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EETE MAY 2015

Redefining Automated Test with open software and modular hardware How we interact with devices is changing. As the world becomes more software oriented, what we can accomplish increases exponentially. This shift should apply to our test equipment, too. Unlike traditional instruments with predefined functionality, the NI automated test platform provides the latest technologies to build complex systems while reducing development time and cost. >> Accelerate your productivity at ni.com/automated-test-platform Follow us on Search National Instruments or LabVIEW Austria 43 662 457990 0 n Belgium 32 (0) 2 757 0020 Czech Republic, Slovakia 420 224 235 774 n Denmark 45 45 76 26 00 Finland 358 (0) 9 725 72511 n France 33 (0) 8 20 20 0414 n Germany 49 89 7413130 Hungary 36 23 448 900 n Ireland 353 (0) 1867 4374 n Israel 972 3 6393737 Italy 39 02 41309277 n Netherlands 31 (0) 348 433 466 n Norway 47 (0) 66 90 76 60 Poland 48 22 328 90 10 n Portugal 351 210 311 210 n Russia 7 495 783 6851 Slovenia, Croatia, Bosnia and Herzegovina, Serbia, Montenegro, Macedonia 386 3 425 42 00 Spain 34 (91) 640 0085 n Sweden 46 (0) 8 587 895 00 n Switzerland 41 56 2005151 UK 44 (0) 1635 517300 ©2015 National Instruments. All rights reserved. LabVIEW, National Instruments, NI and ni.com are trademarks of National Instruments. Other product and company names listed are trademarks or trade names of their respective companies. 14594 Keys to choosing your coherent optical acquisition system By Dean Miles As demand for data grows, network operators continue to search for methods to increase data throughput of existing optical networks. To achieve 100Gb/s, 400Gb/s, 1Tb/s and beyond, complex modulation formats have become prevalent. These modulation formats present new challenges for the designer when it comes to choices of test equipment. The typical coherent optical acquisi¬tion system consists of three major building blocks: the coher¬ent receiver, a digitizer – typically an oscilloscope – and some form of algorithmic processing. www.electronics-eetimes.com Electronic Engineering Times Europe May 2015 29 14594 Redefining Auto Test 91x277 Euro.indd 1 30/04/2015 10:03 Certain performance param¬eters such as the coherent receiver bandwidth or oscilloscope sample rate have an obvious impact on the measured signal quality. However, there are a number of other aspects to the choice of a coherent optical acquisition system that may be less obvious but can play an equally key role in a successful test system. Achieving low EVM Low Error Vector Magnitude (EVM) and Bit Error Ratio (BER) are basic requirements for any coherent optical acquisition system. There are a wide range of system impairments and configurations issues that can affect the final optical EVM. Within the Optical Modulation Analyzer (OMA) – the receiver – EVM can be impacted by a number of receiver issues such as: IQ phase angle errors, IQ gain imbalance, IQ skew errors, and XY polarization skew errors. The good news about these types of errors is that they can be precisely measured and their impacts calibrated out in the algorithmic processing that typically follows coherent detection. The primary impacts of the OMA on EVM measure¬ments can also be corrected. Once the signal is received, the next step is digitization on the electrical signal paths by a multi-channel oscilloscope. With the oscilloscope, a number of instrument factors affect EVM, the most fundamental being oscilloscope bandwidth and sample rate. Most engineers testing 100G coherent optical signals use 4-channel oscilloscopes with bandwidth in the 23 GHz to 33 GHz range and sample rates in the range of 50 GS/s to 100 GS/s. 400G system evaluation requires oscilloscopes with 70 GHz bandwidth at 200 GS/s sample rates. Assuming an oscilloscope with appropriate bandwidth and sample rate is utilized, and all OMA impairments are being corrected algorithmically, the lowest measurable EVM comes down to a function of the effective number of bits (ENOB) of the oscilloscope. EVM definition EVM was recently defined by the IEC in IEC/TR 61282 101. In this article, EVM is defined as follows. The error vector is simply the vector that points from the actual measured symbol to where that symbol was intended in the signal constellation Dean Miles is Technical Marketing Manager at Tektronix – www.tektronix.com


EETE MAY 2015
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