Summary
LeCroy SPARQ Signal Integrity Network Analyzers rapidly characterize crosstalk in interconnects, backplanes and cable assemblies. Both near-end crosstalk (NEXT) and far-end crosstalk (FEXT) can be measured using either single-ended or differential port assignments. With 8 and 12 port SPARQs (model numbers SPARQ-3008E and SPARQ-3012E), signal integrity engineers can validate their crosstalk models with multi-differential lane NEXT and FEXT measurements, at a fraction of the price of a VNA.
The Crosstalk Challenge
The 21st century has witnessed an explosion in the market for cloud computing, streaming video, and in general, on-demand instant access to everything the internet has to offer. To meet this demand, bitrates and continue to rise, and, in turn, signal integrity effects, including crosstalk have become problematic. Crosstalk is the unwanted noise or interference induced on a victim line by an aggressor due to the fringe fields that couple signal and return paths of neighboring transmission lines. Signal Integrity engineers have a difficult task: they must contend with the ever-increasing need for higher channel densities in packages, traces, via fields and connectors, and to predict the extent to which higher densities result in crosstalk that might exceed the design’s margin for noise. In addition to using field solvers and SPICE models to simulate crosstalk, SI engineers should always seek to make S-parameter measurements, since measurements are required to validate the accuracy of any model.
SPARQ Operation
The LeCroy SPARQ Signal Integrity Network Analyzer is easily configured and operated for quick measurements of NEXT and FEXT. SPARQ analyzers use an internal OSLT calibration kit in order to automate the calibration process, avoiding the need for repeated connects/disconnects from external “ecal” modules. The user simply connects to their device, configures analyzer settings such as end-frequency, number of points and ports, and clicks “Go”. The analyzer then acquires TDR and TDT waveforms from the internal OSLT calibration kit, followed by TDR and TDT waveforms from all pairs of DUT ports. The SPARQ calculates the Sparameters of the DUT by applying the calibration and by deembedding the effects of the internal switch matrix and external cabling and fixturing. See the application note SPARQ Sparameter Measurement Methodology for more information. After the measurement and calculation are complete, the S-parameter results configured for display are shown, along with any time-domain views. Note that the SPARQ measures the complete S-parameter matrix. (VNA users would call this a “full crossbar” configuration.)
Configuring to Measure Differential NEXT and FEXT
The SPARQ makes its measurements in a single-ended fashion, by stimulating one port at a time with TDR pulses, and by measuring the signal reflected from one incident port at a time, and from one output port, as is done by a typical VNA. (All pairs of ports are characterized; for example, an 8-port measurement entails 8*7 = 56 permutations of connections to ports for TDR and TDT measurements.) Single-ended S-parameters are converted to mixed-mode results, including differential-to-differential, common-tocommon, and mode conversion S-parameters. The single-ended S-parameters are converted to mixed-mode based on the configuration of the SPARQ Port Configuration dialog. This is a powerful utility for configuring (and reconfiguring for later analysis) the port configuration. Figure 2 shows the configuration used for the network diagram shown in figure 3. The conversion to mixed-mode is done using the standard transformation.
Viewing NEXT and FEXT Results
SPARQ users can then view NEXT and FEXT in both the time and frequency domains. Figure 4 shows the SPARQ configured to show the differential-todifferential and common-tocommon NEXT and FEXT crosstalk in both the frequency and time domains. The four traces on the right side of the screenshot are the S-parameters for NEXT and FEXT crosstalk. The four traces on the left side are time-domain views. To generate these waveforms, the Sparameters are transformed to time domain via an iFFT and then convolved with a step of a userselectable rise time. The resulting time-domain views are easily interpreted. The upper two time domain waveforms show the classic signatures for NEXT and FEXT crosstalk in a microstrip structure. The bottom two are the common signal NEXT and FEXT, which show evidence of impedance mismatches that reflect the crosstalk back in the opposite direction.
The device for this measurement is the SPARQ demo board, which includes 2 coupled differential microstrip pairs. Each differential pair is brought together, where the trace thickness and spacing changes to give a differential impedance of approximately 100 ohms.
