Why Test Instrument Frequency Range Matters When Conducting Signal Integrity Measurements
By Bob Buxton, Anritsu Company
Several 20+ Gbit/s high-speed standards are driving the upper end of the test spectrum to 70 GHz and even 110 GHz. Higher data rates such as these bring with them new challenges for signal integrity engineers. Accurate measurements are needed to better understand the impact on higher order harmonics, as well as to address challenges associated with conductor skin effects and dielectric losses on PC boards, along with design trade-offs related to selecting vias, stackups, and connector pins.
When using Vector Network Analyzers (VNAs) such as the one in Figure 1 to evaluate backplanes and interconnects, engineers need to consider the frequency range over which to make S-parameter measurements. The choice of frequency range affects the ability to locate defects, the correlation between simulations and measurements, and ultimately the ability to make good decisions concerning cost/performance trade-offs.
Higher-speed designs basically translate into higher test frequencies being required to perform measurements to the 3rd or 5th harmonic of the NRZ clock frequency. For example, a 28 Gbps data rate means either a 42 GHz or
70 GHz stop frequency for an S-parameter sweep.
Figure 2 shows a spectrum of a 14 GHz square wave that would be the clock frequency for a 28 Gbps NRZ signal. This example shows the spectrum after the signal has been passed through a connector/cable assembly. Attenuating the harmonics of the clock frequency will distort the signal, creating the need to characterize the frequency response of transmission media to higher frequencies — ideally to at least the 5th harmonic.
It is also important to examine the requirement for the upper measurement frequency from the causality viewpoint. Lack of causality, where the output appears to occur prior to the stimulus, can be observed when poor S-parameter data is transformed into the time domain for use in circuit or other simulations. Non-causal S-parameter data can be the cause of unstable simulations, which may not converge to a solution or may lead to inaccurate results.
One cause of poor S-parameter data is insufficient higher frequency data. For ideal causality, S-parameter data would be available from DC to infinity — not very practical, at least for the upper limit.
Massaging the frequency domain data can reduce these problems, but can lead to potential issues related to distorting measurements of the actual physical behavior of the device. It is therefore often safer and more accurate to use as wide a frequency range as possible just short of the point where DUT radiation begins to distort the measurements and degrade repeatability.
Importance of Low Frequency Coverage
It is important to remember that accurate measurements to the lowest possible frequency are also very important for signal integrity applications. Often, model accuracy can be improved by measuring down to as close to DC as possible. For example, consider the measured S-parameter data for a backplane fed into a software model to estimate the impact of that backplane on the eye pattern.
Figure 3 shows the eye pattern estimate when the low-frequency data has some error. In this example, a 0.5 dB error injected at a lower frequency (<10 MHz) on transmission could convert an 85% open eye to a fully closed eye. Since mid-band (10 GHz) transmission uncertainty may be near 0.1 dB, depending on setup and calibration — and higher at low frequencies — this eye distortion effect cannot be neglected. Figure 4 shows what the resulting eye pattern will look like if the low frequency measurement data is of good quality and extends down to 70 kHz. This prediction correlates very well with the actual eye pattern measured using an oscilloscope, as shown in Figure 5.
Since the non-transitioning parts of the eye diagram are inherently composed of low frequency behavior, the sensitivity of the calculation to the low frequency S-parameter data makes sense. Because the low frequency insertion losses tend to be small, a large fixed-dB error, which is how VNA uncertainties tend to behave, can be particularly damaging.
VNA Time Domain
The time domain performance of a VNA is critical when trying to locate defects. In general, the wider the frequency sweep, the better the time and spatial resolution. Lack of good low-frequency S-parameter data can lead to further complications when converting into the time domain for measuring impedance changes along a line or for modeling.
Resolution is maximized when the Low-Pass time domain mode is used. This mode also permits characterization of impedance changes on the backplane. Low-Pass mode requires a quasi-harmonically related set of frequencies that start at the lowest frequency possible. A DC term is extrapolated to provide a phase reference, so the true nature of a discontinuity can be evaluated. Hence, the lower the start frequency, the better the extrapolation of the DC term.
Considering all these factors, the ideal VNA should have as low a start frequency as possible, and a stop frequency as high as required based on the bit rate and causality concerns.
Getting Both High and Low Frequency Performance from One VNA
A VNA depends on directional devices to sample forward and reverse direction signals to make the various ratio measurements to compute the S-parameters. Typically, microwave couplers are used, however the coupling performance of these devices degrades at lower frequencies, such as 1 GHz. This reduces the dynamic range and uncertainty of lower frequency measurements. A bridge allows frequency coverage to go much lower without performance degradation, since it does not rely on pure geometrical length (in wavelengths) to accomplish the coupling. Rather, the bridge uses lumped impedance sources at low frequencies, and distributed ones at higher frequencies to achieve the coupling. The Anritsu MS4640A VNA family shown in Figure 1 uses this hybrid approach: coupler-based architectures for the higher frequencies and bridge-based coupling devices at the lower frequencies.
As data rates increase, signal integrity engineers require ever-widening frequency ranges and test equipment that maintains the necessary accuracy. VNAs play a key role in helping meet these challenges so signal integrity engineers can make appropriate cost/performance trade-offs. When selecting a VNA, the user should be looking to maximize both upper and lower frequency limits.
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