parameter Definitions

Mode transformation: [Sd; Sc] = M × [S1; S2] where M = [1/sqrt(2), -1/sqrt(2); 1/sqrt(2), 1/sqrt(2)]
SDD11 = differential input return loss
SDD21 = differential insertion loss / gain
SCD21 = common-to-differential mode conversion (ideally < -30 dB)
SDC21 = differential-to-common mode conversion (ideally < -30 dB)

Common Questions

Frequently Asked Questions

What does mode conversion tell me?

Mode conversion (SCD and SDC) indicates how much of the differential signal is converted to common mode (and vice versa) by the device. In a perfectly symmetric differential device: mode conversion is zero. In practice: any asymmetry (unequal trace lengths, asymmetric component placement, impedance imbalance) creates mode conversion. Mode conversion degrades: CMRR (common-mode rejection ratio), EMI performance (common-mode energy radiates more than differential-mode), and signal integrity (mode conversion causes differential signal loss). Target: SCD, SDC < -25 to -30 dB.

Do I need a 4-port VNA?

For complete mixed-mode characterization: yes. A 4-port VNA measures all 16 single-ended S-parameters simultaneously, enabling accurate computation of all mixed-mode parameters including mode conversion. With a 2-port VNA: you can measure the differential ports separately using baluns, but: the common-mode behavior is hidden, mode conversion is not measurable, and the balun introduces errors. For high-speed digital applications: a 4-port VNA is strongly recommended.

How do I handle the ground reference for differential measurements?

Differential signals do not require a ground reference in theory, but the VNA measurement does. Each VNA port has a ground (shield) connection. For differential device measurement: connect the signal (center conductor) of each VNA port to one terminal of the differential pair, and connect the grounds of all VNA ports together and to the device's ground reference (if any). For PCB differential pair measurement: use differential probes (GSG-SGS or GSSG) that contact both the signal traces and the ground reference simultaneously.

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