Introduction to SER-DES Evaluation Methods and SONY GVIF Measurement Service

This column will explain the evaluation of communication channels and the measurements used to evaluate communication quality in SER-DES communication technology, in order to ensure communication quality.

Basic SER-DES technology and explanations are provided below. Please refer to them as well.

Methods for evaluating communication quality and communication paths

The evaluation of high-speed communication technologies such as SER-DES depends on the requirements of the IC manufacturer designing SER-DES, but there are generally three main evaluation methods.

TDR (Time Domain Reflectometry) Evaluation

TDR evaluation is a method for evaluating the discontinuities and locations of impedance by inputting a fast-rising electrical signal into a transmission line (wiring, CABLES, CONNECTORS, etc.) and observing the reflected wave over time.
It is primarily used to check wiring quality, mounting status, and whether the impedance is as designed.

In high-speed communications like SER-DES, even slight impedance disturbances can cause signal reflection or attenuation, significantly impacting communication quality. Therefore, TDR evaluation is often a fundamental and crucial evaluation criterion.

TDR evaluation is measured using a vector network analyzer (VNA), which allows for a visual assessment of the length of the communication signal (transmission) path and where impedance disturbances exist along that path.

S (Scattering) parameter evaluation

S-parameter evaluation is a method for evaluating the transmission and reflection characteristics of high-speed signals in the frequency domain as they pass through a communication transmission path or a particular device.
In high-speed communications such as SER-DES, it is used to quantitatively understand signal behavior such as attenuation and reflection using frequency characteristics.
S-parameter evaluation, like TDR evaluation, is measured using a VNA, and a major feature is that it can analyze the behavior (attenuation and reflection) of high-speed signals ranging from low frequencies to GHz in the frequency axis.

Eye evaluation

Eye evaluation is a technique that involves probing waveform signals under actual communication conditions and superimposing the resulting waveforms to visually evaluate the signal waveform quality in the time domain.
Because the superimposed signal waveforms resemble the shape of an "eye," this is called eye evaluation or eye diagram.

In high-speed communications like SER-DES, signal attenuation, jitter due to reflection, and noise directly lead to communication errors. Therefore, this evaluation method is widely used to intuitively confirm whether the actual signal waveform matches the expected communication waveform.

For eye evaluation, measurements are taken using a high-speed oscilloscope capable of handling high-speed signals and an active probe with low parasitic capacitance.

In the following chapters, we will explain each evaluation item and its methodology, along with evaluation examples and results.

TDR rating

As mentioned above, TDR evaluation visually assesses the length of the communication signal (transmission) path and where impedance disturbances exist along that path.
The principle is that a step-like (or pulse-like) signal is applied to the transmission line, and the change in impedance can be determined from the amplitude of the reflected wave, while the position (distance) can be determined from the time it takes for the reflected wave to return.

This allows us to visualize "where" and "to what extent" the impedance is disturbed, which is effective for validating the design of the circuit board and for isolating defects in fault analysis.

What can be learned from the evaluation?

Characteristic impedance

Deviation from the design value (e.g., 50Ω for coaxial, 100Ω for differential)

Location of the impedance discontinuity

Connection points such as CONNECTORS, vias, layer transitions, component lands, and extended wiring are prone to becoming disorganized, and the disorganized areas are particularly prone to this.

Termination state

Open, short, excessive or insufficient termination resistance.

Understanding the length of the wiring

The length of the signal path is calculated from the propagation time.

Measurement procedure overview and example measurement results

1. First, after powering up the device, perform calibration. Calibration is performed by making connections such as open, short, load, and through.
The E-Cal kit, which completes calibration with a single button, is very convenient.

2. On the VNA screen, configure various settings such as setting the TDR mode and aligning the center of the graph.

3. By connecting the VNA's SMA CONNECTORS to the device under test (DUT), a graph will be displayed with time on the horizontal axis and impedance on the vertical axis.

The graph below shows an example of measurement results under the following conditions.

Differential (100Ω center) signal
Use an SMA to differential CONNECTORS conversion board.
The communication IC is not installed, and the entire communication line path from the differential CONNECTORS is shorted with a 0Ω resistor.

The line shown in light yellow (①) is a graph that measures only the SMA to differential CONNECTORS conversion board.
This graph will serve as the basis.
Beyond the differential signal CONNECTORS, the graph spikes, indicating that the impedance becomes infinite (∞), resulting in an open (unconnected) state.
The line shown in dark yellow (②) is a graph of measurements taken when the test unit (DUT) was connected to an SMA to differential CONNECTORS conversion board.
The area to the right of the light yellow line (the graph showing measurements only for the SMA to differential CONNECTORS conversion board) is the part we want to measure.
Since the SER-DES IC is not actually implemented, we can see that the impedance jumps to infinity (∞) beyond the IC.

This graph, shown in dark yellow, reveals that the impedance remains around 100Ω from CONNECTORS end of the circuit board to just before the IC end, but drops to around 85Ω just before the IC. (Red box)

S-parameter evaluation

First, regarding what S-parameters (hereinafter referred to as S-para) are, please refer to the explanation page provided by Rohde & Schwarz, the manufacturer of VNA equipment.

https://www.rohde-schwarz.com/jp/products/test-and-measurement/essentials-test-equipment/spectrum-analyzers/understanding-s-parameters_257831.html

S-parameter evaluation is a method for evaluating the transmission and reflection characteristics of high-frequency signals in the frequency domain as they pass through transmission lines and devices.
For the communication path being measured, S11 (reflection characteristics) and S21 (attenuation characteristics) can be visually perceived in terms of frequency.

Measurement procedure overview and example measurement results

1. In S-parameter measurements, it is necessary to prepare a test coupon, which is a circuit board with only the communication path cut out, in order to measure the characteristics of the communication path on the board.
Additionally, the VNA uses CONNECTORS CONNECTORS Therefore, CONNECTORS conversion board (also called a fixture board) is required to connect it to boards actually used on the SER-DES board, such as coax CONNECTORS or differential signal CONNECTORS.
Once these circuit boards are prepared, connect them as follows for measurement. (This diagram assumes a differential connection.)

Measuring instrument (VNA) ---CONNECTORS conversion jig (SMA to measurement board CONNECTORS) --- Measurement target board (DUT)
---CONNECTORS conversion jig (measurement board CONNECTORS to SMA)--- Measuring instrument (VNA)

When using the above measurement method, the characteristics of not only the substrate being measured but also those of the two CONNECTORS conversion jigs are taken into account.
Therefore, de-embedded is necessary.

What is "de-embedded"?

This refers to removing elements other than the actual measurement unit (DUT), such as CONNECTORS conversion boards (fixture boards) and SMA CONNECTORS for VNA connection (parasitic elements).
By removing elements other than the test substrate (DUT) that you want to measure, you can accurately evaluate the performance of the test substrate itself.

2. Similar to TDR evaluation, after starting up the VNA device, perform calibration and make various settings, then measure the target substrate to obtain a frequency response graph.

The following is an example graph of the measurement results.

In the S11 evaluation, the reflection characteristics can be read in the frequency axis.
Since it's desirable to minimize reflection, the closer the reflection characteristic graph value is to -∞dB, the lower the reflection.
In the S21 evaluation, the attenuation characteristics can be read in the frequency axis.
Since attenuation should be as small as possible, the closer it is to 0 dB, the less attenuation there is, and the better the attenuation characteristics.
In this evaluation, measurements were performed using a Keysight network analyzer and its AFR (Auto Fixture Removal) function. It was confirmed that de-embedding redundant parasitic components, including the conversion board, slightly improved the performance.

Eye evaluation

Eye evaluation is a technique that involves probing waveform signals under actual communication conditions and superimposing the resulting waveforms to visually evaluate the signal waveform quality in the time domain.

While the two evaluations mentioned above primarily focus on evaluating hardware characteristics and design values, such as circuit boards and communication paths, the eye evaluation involves probing and observing signal waveforms under actual communication conditions. Its advantage lies in its ability to evaluate under more realistic conditions.
On the other hand, since the probe and measuring device have a certain degree of influence, some ingenuity and skill are required to prevent the probing from becoming parasitic capacitance. Furthermore, with faster signals, the influence of the measuring device becomes significant, making it difficult to observe accurate waveforms, so attention must be paid to the specifications of the measuring instrument and probe, as well as the speed of the signal to be observed.

Measurement procedure and example of measurement results

1. Start up the high-speed oscilloscope, connect the corresponding probe, and perform calibration.

2. Place the probe tip on the probing point of the DUT (Device Under Test), set up real-time eye observation on the oscilloscope, and check the waveform.

The procedure is very simple, but settings on the high-speed oscilloscope and where to place the probing point on the communication line often vary depending on the IC manufacturer. In some cases, a configuration file is provided as part of the evaluation module, so it is necessary to check with the IC manufacturer to find out what conditions and settings should be used for evaluation.

Regarding probing, there are various types, such as needle-type and solder-type probes. Needle-type probes must be installed in a way that prevents contact chatter, and solder-type probes must be attached with minimal solder to avoid parasitic capacitance, requiring skill and experience.

The graph below shows an example of measurement results under the following conditions.

The signal speed is 4.8Gbps.

The image above shows an eye diagram measured with a 2m CABLES. Naturally, extending CABLES increases signal attenuation, causing the eye to close. Below is the eye diagram when CABLES is 10m long. You can see that the eye is more closed compared to the 2m cable.

Case Introduction evaluation service for SONY SER-DES GVIF.

While various SER-DES products are being released by different manufacturers, Sony has also released a SER-DES GVIF (Gigabit Video Interface) for automotive use.

https://www.sony-semicon.com/ja/technology/automotive-if/index.html

Nexty Electronics offers evaluation services for circuit boards designed using this IC, and has the equipment (VNA and high-speed oscilloscope) and skilled measurement technicians capable of performing the three evaluations mentioned above.

For SONY GVIF, we provide an environment where you can start evaluation immediately, including evaluation boards for SER or DES, various configuration tools, and configuration files tailored to your environment.

If you are interested, please contact us.

Summary

We have so far explained the evaluation criteria and methods for high-speed communication, such as SER-DES technology.

Evaluating and measuring high-speed communication often yields different results depending on the settings and even minor errors, and requires expensive equipment.

At Nexty Electronics, we maintain our own evaluation environment and have engineers on staff who can handle customization and meet your specific needs. Please feel free to Inquiry not only for evaluation requests but also for any technical consultations.