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Test & Measurement diagram. The ‘reference’ or intended symbol location is defined by the modulation type except for the overall signal magnitude. For a group of symbols, the reference magnitude is taken to be that which results in the lowest EVM for the group. Once this magnitude is determined, both the signal and the reference symbols are divided by the magnitude of the largest reference symbol to normalize the data. Normalizing the data in this way has the effect of presenting EVM as a fraction of the largest reference symbol magnitude. This makes comparisons between QPSK and QAM EVM more difficult. Wireless standards have chosen to use square root average symbol power for the normalizing factor. It may be that the optical standard will change to this method over time. The Tektronix OM4000 software allows the normalizing factor to be customized for this reason. The default definition follows the TR – see figure 1. These considerations provide the following formula for EVM when expressed as a percent, shown in equation 1: Where EVM(n) is the normalized error-vector magnitude of each symbol and N is the number of symbols in the group. As stated above, the TR assumes normalization by the largest reference symbol. EVM accuracy and reproducibility There are several factors that limit EVM accuracy and repro¬ducibility. These can be generally categorized as systematic or random noise contributions. Systematic error is primarily a function of coherent receiver imperfections. Receiver imperfec¬tions include I-Q phase error, I-Q amplitude mismatch, Fig. 3: Transitioning from 100G test to 400G test skew, cross-talk, and frequency response. These errors are cor¬rected in data post-processing, but a residual error remains since there will be some uncertainty in measurement of the imperfections. The random EVM noise contribution is the input-referred rms noise divided by the largest symbol magnitude. Increasing the signal power thereby reduces this random-noise contribution until the digitizer dynamic range limit is reached. Digitizer instantaneous dynamic range is usually measured by effective number of bits or ENOB which is the number of bits required for an ideal digitizer to have the same noise level as the actual digitizer. Digitizer ENOB can usually be improved with a digital low-pass filter if the entire digitizer bandwidth is not required. Low ENOB, then, is key to achieving the lowest possible EVM. Asynchronous time interleaving Interleaving is not a new technology in oscilloscopes. As soon as the bandwidth requirements extend beyond the sample rate capability of the commercially available analog-to-digital converter (ADC) components, it becomes necessary to find other techniques to utilize available components to meet those extended requirements, or design a new generation ADC. Both LeCroy and Keysight deploy in their oscilloscopes frequency interleaving techniques that extend bandwidth, but do so at the cost of increased noise in the measurement channel. For many applications, the degraded signal fidelity provided by frequency interleaving is problematic, and as a consequence, Tektronix has chosen to take a different approach. The limitation of the frequency interleaving approaches lies in how the various frequency ranges are added together to re¬construct the final waveform, a step which compromises noise performance. In traditional frequency interleaving, each ADC in the signal acquisition system only sees part of the input spectrum. With Tektronix’ patented ATI technology – see figure 2- all ADCs see the full spectrum with full signal path symmetry. This offers the bandwidth performance gains available from interleaved architectures while preserving signal fidelity and ensuring the highest possible ENOB. Future proof for next generation 400G and 1Tb testing As technologies progress and testing requirements evolve, it’s common to re-deploy instruments from one lab or develop¬ment team to another within the company or institution. Here again, the modularity of the Tektronix ATI Technology offers a significant benefit. Systems can easily be scaled down with multiple units di¬vided and redeployed to other projects as needed, maximi- Fig. 2: Tektronix’ patented ATI architecture delivers the lowest noise. 30 Electronic Engineering Times Europe May 2015 www.electronics-eetimes.com


EETE MAY 2015
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