S-parameter measurements for the masses

-March 15, 2011

S-parameters were king at DesignCon 2011. This topic dominated many of the talks and dominated many of the new products on display.

LeCroy showed off one of their new 4-port SPARQ TDR-based network analyzers. The 4-port version was announced four months ago and was showcased at DesignCon 2011. Also previewed at this show, and expected to be released in the spring, was a 12-port SPARQ.

SPARQ stands for "S-PARameters Quick," in reference to their goal of bringing routine S-parameter measurements to all signal integrity engineers, especially those not able to afford a conventional Vector Network Analyzer (VNA).

While on the outside, it looks like a 4-port network analyzer, that's where the similarity with a conventional VNA ends. Inside is the same time domain hardware technology LeCroy is well known for.

"All the measurements are done in the time domain. We convert the measurements into the frequency domain, but the user can still view the live TDR measurements, especially useful for de-bugging," said Alan Blankman, technical product marketing manager at LeCroy.  

The key reason to use time domain measurements is the lower cost to implement a multi-port instrument compared with a VNA, operating in the frequency domain.

Inside are one TDR pulser and one sampler. High performance switches are used to route all the signals to and from the ports on the front of the instrument. This architecture dramatically reduces the costs of an instrument and scales well into multiport systems, according to Blankman.

The SPARQ is "a network analyzer designed for the SI market," Blankman said. "It is 1/3 the price of a conventional VNA and designed to be easier to use."

In a conventional VNA, calibration is done by measuring standards connected to the ends of the cables, in various configurations. While this option is also available for the SPARQ, a simpler approach is also offered. The unit comes from the factory calibrated to the front of the ports. Each cable supplied is fully characterized and comes with its own unique S-parameter file. This file is used to de-embed it from all measurements.

If the user has an S-parameter model for other fixtures or adaptors leading to the DUT of interest, it can be brought into the SPARQ and used to de-embed the fixture from the measurement. This simplifies the set up process and reduces the time to get first data.

The 4-port SPARQ uses an external laptop through a USB interface to run the controlling software. In addition to being able to display the live TDR response, it records and displays the 4-port S-parameters as single ended or differential, in time or frequency domains.

Blankman says LeCroy's standalone software tool, Signal Integrity Studio, due for release in the spring, works seamlessly with the SPARQ. The measured S-parameter channel response can be imported into SI Studio. Any serial data pattern can be synthesized as a stimulus and its response through the measured channel simulated. Even multi tap FFE, DFE and CTLE equalization can be applied to the waveform to evaluate the response of the channel with a range of transceiver features. For the specific channel performance, the equalization settings can be automatically optimized for best received eye.

"Other than the slightly reduced signal to noise ratio at high frequency, the measurements are exactly the same, whether done in the time domain or the frequency domain," Blankman said. LeCroy specs the SPARQ with a 50 dB dynamic range up to 40 GHz.

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