SPARQ SPARQ系列述评系列述评系列述评之之九
——普罗众生的S 参数测量
Eric Bogatin            S 参数方面的话题显然是DesignCon 2011上最受到关注的话题。很多主题讲座，很多参展的新产品都和此话题相关。
力科在Designcon 2011上展示了其4个月前发布的4端口，基于TDR 原理的网络分析仪SPARQ ，并且预先展示了即将发布的12端口SPARQ 。
SPARQ 表示“S-Parameters Quick”，表达了力科期望能通过该产品使所有的SI 工程师，特别是那些不能购买传统的VNA 用户能进行日常的S 参数测量。
SPARQ 的外观看起来就是一个4端口的网络分析仪，和传统的VNA 的端口看起来是一样的，而其内部硬件技术则是基于力科擅长的时域技术。
“所有的测量是在时域完成的，我们将测量结果转换为频域，但是用户仍然可以观察到实时TDR 测量结果，
这对于做调试工作很有帮助。 ”力科公司的技术产品市场经理Alan Blankman 说道。
采用时域测量的主要原因是可以通过相对于传统的基于频域的VNA 来说以更低的成本来实现多端口的S 参数测量，
Blankman进一步介绍到，SPARQ的内部采用了一个TDR脉冲发生器和一个采样头。通过高性能的开关
来切换仪器端口信号的进出路径。这种架构极大地降低了仪器的成本，并可以很好地扩展到多端口系统。
SPARQ是“为SI市场设计的网络分析仪”，Blankman说，“它的价格只有传统的VNA的1/3，而且在设计理念上就为了使其容易使用。”
传统VNA的校准是通过测量连接到电缆端口的不同配置的标准件来完成的。这些校准件及传统的校准方法同样可以用于SPARQ，但同时SPARQ提供的一种更加简便的方法。仪器在出厂时会被校准到外部的端口部分。每个出厂的电揽也都严格地进行了校准并提供了各自的S 参数文件。通过该文件可以在测量时去除电缆的效应。
如果用户有其它连接到DUT的夹具或者适配器的S参数模型，SPARQ也可以利用它来去除夹具或适配器的效应。这样使测量设置的过程变得简化，而且减少了得到初始数据的时间。
4端口SPARQ利用外部的电脑通过USB接口运行软件来控制。除了能够实时显示出TDR 波形，它还可以记录和显示时域和频域的单端或差分的4端口的S参数。
Blankman说力科即将发布的软件工具Signal Integrity Studio（SI Studio）将可以无缝地和SPARQ配合工作。测量出来的S参数通道响应可以输入到SI Studio。任何串行数据码型都可以作为激励输入，利用SI Studio可以仿真出通过测量出的通道模型后的响应。
“除了在高频部分略微地减少了信噪比之外，通过时域或频域方法的测量结果也几乎完全是一样的。”Blankman说道。在力科SPARQ的指标中说明了在40GHz时的动态范围可以达到50dB。
（美国力科公司汪进进译）
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英文版:
imes/electronics-blogs/other/4214074/S-parameter-measurements-for-the-masses
S-parameter measurements for the masses
By Eric Bogatin
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,
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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 impo
rted 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.