2
\$\begingroup\$

What major criteria should I consider when buying an oscilloscope to test compliance for an SFP+ host board? Examples would be helpful as well.

I am looking for a high-end product since this is for a company, but I'd like to know the minimum requirements as well.

(I have seen a lot of articles online but one way or another they lead to advertising a certain product. I would appreciate unbiased information to mainly understand the big picture of what I should look for.)

Updates: This is to measure Total jitter, voltage range, Crosstalk rise/fall time, DDPWS(Data Dependent Pulse Width Shrinkage), Eye mask hit ratio
More info: 10GE Ethernet will be used. Transceiver max data rate is 10.3125 Gb/s with min rise time 34ps. All measurements are for the electrical interface.

\$\endgroup\$
  • 1
    \$\begingroup\$ You should first decide what you want to do with it. Try decoding? Measure signal quality parameters? What exact wire protocol and speed are you going to look at it? What bandwidth do you need to look at it with to see what you want to see? \$\endgroup\$ – PlasmaHH Dec 9 '15 at 22:22
  • \$\begingroup\$ Yes this is to measure signal quality parameters. 10GE Ethernet will be used. I will keep updating the question as I learn more about it. \$\endgroup\$ – Arash Dec 9 '15 at 22:32
  • \$\begingroup\$ Look at a scope spec that you think might be useful and if there is something in the spec you don't understand ask about it here. Don't expect everyone to understand 10GE - try explaining data rates in real numbers plus types of signalling i.e. differential as opposed to single ended etc.. \$\endgroup\$ – Andy aka Dec 9 '15 at 22:36
  • 1
    \$\begingroup\$ You might like to look into each of the connections between an SFP module and host and determine what they are used for, how fast they are and what parameters you would test (voltage, noise, rise time, jitter, eye diagram etc). This would make a good update to the question. \$\endgroup\$ – David Dec 9 '15 at 22:38
  • 1
    \$\begingroup\$ If you need to probe these signals in-circuit, remember to factor in the price of probes. \$\endgroup\$ – The Photon Dec 10 '15 at 0:57
1
\$\begingroup\$

You'd want to look into either the optical end or electrical end. Probably you are mentioning the electrical end here, and may already have a setup, but mentioning some points either way.

1. The first step is to have the original spec. And the compliance test document MOI (Method Of Implementation) either from Agilent, Tektronix, TD Lecroy or R&S etc. Unfortunately it is the only commercial companies who have dedicated expertise in assessing the complexities in high speed measurements. And MOIs are published by these companies for most High Speed Serial/HSS standards.

(Well, semiconductor and Test & Measurement have a strange symbiotic relationship, and both ends contribute to the original standard! It's not like a semicon company proposes a standard and these companies build boxes for them. And specs are adjusted from both ends. it's all sensible business!)

There are 4 channels of apprx 2.5Gbps each, together forming 10G speed. The choice of oscilloscope depends on high speed serial characteristic of the signal-

A. The spectral content of interest (Scope bandwidth)

B. The Rise-Time / Fall-time response (Scope or probe rise time)

2. Though bandwidth and rise-time are interrelated, the commercial spec on the scope bandwidth does not always promise high rise-time response. However high rise-time response always gives promising bandwidth.

Better bandwidth spec would offer accuracy in vertical measurements and complements the available dynamic range (set by vertical scale on panel, in turn on ADC hardware and amplifiers).

Though high bandwidth offers basic timing measurements like UI/ unit interval or period measurements, this alone is not good-enough for constructing EYE diagrams and decomposing the Jitter components. (Random Jitter + Deterministic Jitter)

For cable testing / characterization and Jitter measurements, you'd need lot of spectral response and rise-time response too. We need to go with another beast called Sampling type oscilloscope. It captures just ONE complete waveform/s on multiple triggers and gives a super-high time resolution capture. It actually does some math magic inside with a lot of waveform captures at regular sampling rate (like a comparable DSO) and reassembles it to appear super-high sampling rate.

3. Plus one would need ready-made compliance package (like SFP+ compliance application for Infiniium of Agilent/Keysight or the fancier TekExpress package of Tektronix) Each costing a few grands.

Or develop your own compliance measurement analysis algorithms in Matlab, LabVIEW or Python for each measurement, which is taller order!

(Because each of those measurements are not simple and direct. They are not directly available on scope display. There needs further detailed analysis on the captured waveform, spectrum or EYE etc. with proper modeling technique)

The commercial ads you mentioned are for a reason. Unfortunately none of these measurements can be achieved by simple lab oscilloscopes for a pi testing. There is a category high performance scopes. And they cost a lot.

I'd suggest DSA91204A or DSA8200 type sampling scope that will cover most measurements. Or DPO/DSA 70000 to cover a few of them

\$\endgroup\$
  • 1
    \$\begingroup\$ Thanks for the detailed answer. I talked to some Agilent, Tektronix, and Lecroy engineers and all seem to agree that a higher bandwidth scope >20GHz is needed. I have looked into wilder-tech for SFP+ compliance host board to make the connection between the scope and the SFP+ cage. I'm following the MSA for the test setup. I have seen some documents from tektronix and Agilent but have not heard of the MOIs. Is there any links to an MOI you'd be able to provide? Agilent recommends 86100D. What do you think about that? We actually have the DSA91204A \$\endgroup\$ – Arash Mar 4 '16 at 15:49
  • \$\begingroup\$ @Arash Rowshan Here are couple of them one from Agilent's exclusive package (additional software installable on scope's machine) keysight.com/upload/cmc_upload/All/… And this one is from Tektronix. DPOJet is pluggable package within their main Scope's software tek.com/document/method-implementation/sfp-qsfp-dpojet-moi \$\endgroup\$ – seshu Mar 15 '16 at 20:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.