February 21, 2017
RF and microwave engineers face new challenges because today’s communications systems are operating at faster Gigabit data rates. To overcome these hurdles, digital designers must make evaluations that assure pulse fidelity, measuring aspects such as standing wave ratio (SWR), insertion loss and signal leakage between printed circuit board (PCB) traces and delay times.
Parameters must be measured as differential lines with circuits being used to reduce interference. Among the issues are that many complex circuits are compressed onto multilayer PC boards and the system can be damaged by improperly contacting the desired points to test the circuits. Additionally, test connectors occupy valuable space and can adversely affect measurement reliability.
As we have discussed in previous posts, it is no longer sufficient to independently measure bit error rate (BER), jitter and waveform quality of each lane. Quantitative measurements of high-speed multilane serial communications require multichannel test solutions and RF/microwave instruments, most notably vector network analyzers (VNAs), with sufficient performance. In this post, we will go more in depth on some key VNA features that we have touched on in the past.
Before we do that, however, it is important to fully understand the technologies that the new generation of VNAs are measuring. Data rates have been advancing from 15 GB/s to 50+ Gb/s per channel for 40 Gb/s to 400 Gb/s systems. One by-product is that conventional logic-emulating non-return-to-zero (NRZ) signaling is being replaced by PAM4, a four-level pulse amplitude modulation scheme that requires half the bandwidth of NRZ to transmit the same signals. As any signal integrity, test and design engineer responsible for SERDES components, interconnects, backplanes, cables, connectors, circuits and complete systems knows, PAM4 – while provides distinct advantages – creates new test issues.
Figure 1 shows a typical SERDES block diagram. The main objective is to transmit data at the highest speed and lowest BER. The main issues are frequency dependent losses, understanding materials properties at high frequencies and knowing the limits of the channel.
Why VNAs Are Necessary
While BERTs, oscilloscopes, and time domain reflectometers can find problems in these high-speed designs, VNAs can locate the actual cause of a signal integrity issue. For example, they can help determine what is causing an eye closure in a high data rate system. Wide frequency bandwidths of certain VNAs help build accurate models as well as provide accurate resolution in the time domain for locating channel defects. Time domain performance is also a vital asset of a VNA’s ability to locate defects.
Time Domain Resolution
High-speed passive components must be measured in frequency and time domains to ensure that transmission characteristic standards are met at each measurement point. By using the best time domain resolution capability, it becomes easier to locate discontinuities, impedance changes and crosstalk issues.
Generally speaking, wider frequency sweeps produce better time and spatial resolutions. Figures 2 and 3 show the differences in time domain resolution for three different frequency spans of 40 GHz, 50 GHz and 70 GHz.
With only a 40 GHz span, it is almost impossible to determine that the channel contains two mismatches, which is clearly shown by the two peaks in the signal with a 70 GHz frequency span. Higher resolution plots can also indicate the dominant mismatch.
Low Frequency Accuracy
We cannot emphasize enough that low frequency S-parameter data is important because it converts the time domain for measurement of impedance changes either along a line or for modeling. Longer electrical length DUTs have more ripple in the lower frequencies, which can make DC point extrapolation difficult. The rule of thumb is to measure as close to DC as possible to ensure better extrapolation. VNAs that use directional couplers lose dynamic range at low frequencies, which can translate into noise and instabilities on that data. If severe enough, this can have a significant impact on the time domain response. The Anritsu VectorStar® VNA family eliminates this issue for greater measurement confidence.
Using a Low-pass Mode
Anritsu recommends using a low pass mode for time domain analysis, as it has the highest resolution and can show real impedance as a function of distance. Low-pass mode also assumes the existence of data near DC, which allows for computing a step response in order to create a pure real transform. Noise, instability and uncertainty in low frequency data can affect the time domain response, which can cause errors in impedance information. Because of this, the extrapolation of the DC is better with lower start frequency and good dynamic range, which are provided by Anritsu VNAs.
VNA time domain transformation is a circular function that repeats at a value termed Tmax, which determines the alias-free range. The step response repeats, or aliases, after that maximum value. It is important to have small frequency step sizes when looking at longer structures, such as cables or a medium that has a high dielectric constant. For example, if a PC board with an alias-free range has a dielectric constant of 4, and the step size is 40 MHz, the resulting alias-free range would be 75 inches, meaning that measuring anything greater than 75 inches would be impossible. However, reducing the step size to 4 MHz would result in a 750-inch alias-free range.
Anritsu offers a wide range of VNAs to meet budget and performance needs for time domain transformation. To learn more about these VNAs and how to use them to conduct RF and microwave measurements for signal integrity (SI) applications, download this white paper.