VNA Reflections

VNAs Provide Insight to Solve Signal Integrity Design Challenges

October 26, 2016

Signal integrity engineers working on high-speed digital designs have time-domain-based instruments, such as oscilloscopes and Bit Error Rate Testers (BERTs), as part of their tool box. While these instruments can identify signal integrity issues by showing an open or closed eye diagram, they can’t determine the cause of the problem. That is where a vector network analyzer (VNA) adds value. For this reason, a VNA needs to be used during the design stage of today’s high-frequency components and systems to assure measurement and simulation correlation.

Many challenges associated with signal integrity are typically caused by SERDES. BERTs are adept at measuring jitter, BER and eye openings related to the parallel data being serialized and deserialized. VNAs are a complement because they characterize the high-speed channels, which are typically passive structures that support the high-speed digital stream. Because of this, the channels need to be characterized in the frequency domain to ensure the quality of performance for the given digital systems.

High-frequency Loss

High-frequency losses typically have the effect of closing eye diagrams of systems.

Figure 1

Figure 1 displays a transmitter and a receiver sending data over an FR4 backplane, which has a loss associated with it. As a low-pass structure, backplanes typically record more loss at higher frequencies. Attenuation in a high-frequency signal has the effect of closing the eye diagram. This is often corrected by using shorter path lengths, materials that have lower loss at higher frequencies, eye-openers that equalize the high frequency, or emphasis on the digital signal itself.  

Setting the Correct Emphasis

Emphasis provides additional high-frequency energy to the transmit waveform by accentuating the rise and fall times of signals. It can be used by engineers to overcome the loss in that channel. Setting the ideal emphasis depends on the loss characteristics of a channel.

One of the quickest and most efficient ways to do this involves using the BERT and VNA together. The VNA measures the channel loss characteristics and exports them to the BERT, which then sets the ideal emphasis for the channel.

Inter-symbol Interference (ISI)

Various frequencies can travel at different velocities for certain media types, causing inter-symbol interference. When low-frequency components travel faster than their high-frequency counterparts, bunching of the data stream occurs, closing the eye.

VNAs are exceptionally useful for characterizing this type of dispersion. The channel attribute that results from the data bunching, called group delay, can be measured by the VNA as the slope of the phase vs. frequency curve. For a nondispersive channel, having a flat group delay vs. frequency channel is ideal, as it indicates minimal dispersion. Having a non-constant group delay over the frequency means that ISI may exist in the system.

Time vs. Frequency Domains

Time-Domain Reflectometer (TDR) architecture causes several limitations. TDRs tend to be wideband instruments, meaning noise in the system is proportional to the instrument’s instantaneous bandwidth. The wideband noise can limit the dynamic range of the S-Parameters to about 40 dB.

As narrowband systems, VNAs have much lower noise floors. Because of this, VNAs provide S-Parameters with higher dynamic range. For example, the VectorStar® has typical dynamic range of 110 dB for a typical noise floor of -100 dBm, and the ShockLine™ features 125 dB dynamic range and -115 dBm noise floor. This level of high-resolution time domain allows engineers to better locate channel defects.  

VNA Transforms Into Time Domain

Transitioning from time to frequency domain involves making an eye diagram simulation of the frequency domain data. Noise, fixture effects, measurements uncertainties and data quality can affect this transformation.

In cases when the frequency is low, the time domain response changes slowly, which effects eye diagrams. Because no VNA can measure to DC, it must be extrapolated. If this is done incorrectly, it can cause step responses with associated slope. Any noise, instability and uncertainty in these low-frequency S-Parameter data tend to affect the flat tops of the time domain step responses. Figure 2 shows the impact these factors can have on the eye diagram. The eye on the left is not as clean as the one on the right due to improper transformation.

  

Figure 2

A time domain transformation is often used to locate defects. Low Pass (LP) and Band Pass (BP) modes are available on some VNAs, including the Anritsu ShockLine and VectorStar families, with LP having the highest resolution. To conduct this measurement, which can provide TDR-like step responses and show impedance changes in the channel, a quasi-harmonically related set of frequencies is required. This means that the start frequency and step size are the same, causing a symmetric calibration in measurement frequencies. A smaller step size allows more points to be used, while a lower starting frequency can allow users to get closer to DC.

The Anritsu VectorStar ME7838x broadband VNAs have the lowest starting frequency of 70 kHz in the industry. When it is coupled with the highest stop frequency of 145 GHz, the ME7838x has the widest single sweep coverage to minimize DC extrapolation errors as well as causality issues.

S-Parameter Quality Metrics

There are three primary methods to verify S-Parameters. Let’s briefly discuss each.

Reciprocity – This is defined as the forward and reverse transmission parameters being equal in both magnitude and phase. Due to their architecture, VNAs allow for excellent reciprocity. It is more of an issue for TDRs and oscilloscopes, as they have trigger jitter from forward and reverse directions.

Passivity – A lack of gain across the band of a passive structure that can cause issues brought upon by calibration, de-embedding and contact repeatability. A VNA with a wide range of de-embedding tools is an advantage for ensuring passivity. VectorStar has seven different de-embedding techniques, as well as hybrid calibrations to solve this issue.

Causality – Causality appears as output energy happening before the input stimulus occurs in the time domain. It can be verified by examining S-Parameters on a polar display, ensuring that they are rotating clockwise in the frequency domain. If it spins counterclockwise, there may be causality issues. VectorStar achieves the best time domain measurement accuracy that, when combined with the unique low-frequency coverage and up to 100,000 measurement points make it all but immune to causality problems, as well as aliasing. The more economical ShockLine VNAs also have excellent low frequency coverage and up to 20,000 measurement points.

For a more in-depth explanation of how to use VNAs as signal integrity tools, visit a new Signal Integrity Technology page.

PAM4 Demands Accurate S-parameters that Only a VNA can Deliver

October 4, 2016

As high-speed serial data rates advance beyond 50 Gb/s, NRZ signaling is being replaced by PAM4. While the modulation scheme of PAM4 takes half the bandwidth to transmit the same payload as an equivalent NRZ signal, it does pose design challenges, particularly when it comes to test. Using the proper vector network analyzer (VNA) can solve these issues.

The bottom line is that PAM4 requires more accurate measurements. Designs have to be verified before they’re produced. Simulations and measurements have to agree on a point-by-point basis. Since SNR is about 10 dB worse for PAM4 than NRZ, qualitative agreement is no longer acceptable. Simulation accuracy and channel evaluation begin and end with accurate S-parameter measurements.

A Closer Look at PAM4

PAM4 brings loss levels down to acceptable specifications but the trade-off is an incremental leap in signal complexity. PAM4 also introduces potential nonlinearities. Variations in the eye heights of the three eye openings is called “eye compression.” Similarly, “timing skew” occurs when the centers of the three eyes are misaligned. Figure 1 shows an eye diagram that suffers both eye compression and timing skew.

Figure 1

Of course, signal integrity engineers must still contend with jitter, noise, and crosstalk when measuring PAM4 signals. Very clean clocks, robust clock recovery circuits, and sensitive symbol decoders/voltage slicers are all necessary. In fact, three slicers are required to decode the four possible symbols. Better equalization schemes at both the transmitter and receiver to address inter-symbol interference (ISI) caused by the channel are necessary with PAM4 designs.  

While equalization can compensate for the ISI caused by channel frequency and phase response, it causes trouble in the presence of crosstalk. The parameters that comprise ISI, crosstalk, and equalization present a multi-variable optimization problem. Signal integrity engineers can think of it as a multi-dimensional design space where they search for that point where BER is minimized. S-parameters provide the tools to predict the combined effects of ISI, equalization, and crosstalk.

The introduction of PAM4 signaling complicates design and test of high-speed components and systems at every step, from design to development to manufacture. The key measurements demand accurate S-parameters that can only be measured on VNAs. Since VNAs are high-quality precision instruments that can be major investments, it’s important to use the appropriate equipment for the product stage. For SERDES and interconnect design, a full featured VNA, such as the Anritsu VectorStar™, satisfies the performance requirements. For system design and development, a 4-port ShockLine™ Performance VNA is probably more than adequate, and for verification and manufacture, a ShockLine Economy VNA might be all that is necessary. Table 1 shows important features for PAM4 applications for two such VNA models.

Table 1

VNA Benefits

VNA measurements are more accurate in both the time and frequency domains than TDT/TDR measurements for three reasons:

• Noise floor: The noise floor of any measurement is proportional to the bandwidth over which the quantity is measured. By converting a step response to an impulse response, TDT/TDR systems make measurements over 35 GHz. A VNA can make many narrowband measurements each over tens of hertz, making TDT/TDR measurement noise floors inherently higher.
• Dynamic range: The typical dynamic range of a TDT/TDR test set is around 40 dB. The dynamic range of the VNAs shown in Table 1 are typically over 100 dB—a million times that of TDT/TDR!
• Reciprocity: With their short rise-time voltage steps and intrinsic timebase uncertainty, TDT/TDR measurements suffer synchronization problems that can lead to inconsistent S-parameters and make it impossible to model PAM4 problems like timing skew.

While TDT/TDR can be used to measure crosstalk S-parameters, at least in principle, their limited dynamic range and high noise floors make it difficult for them to access the weak coupling between victims and aggressors with any accuracy. Precise NEXT and FEXT S-parameters are necessary to perform accurate IBIS-AMI models or COM measurements.

Since PAM4 signaling is used at high data rates in high loss environments, the dynamic range and accuracy of a VNA are necessary. Figure 2 shows a measurement of the VectorStar ME7838D dynamic range from 70 kHz to 145 GHz, as an example.

Figure 2

For a more thorough explanation on the challenges associated with PAM4 signals and how to measure them using a VNA, download a new white paper from Anritsu.