A Simple, Powerful Method to Characterize Differential Interconnects

Abstract 

The Automatic Fixture Removal (AFR) process is a new technique to extract accurate, high bandwidth models of interconnects that are both simple and accurate. This technique can be applied to all interconnects such as connectors, IC packages, cables, circuit board traces, and even vias. It has comparable accuracy to traditional TRL techniques but is much simpler to implement. This app note describes the AFR technique and why you will want to use it in your next measurement project. 

Measurements in Perspective 

Every high-speed digital product designed and built today, operating above 50 MHz, has one problem in common: the interconnects between the chips are not transparent. This means if they are not optimized right from the beginning of the design process, the product probably will not work. In fact, the chance of success, if you do not optimize the interconnects, diminishes rapidly as data rates exceed 1 Gbps. This is the high-speed digital regime. 

The ability to accurately simulate the performance of the interconnects is a critically important capability to assure optimized cost-performance before you build the product. 

The key element here is “accurate”. How do you know how accurate your models, material properties, or simulation tools really are? The only way to gain this sort of information is by comparison to measurement, your anchor to reality. 

While measurements are not a substitute for simulation, they can complement simulations, for example, by using them to: 

• Emulate system performance using a measured behavioral model 
• Characterize a component 
• Verify a component to a spec 
• Validate a simulation/model to hardware 
• Extract material properties 
• Debug a problem 

But all measurements, especially at high frequency, have a critical limitation: what you want to measure is often buried deep inside an interconnect and not easily extracted. 

The fundamental problem in all measurements is this: how do we separate out just the DUT we want from the total measurement of the DUT + fixture.  

This is where the Automatic Fixture Removal (AFR) technique, a new calibration routine recently introduced by Keysight Technologies, Inc. can play an important role. 

There are already a number of calibration techniques commonly used in the industry, such as 

• SOLT (short, open load, thru) 
• TRL (thru, reflection, line) 
• LRM (line, reflect, match) 

They each offer a balance between complexity to implement and the ability to extract the DUT properties from a DUT + fixture measurement. Unfortunately, for TRL and LRM, while they are widely used to remove the fixture effects from a measurement, they are difficult to implement correctly and have a number of subtle opportunities for artifacts to be introduced. 

Historically, calibration techniques have followed the old saying, “The better a medicine is for you, the worse it will taste.” The more accurate a calibration technique, the harder it has been to implement. The Automatic Fixture Removal (AFR) technique breaks this pattern and offers both accuracy and ease of use. 

List of content: 

• De-embedding Techniques 
• The Automatic Fixture Removal (AF) Technique 
• Evaluating the Accuracy of the AFR Technique 
• A Real World Differential Example 
  

The Automatic Fixture Removal (AF) Technique 

The most accurate results of the AFR technique are realized if the fixture elements on the two ends of the DUT are mirror-symmetric. Furthermore, it’s necessary to have a separate structure that is just the two fixtures connected together as a single thru. Since this thru reference structure is twice the length of either fixture connected to the DUT, it is often referred to as a 2x thru reference fixture. 

Each fixture end by itself may not be symmetric. For example, one end of the fixture on port 1 has an SMA connector, while the other end of this fixture connects to the DUT with a section of the uniform transmission line. However, when the two fixture pieces are connected together, end to end, the resulting 2x thru the reference fixture is mirror symmetric. 

The resultant measurement is the composite S-parameters of the 2x thru reference fixture. This is described in terms of the two S-parameter sets that make up the fixture. In this example of a single-ended fixture, the measurement is the composite S-parameters of the series combination of the two fixture sections, S11, S22, S21, and S12. These four S-parameter elements and the fact the structure has mirror symmetry are still not enough to uniquely extract the S-parameters of each fixture half. 

An essential element to the AFR technique is leveraging signal processing in the time domain to extract the unique values of S11A and S22B. With these three features, each element of the S-parameters of each fixture can be uniquely extracted. 

Of course, once the measurement of the 2x thru reference fixture is performed, these operations are all done automatically by the AFR software built into PLTS. The intuitive user interface of AFR transforms this normally complex error correction algorithm into a simple task.