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Achieve consistent differential interconnect measurements

Posted: 12 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:impedance? differential interconnect? CCN? nTegrity? S-parameters?

There was a time when it was possible to test controlled impedance circuit boards after production to verify that they met a characteristic impedance spec. Then, differential impedance became a critical performance feature. Now, differential interconnect loss is an important performance metric for production boards.

As these tests become more and more sophisticated, the correlation between measurements made by a fabrication vendor and those made in a designer's lab becomes harder and harder. While the test structures may be the same, the probes used, the instrument used, and even the type of measurement itself may be different in these two environments.

Because of these differences, making sense of a production floor test result in terms of the R&D environment result is difficult. Is the board good or bad? Can the fabricator's test result be used to make design decisions or is it just a relative metric of this board compared to the last board?

A new measurement system recently introduced by CCN, called nTegrity (figure 1), solves this problem. "It is the first dual use instrument that uses the same probes, the same measurement instrument and the same algorithm to interpret the results on the production floor as in the R&D lab. It's the user interface and automation that are tailored for the two different applications," said Don DeGroot, President of CCN.

Figure 1: The nTegrity PCB characterisation measurement system recently introduced by CCN.

The nTegrity circuit board probe is designed to work up to 20GHz with the same circuit board footprint as the SET2DIL test coupon, introduced by Intel in 2010. Key to the nTegrity system is the use of a replaceable interposer (figure 2) that touches the test coupon and connects the devices under test to the high performance measurement system. This low-cost interposer is easily field replaceable.

Figure 2: Close up of the fixture to hold the test coupon to the interposer.

The second key innovation is a mechanical clamp fixture that removes the variability of hand-held probes. This fixture enables a variety of coupon shapes and sizes and can probe both two-port and four-port test coupons.

CCN chose the Teledyne LeCroy SPARQ Signal Integrity Network Analyser for nTegrity. The SPARQ is used to de-embed the test fixture and launch structures from the device measurement, then to acquire both time-domain impedance and frequency domain S-parameter characterisations of the test coupon.

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