VNA Reflections

Improved Low Insertion Loss VNA Measurements

November 12, 2018

Engineers rely on vector network analyzers (VNAs) to measure the performance of passive and active RF and microwave devices. Characterizing passive components has generally been more straightforward than testing active devices but there are a few exceptions.  One interesting example of such an exception is measuring precision adapters, airlines, or similar devices with very low insertion loss. These devices present a difficult challenge to characterize on a VNA because of the need for extremely low uncertainties on very low loss insertion measurements.

A method Anritsu VNAs have to improve these types of measurements is secondary match correction (SMC).  SMC augments existing calibration techniques by essentially adding additional correction to the assumptions made about match terms in the VNA calibration error models that may not be entirely valid for low loss devices. 

Precision Adapter Example

Figure 1 is an example of an insertion loss measurement of a precision adapter. It shows about 0.05 dB peak ripple, which based on general specifications, is significantly below the measurement uncertainty of ~0.1 to 0.12 dB (peak) for the VNA used to make the measurement. Even though the data shows a small amount of ripple within the measurement uncertainty of the VNA, the reality is it may not be an acceptable result. 

Figure 1: Insertion loss measurement of a precision adapter.

The reason? Residual error terms are on the order of 30-35 dB and the device under test (DUT) match may be on the order of 25 dB. This means the ripples may be rationalized as multiple reflections between the interfaces.

There are several potential causes for the measurement ripple. The most common are high DUT reflections, reference plane problems from pin depth, and connector mating issues. Other causes include DUT- or cal-kit-related explanations. One ripple source unrelated to the physical test setup is how the VNA calibration model corrects for the measurement match. 

Importance of Source Match Correction

A reflectometer diagram is shown in Figure 2 representing a simplified view of a VNA port. If the dominant source of VNA port mismatch is at position Y, then the signal propagates from the source and some energy is reflected back, while the rest is transmitted. Some of the energy transmitted is reflected off the DUT and re-reflected off Y.  The product of all those reflections (approximated by eps*S11 in the classic 2-port VNA error model) is important because the product repeats on multiple re-reflections to form a geometric series.

Figure 2: Reflectometer diagram.

For the measurement case where Y is the dominant mismatch, the result from the standard error model is very close to the actual measurement. The reason is that the test coupler sees all the multiple reflections. 

If a significant reflection happens at position X in figure 2 the reference and test couplers will see the mismatch. The ratio that forms S11, test/reference, now represents a distorted picture of the reflection product. The test coupler recognizes the effective series of reflections, but the reference coupler also is affected by some of the reflections. The ratioing of the two results produces partial cancellation or amplification, depending on the phasing.

The effect of the X source mismatch location usually represents only a small perturbation on the effective port match. If the DUT is extremely lossy, the likelihood of these uncorrected mismatches interacting with the DUT decreases and the benefit of SMC may be small. Similarly, if the DUT is highly mismatched or has multiple moderate mismatch reflection centers, those structural effects in the data may swamp any possible correction by this method.

For low insertion loss devices, the importance of the X and Y reflection locations can vary greatly, depending on the measurement setup. There are cases where the X mismatch location will have a noticeable effect on the measurement. 

Calibration Process

Additional calibration standards can be added to achieve a more elaborate match model as an alternative to SMC. While this will account for mismatch, it will lengthen the calibration process, which is not desirable. SMC uses the phase information in the existing calibration residuals to localize the mismatch elements to process a second-tier correction that primarily impacts the match terms. Thus, SMC improves the match model without incurring additional calibration time.

If SMC is applied to the precision adapter insertion loss measurement example describer earlier, a significant reduction in the ripple that was not part of the DUT behavior will be realized (see figure 3).

Figure 3: An adapter insertion loss measurement without SMC (darker, brown trace) and with SMC (lighter orange colored trace).

SMC for Moderately Lossy Devices

SMC can be used to improve the measurement of moderately lossy devices in some situations. If the component has a reasonable amount of loss but its main mismatch centers are near the measurement reference planes, SMC can provide benefits since DUT mismatches can interact with the uncorrected/partially corrected mismatches in the instrument. 

Figure 4A/4B is an example of this measurement. Results of a clean PC board-based microstrip line measured without SMC are shown in Figure 4A. There is a structural resonance in the DUT response but also some high-spatial frequency ripple. In this case, ripple arose from the PC board-based connector launch mismatch interacting with the remaining instrument mismatch. The same measurement was performed with SMC applied and that result is shown in Figure 4B.

Figure 4A: No SMC applied.

Figure 4B: With SMC applied.

The “after” result looks like it could have been achieved via smoothing or a similar method. Bulk processing techniques like smoothing treat all high-spatial frequency distortions identically and will remove all on a sliding scale. If those distortions were due to interior, uncorrected mismatch, that would not be a problem. In many cases, however, high-spatial frequency distortions may be caused by the DUT and should not be removed. In those cases, SMC is a more effective method to accurately improve the measurement ripple.

Anritsu’s VectorStar™ and ShockLine™ VNAs have SMC capabilities as part of the standard software. This feature enables both VNA families to conduct more accurate measurements on low insertion loss devices. The improvements are often on the scale of hundredths of a dB in insertion loss and picoseconds in group delay, but for low-loss adapter and fixture characterization those enhancements can be valuable.

To learn more about the benefits of SMC, download our free application note: Technique for Improving Low Loss VNA Measurements.

How High-frequency Designs are Changing De-embedding Approaches

November 1, 2018

Signal integrity engineers have increasingly been reliant on de-embedding methods, particularly in PC board and cable assembly spaces. Constant challenges are always repeatability issues and standards availability problems at the device-under-test (DUT) plane.

Some of the same issues occur in millimeter (mm-wave) fixtures, where repeatability can be even more of a challenge. For certain fixtures, the repeatability and standards sensitivity associated with newer de-embedded methods can be orders of magnitude better than classical methods, while showing similar sensitivities to first tier calibration issues. The absolute errors can, however, be substantial for certain distributions of mismatch within the fixture.

Given the need for conducting de-embedded measurements at higher frequencies, especially for emerging 5G and advanced wireless designs, an improved process is necessary. In this post, we will outline limitations associated with conventional methods, how to address them, and which techniques are best suited for today’s high-speed emerging designs.  

mm-wave De-embedding Techniques

Dozens of de-embedding routines are commonly used at mm-wave frequencies. Most are based on limited assumptions, often reciprocity and perhaps symmetry, about the fixture parameters. Engineers have found at lower frequencies that if the DUT interface has repeatability or standards issues, these classical approaches may be sub-optimal for various reasons:

1. Some classical methods (e.g., Thru-reflect-line [TRL]) are very sensitive to changes in launch impedances/admittances
2. Some classical methods (e.g., defined-standards approaches) rely on geometrically well-controlled structures at the DUT plane (not always possible)
3. Classical methods deterministically solve for inner and outer plane match and, as those match levels degrade, there is a nonlinear coupling to insertion loss extraction

All these potential problems are compounded when designing for mm-wave frequencies.

A second approach - partial information de-embedding – has also been used at lower frequencies. It may be more useful in addressing the mm-wave fixture problem. While there are many permutations on these methods, one variety that uses spatial separability of dominant mismatch centers from the main path appears to have more advantages.

Some Different Approaches

While there are many different de-embedding methods, they can be categorized by two types: classical (or full information; i.e., the fixture network is solved with minimal assumptions) and partial information (where more assumptions and structural information about the fixture are used). Examples of the classical methods are TRL and Bauer-Penfield. TRL uses a two-tier calibration based on transmission line characteristics to allow computations of fixture parameters while Bauer-Penfield is based on one-port calibrations for each fixture arm at both inner and outer planes. The latter technique can be viewed as a generalization of open-short de-embedding that is popular in on-wafer measurements.

In partial information techniques, fewer standards are used but more assumptions are made, including the distribution of mismatch within the fixture. The reflection data is correlated with a series of propagation kernels to separate the response portions due to the reflect standard and those caused by the internal mismatch centers. The process is shown schematically in figure 1. Because the contribution separation is based on phase resolution, there is a limit when a mismatch center near the DUT interface cannot be separated from the reflect standard at the DUT interface. There are also variations on this approach that use time domain processing. 

 

Figure 1

Let the Experiment BeginWe conducted a somewhat controlled measurement experiment using the two method classes on a WR-10 fixture to show the results when repeatability is poor. The DUT plane did not have an anti-cocking support and had about 200-μm concavity of the flange surface. Two additional holes were drilled in the flange surface, each 2 mm from the aperture narrow walls, to mount the DUT.

For each connection, the waveguide screw torque was randomized with a uniformly distributed range of 0.5-0.7 cN-m on each screw. The same first tier calibration was used for all runs and 20 second tier runs were done.  Figure 2 shows that the scatter was limited to largely one portion of the frequency range using the partial information method.

 

Figure 2

The scatter on Bauer-Penfield, which uses a short-short-load (SSL) set of standards, was larger (figure 3). Not surprisingly, the distribution is not particularly well-behaved, as there are multiple non-linear geometrical mechanisms involved with both positional alignment and skew gap formation.

Figure 3

The absolute errors of the partial information methods are a function of the spatial structure of the fixture because of the phase correlation techniques used. Another significant factor is the processing details, making it difficult to come to a 100% certain conclusion but the relative repeatability immunity trends seem to hold.

Sensitivity to Standards Errors

Another point of interest was the sensitivities to standards errors. For the Bauer-Penfield technique, SSL standards were the basis when we conducted the experiment but conclusions for an SSS set are similar. As one of the shorts is common to both methods, a basis for comparison was a simulation based on an error in that short offset length. Plots are shown in figure 4 that yielded roughly equal peak excursions but the distributions of the length errors were wildly different for Bauer-Penfield (+/- 10 μm) and for the partial information method (+/- 2 mm).

Figure 4

The reduced sensitivity of partial information method is not surprising since the phase interval used in the correlation was on the scale of cm. The result is that an offset length error resulted in relatively minor magnitude impact for the new method. S21 phase is transparently affected for the partial information method if the offset length was used explicitly, rather than an auto-rotation scheme. Two aspects of the Bauer-Penfield behavior need to be noted:

1. The two offset short lengths were chosen for a 180° reflection phase difference at 90 GHz, so the sensitivity to length error is minimized at that frequency
2. Altering the length entry directly effects the S22 as well as the S21 extraction and there is feedback between these terms

Conclusion

As the experiment indicates, classical de-embedding techniques can cause noticeable measurement problems in some repeatability-challenged mm-wave fixtures. To address this challenge, Anritsu has developed a measurement technique for its VectorStar^®  vector network analyzers (VNAs) that can help engineers more accurately measure design performance. You can learn more by downloading this white paper entitled MM-wave Partial Information De-embedding: Errors and Sensitivities.