Last February, at DesignCon 2010, Anritsu made a splash with their new 12-Port 70 GHz vector network analyzer (VNA) System, a new member of their VectorStar VNA product line. The new analyzer measures multi-port balanced differential and mixed-mode networks used in today’s high-speed bus designs, covering an extremely wide band of frequencies (40 MHz to 70 GHz).
Traditionally, VNAs measure S-parameter on single-ended, 50 ohm components and devices, however as digital communications systems and buses increase in speed (and frequency), multi-port, mixed-mode S-parameters have become an effective tool for characterizing the signal integrity (SI) of signal lines, buses, and components operating at high digital speeds.
VNA’s can directly measure crosstalk on high-speed channels, such as those found on a high-speed backplane. While these channels are designed to be independent of each other, they often suffer from crosstalk when operating at high-speed/ high-frequency signals. For digital communications standards exceeding speeds of 10 Gb/s, such as USB 3.0 or PCI-Express Gen 3, the availability of a 12-port, 65-GHz VNA test system can provide meaningful SI measurements under full-speed conditions. Valid measurements over a wide bandwidth are critical to accurate signal integrity analysis of high-speed electronic designs. Thus Anritsu’s new VNA addresses the need for bandwidth with extended range well into millimeter-wave frequencies. To represent the frequency-dependent behavior of a complex structure, S parameters, which have long been used in the microwave community, are beginning to gain popularity in the analog and digital designs.
A lot of information is actually embedded in the S parameters, and there are many methods to manipulate the S parameters to reveal the structure's property in detail. What microwave engineers have known for some time, many high-speed digital and signal integrity designers, who work in the time domain, are just beginning to grapple with.
In addition to high-speed backplanes, an increasing number of wireless components and devices rely on differential (balanced) architectures for reduced susceptibility to electromagnetic interference (EMI). While a four-port VNA system can accommodate measurements on a single differential channel or device, more complex devices or components require a greater number of measurement ports. In fact, single-ended measurements for high-speed transmission lines can provide misleading results of loss performance since those lines are designed for differential operation.
As with high-speed backplanes, crosstalk between adjacent differential channels can degrade performance. In a pair of differential channels, crosstalk will be caused by energy from one channel, referred to as the aggressor line, coupling to an adjacent channel, referred to as the victim line.1 In order to characterize the crosstalk of two differential channels with a VNA system, four test ports would be needed for the aggressor line and four test ports for the victim line. Of course, in a multichannel communications system or set of differential lines, pairs of lines cannot realistically be considered as isolated from surrounding lines. It is generally more practical to characterize the crosstalk from the two adjacent aggressor lines on a victim line, which requires four test ports for each line, or a total of 12 test ports.
S-parameter Measurements with Multiport Balanced Test Sets
With the proliferation of multi-port high frequency devices, VNAs are increasingly being used to perform S-parameter characterization. The new 12-port VNA extends existing 2 and 4 port systems that are commonly used for RF/microwave devices toward a system more readily equipped to address high-speed interconnects, which often have more I/O connections than most microwave circuitry. While the measurements are similar to conventional VNA approaches, there are architectural, calibration, and performance differences that should be considered carefully. The basic function of the test set in multiport measurements is to provide multiplexing from the M VNA ports to the N DUT test ports. The fundamental need is the ability to measure all N2 S-parameters at the DUT plane. Certain other measurements would benefit from additional connectivity within the test set (e.g., every VNA port can connect to every DUT port) and that does provide additional calibration flexibility but that will not be the focus of this discussion. With this basic functionality in mind, there are still different ways of executing the system.
There are at least two common architectures of external test sets and each has its advantages and disadvantages. The first is derived from the classical VNA structure in which a test coupler or bridge is associated with each port. Typically the coupled arms would be multiplexed before being sent to the receivers of the VNA. The drive side is also multiplexed and this may be done before or after reference couplers/bridges. The coupler test set, is shown in figure 1 for the case of a 4 port test set linked to a 2 port VNA.
Variations on this concept are possible in which some of the couplers are in the VNA unit and some are in the test set, as well as other multiplexing combinations. The important point is that the drive lines to the test couplers (at least) are after the multiplexing switches.
A simpler structure has no couplers in front of the multiplexing; the test set consists entirely of switching. There may be differences in the connection of VNA ports to test ports that can affect some more elaborate measurements but will not, in principle, affect S-parameter measurements as long as every test port pair (all N(N-1) paths) can be measured. The calibration schemes used may be affected by the level of connectivity. This concept, termed the no-coupler test set, is shown in figure 2 also for the case of a 4 port test set linked to a 2 port VNA. This concept is most easily extendable for large N test sets linked to 2 or 4 port VNAs.
With the new 12-port VNA, mobile port modules (fig. 3) are connected to the interface test set with flexible V cables, allowing for the placement of the ports as close to the device under test (DUT) as possible, resulting in improved measurement stability compared to alternative systems. Furthermore, low-loss solid state 70 GHz switches have been developed for the VNA system to improve performance. The low insertion loss of these switches help to maximize the dynamic range of the multiport system. The combination of low-loss components and high directivity architecture allows the system to enhance the accuracy of the analyzer.
For more on multi-port VNA measurements from Anritsu.
Additional Tips on SI Measurements with a VNA
But an accurate measurement instrument still requires care on the part of the test engineer. Such a warning was given in a paper presented at Design Con 2009, by Don DeGroot of CCNi Measurement Services, Guidelines for multiport and mixed-mode S-parameter measurements in high-speed interconnection design . Degroot notes that, “subtle features of a measurement set-up that can affect the accuracy of high-frequency scattering parameter measurements made with VNAs. Since some of the errors at the upper frequencies may be less significant than others when determining waveform distortion in interconnections, the overall accuracy requirements for S-parameter measurements are still an open debate in the high-speed signal integrity (SI) world.”
Measurement inaccuracy can result from misapplying differential-, common-, and mixed-mode S-parameter methods to the characterization of high-speed interconnection networks, and these measurement errors made in mixed-mode S can be significant, adversely affecting an engineers understanding of signal visualization vs. network identification, wave propagation mode vs. signaling mode, and multiport S vs. mixed-mode S.
The author’s summary of Multiport and Mixed-Mode Measurement Guidelines (emphasis on VNA measurements) includes:
1. Identify the true electromagnetic ports for the interconnection network being characterized.
2. For true one- and two-port networks, do not use two TDR signal sources configured in differential or common-mode drive.
3. For true one-, two-, three-, and four-port networks single-ended multiport network analysis will most often work, using either VNAs or TDR scopes. Drive one port at a time and measure all port voltages. All ports must be terminated in the reference impedance (normally 50 Ω).
4. All VNA measurement systems must be fully calibrated. Ideally this calibration is made using a full kit of calibration standards, like Open, Short, Load, and Thru (OSLT) standards, or using an automated electronic calibration module with sufficient impedance states.
5. Ideally, all measurement set-ups should allow for calibration devices to be connected at the measurement reference plane, that is, the same points used when making connection to the network under test. This may require the construction and characterization of a custom calibration kit for the particular network being characterized.
6. For all measurement set-ups, great care must be taken to not disturb the cables connecting the sources and samplers to the network. Slight bending and motion of the test cables and connections may invalidate the calibration.
7. If the interconnection network under test is coupled to adjacent transmission lines and conductor systems beyond the ports being measured, the other conductor systems must be electrically terminated as they would be in the target application.