Differential vs. Single-Ended Insertion Loss

Key Takeaways

• Differential and single-ended insertion loss are used to describe the signal loss in a device, depending on the type of signaling used. 

• Insertion loss in a single-ended or unbalanced network is called single-ended insertion loss. 

• Insertion loss in a differential interconnect or balanced network is called differential insertion loss. 

Signal integrity is of great concern in high-speed digital communication systems

As high-speed digital communication systems become more prominent, the signal integrity in high-speed interconnects becomes a concern. The signal integrity in such systems can be identified using S-parameters. While using single-ended devices, single-ended S-parameters are used to quantify reflections and transmissions. If a device is using differential signaling, differential S-parameters determine the attenuation and losses. Differential and single-ended insertion loss are used to describe the signal loss in a device, depending on the type of signaling used. In this article, we will explore differential vs. single-ended insertion loss a little further. 

Differential vs. Single-Ended Insertion Loss

Signal integrity is a key factor guiding signal transmission. The performance of the signal transmission paths or interconnects is critical in maintaining signal integrity. In high-speed digital communications, single-ended and differential signaling are used. In such systems, interconnects are designed so that they offer nearly the same performance for both differential and single-ended signals. 

Frequency-Dependent Parameters

The signal integrity of high-speed communication systems is frequency-dependent. The frequency-dependent parameters, called scattering parameters or S-parameters, are perfectly suitable to define both single-ended and differential signaling systems. Here are two key definitions to know:

Single-ended insertion loss—the insertion loss in a single-ended or unbalanced network. The single-ended insertion loss is obtained from a single-ended S-parameter.

Differential insertion loss—the insertion loss in a differential interconnect or balanced network. Differential insertion loss is obtained from the mixed-mode S-parameter.  

A Single-Ended S-Parameter 

A device that handles a single-ended signaling scheme is most preferably represented as a single-ended S-parameter matrix. A single-ended signal is referenced between a single transmission line and the return path. The devices using single-ended signals have one pin in both the input ports and output ports. Single-ended S-parameter insertion loss characterizes the frequency dependence of signal loss in the device under test, which is single-ended and unbalanced. 

The single-ended S-parameter is not capable of characterizing differential signals or differential or balanced devices accurately. To describe signal transmission and reflections in balanced or differential systems, mixed-mode S-parameters are defined. Usually, the single-ended S-parameter data is mathematically converted to differential S-parameters and the matrix is called mixed-mode S-parameter.

A Mixed-Mode S-Parameter 

Differential signals require two identical conductors, which carry two complementary signals switching simultaneously. The devices that use differential signaling are called differential or balanced devices and they have two pins in the input and output ports. The input and output ports are also called stimulus and response ports when describing differential and common-mode responses. The differential signal is not referenced to the ground plane. The mixed-mode S-parameter gives both the common-mode as well as differential S-parameter data of the balanced systems. 

Calculating Differential Insertion Loss From Single-Ended S-Parameters

The Single-Ended S-Parameter Technique

The single-ended S-parameter technique can be used to measure differential and common-mode responses of the differential devices. The differential two-port device is modeled as a four-port device, carrying four simultaneous signals and their associated ground signals. The measurement of differential S-parameter insertion loss from single-ended measurement is difficult. If the differential device input ports are numbered as 1 and 3, and output ports as 2 and 4 for single-ended measurements, then the differential insertion loss can be calculated as:

The differential vs. single-ended insertion loss of high-speed interconnects gives the performance of it under the differential and single-ended signals. Luckily, Cadence’s software offers a suite of design and analysis tools for measuring insertion loss. 

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