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In today’s world of double-digit gigabit-per-second data rates it is imperative that engineers properly design and characterize their system to meet standards compliance. This sounds simple enough but actually simulating a high-speed serial link with all its impairments and to check if it meets compliance is no easy task. Furthermore, traditional methods of compliance checking involving comparisons of different frequency domain parameters (such as insertion loss, return loss, crosstalk, etc.) to mask limits has led to overdesign and higher cost products because the method did not consider the interdependencies of the different parameters and allow for trade-offs. For example, if a design meets the insertion loss and crosstalk specifications by a large margin but barely fails the return loss specification then it would be deemed non-compliant even if it works perfectly in the real world.
Enter Channel Operating Margin (COM), a compliance check method introduced in the IEEE 802.3bj-2014 100 Gb/s Operation Over Backplanes and Copper Cables Ethernet standard. This is a time domain specification defined as:
COM = 20 log10(As/Ani)
where As and Ani are calculated signal and noise amplitudes at the output of the receiver and considers the effect of the entire end to end channel characteristics including the transmitter (TX), receiver (RX), and channel impairments. As is calculated from s-parameters of the channel including near-end and far-end crosstalk paths, TX and RX device package models, TX FFE and RX CTLE filters, and models of the TX and RX output/input impedances and the RX DFE. The equalization taps of the FFE, CTLE, and DFE are optimized such that As is maximized relative to the noise terms coming from the transmitter, residual ISI after equalization, jitter (converted to amplitude noise), peak crosstalk, and CTLE noise cascaded with a COM specified RX noise filter. The noise amplitude Ani is calculated according to the combined distribution function of all the noise sources propagated to the receiver output, assuming Gaussian noise distributions, and solved by equating the cumulative distribution function to a detector error rate (DER0) such that the resultant noise level corresponds to that statistical error rate. Compliance is achieved when the COM value exceeds a certain threshold, i.e. 3dB for the IEEE 802.3bj standard. The key point here is to notice COM is composed of many interrelated signal variables which allows engineers to make design trade-offs while meeting BER or DER0 requirements.
JCOM is the compliance method for the JEDEC JESD204C category C physical layer specification of high-speed serial links connecting data converters (ADCs and DACs) to logic devices (such as ASICs, FPGAs, etc.) with data rates ranging from 6.375 Gbps up to 32 Gbps.
JCOM = 20 log10(As/Ani)
Like COM, JCOM is also a time domain measure of calculated signal-to-noise ratio (SNR) at the output of the receiver and allows flexibility in managing the trade-offs between the TX, RX, and channel impairments in the link. It includes several improvements over COM with custom device package models and frequency dependent output/input impedances of the TX/RX. Compliance is achieved when the JCOM value is greater than 2 dB threshold.
So what are the benefits of using COM or JCOM? Here are three key benefits:
Overall compliance checking with COM/JCOM is fast, efficient, and reduces overdesign which is critical for today’s and tomorrow’s high-speed serial links. Cadence’s SigrityTM SystemSITM now includes COM channel compliance enabling engineers to quickly and easily check if their system is compliant. JCOM compliance is in the works and will be available soon. So, if you’ve been struggling with your channel compliance simulations, contact us and we’ll show you how fast and easy it is to get you running COM channel compliance with SystemSI.
For detailed information on COM calculations, you can refer to Annex 93A of the IEEE802.3bj-2014 specification and to section 5.12 and Annex E of the JEDEC JESD204C standard for JCOM.