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Test System Sets Sights on OC-192 Designs
By Janine Sullivan, Contributing Editor
New 60-GHz analyzer operates in the frequency and time domain to test optical and semiconductor devices employed in next-generation optical communication systems.
Developers of leading-edge, high-frequency optical communication systems struggle to find test equipment that can produce, view, and analyze frequencies of optical components in the 40- to 50-GHz ranges and higher. A new entry in the test equipment market promises to make things easier. Featuring a 60-GHz bandwidth, the MCA1060 microwave communication analyzer from LeCroy Corp. (Chestnut Ridge, NY) combines the analysis capabilities of an oscilloscope and a spectrum analyzer with the signal-generation capabilities of a frequency synthesizer. The analyzer offers designers of high-frequency components a versatile test suite.
LeCroy's first entry into high-bandwidth applications, the MCA1060 performs measurements in both the time and frequency domains (see Figure 1). This lets designers characterize a device in the frequency domain and then detect specific circuit problem details using the time domain (analysis traditionally associated with an oscilloscope). This flexibility allows measurements of rise time, pulse width, amplitude, group and phase delay, and transmission and return loss for optical and semiconductor components such as optical receivers and modulators, high-speed logic blocks, and high-frequency mixers, filters, and amplifiers.
Signal acquisition
It can be difficult in high-frequency circuit design to ensure that testing involves an accurate signal that has not been influenced by the testing instrument itself. The MCA1060 addresses this concern by employing a new signal acquisition technique. By incorporating a frequency synthesizer and trigger within the unit, the tool does not have to trigger directly on an input signal. Instead, the integrated synthesizer, with sweep speeds settable in MHz units, is tuned to the frequency under observation. As the synthesizer slowly sweeps through the test signal, it builds up a profile of the signal under observation. Since the instrument does not have to trigger on an input signal directly, timing accuracy can be specified at 2 picoseconds (ps) or better. Eventually, the company expects this specification to fall into the 100-femtosecond (fs) range.
Since this signal acquisition technique uses sequential sampling that moves through the waveforms, problems related to overshoots and undershoots in voltage value are essentially eliminated. Because every point is similar in voltage value, the actual displayed waveform is quite precise. This results in high accuracy (0.0000525-Hz frequency resolution at 60 GHz) and a low noise floor (-90 dBm). The resulting screen display depicts a clear rendering of the waveform (see Figure 2).
Using the integrated synthesizer, signal acquisition and analysis on the MCA1060 can take one quarter of a second to make a 51-point step sweep. By comparison, a typical vector network analyzer (VNA) would take more than one full second to perform the same task.
Performance
High-bandwidth test instruments are crucial to the development of next-generation optical communication systems. Designers testing a 40-Gbps OC-768 signal for high-speed optical communication networks, for example, can benefit from a 60-GHz bandwidth tool because it allows them to analyze small details in the signal, such as the amount of overshoot or the presence of artifacts in rising or falling edges.
Bandwidth is not the only factor to consider for detailed analysis. Designers also need a tool with high sensitivity and a low noise floor to study low-level signals. While typical oscilloscopes have a -45-dBm noise floor, the MCA1060 was designed with a -90-dBm noise floor in order to enable careful observations of the complete signal.
Multiple channels are another benefit to signal analysis. The dual-channel MCA1060 instrument permits observation of two test points, which is helpful when performing tasks such as jitter analysis, timing variations between two signals, or transmission/ return loss. Different measurement modes, such as time domain, frequency domain, or time-domain reflectometry (TDR) can be combined with numerous display modes (including linear, log magnitude, Smith Chart, polar, group delay, and eye diagram) for full signal characterization.
A tool that performs analysis in both the time and frequency domain and incorporates internal signal synthesis can simplify testing, which offers time and cost savings advantages. For example, designers performing TDR, transmission loss, return loss, group, and phase delay measurements need to generate a precise test signal, put it through the device under test, and measure the result. The integrated signal generator can produce emitter-coupled logic (ECL), transistor-transistor logic (TTL), or frequency comb signals up to 1,600, 200, and 60 GHz respectively, avoiding the need to purchase an additional synthesizer for many applications.
Technical advances
Test instruments with 60-GHz bandwidths and -90-dBm noise floors are certainly a product of the technological advances that have come before them. To build their new tool, the engineers at LeCroy relied not only on technical expertise, but also on some new high-performance components. For instance, the instrument incorporates broadband samplers and connectors, low-capacitance GaAs diodes, high-speed transistors, as well as low phase noise synthesizers with high resolution that simply were not available until the late 1990s.
Undoubtedly, tools of the MCA1060's caliber will enable the components for tomorrow's high-speed communication systems. As new products and technologies migrate from advanced R&D to product development and finally into manufacturing, the MCA1060 can be used to look for imperfections in cables and components, allowing designers to efficiently characterize and optimize their next-generation products.
The MCA1060 is priced at $39,995. Accessory kits, adapters, and connectors are also available. For more information, contact LeCroy Corp. at 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977-6499. Phone: 914-425-2000. Fax: 914-425-8967. URL: www.lecroy.com.
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