Video Tutorial on Passive Filters, Data Transmission and Equalization - Page 1
Forum section: electronics / projects

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 13, 2012, 01:39:18 pm
content: I am back! After being away for a while due to work, I have uploaded another video to "The Signal Path". This video features some more advanced topics and a 1.5Gbps data link:In this episode Shahriar explores the world of filters! Starting from a simple lumped RC filter, he briefly covers the theory before moving onto measurement techniques. The bandwidth of the filter is verified experimentally in the time domain. A more complex RLC band-stop filter is also demonstrated with a tune-able inductor which is measured using an RLC meter. Using a Rigol spectrum analyzer with built in tracking generator and an active probe, the frequency response of the filter is measured. Several other packaged filters are also demonstrated and a microwave band-pass filter is disassembled to reveal its internal construction.Moving onto "undesired filters", a Tyco backplane board is presented and the bandwidth limitation of the backplane traces are measured. A Xilinx FPGA board equipped with a Vertex II PRO is used to generate a 1.5Gbps PRBS data stream through the Tyco board. After observing the frequency composition of the date, pre-emphasis equalization is used to compensate the backplane frequency limitations. Eye diagram measurements verify the benefit of pre-emphasis equalization to combat inter-symbol-interference (ISI).Please visit:http://www.TheSignalPath.com

author: EEVblog
author_rank: Unknown
timestamp: February 14, 2012, 12:25:28 am
content: Hi ShahriarGreat work.Is that your personal home lab or work/university lab?Dave.

author: olsenn
author_rank: Frequent Contributor
timestamp: February 14, 2012, 01:26:25 am
content: I actually have a couple of questions about that spectrum analyzer if you have the time to answer... I was thinking of purchasing a Rigol DSA815 1.5GHz spectrum analyzer (~$1500) but I would like to know how it differs from the one you have, apart from half the bandwidth)? Also, I keep reading that the front-end to the device can be damaged if your input signal is too large; I'm not farmiliar with the dbm scale, but what kind of RMS voltage (sine wave) and DC offset can you have on those things? Why aren't they high input impedance like oscilloscopes?

author: kaz911
author_rank: Super Contributor
timestamp: February 14, 2012, 04:16:39 am
content: Quote from: olsenn on February 14, 2012, 01:26:25 amMost SA's are limited to 16-24dBm CW (Continuous Wave) on the input and and XX Volt DC. Then you use DC blockers to filter out DC - and fixed or programmable dB attenuators to lower the input dBm. Or attenuators with DC block built in.DC Blockhttp://www.testequity.com/products/1501/My BK 2650 can take signals up to 20dBm (so 100mW or 2.23 Volt RMS) - and max 25V DC.Every 3dBm double or halves the signal - so 23 dBm is 200mW and 17dBm is 50mW)/Kasper

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 14, 2012, 04:40:22 am
content: Quote from: EEVblog on February 14, 2012, 12:25:28 amHi Dave! It is nice to talk to you. I hope you and your family are doing well.Yes, this is my personal lab at home. What you see is actually in my bedroom!This is one of the reasons why I keep it clean all the time.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 14, 2012, 05:02:36 am
content: Quote from: olsenn on February 14, 2012, 01:26:25 amContinuing on what "kaz911" has already mentioned,The input to most spectrum analyzers is good up to 30dBm (~ 20Vpp Sine), so it is difficult to kill them with an AC input. Some spectrum analyzers are DC coupled at the input and can't tolerate any DC input. These spectrum analyzers typically go to very low frequencies (< 10Hz). My spectrum analyzer starts at 9kHz and is AC coupled. It is able to tolerate up to 40V DC at its input.As for the specs, the lower end spectrum analyzers have higher noise floor, higher internal LO phase noise (thus can't go as low on RBW) and also lower frequency range. But there is still a lot that can be done with them!

author: chscholz
author_rank: Regular Contributor
timestamp: February 14, 2012, 06:26:41 am
content: Hi Shahriar,great nice tutorial, thank you very much!BTW, there should not be a spur at the null in the spectrum of an NRZ signal. You probably see this spur because you looking at a single ended signal.I tried to emulate what a 1.5 Gbps differential signal looks like on a 1 GHz scope. I believe your scope's sample rate is in the order of 5 GS/s? In lieu of an spectrum analyzer I used the FFT function (for this kind of application you really don't need a spectrum analyzer!). I also believe you had sin(x)/x interpolation enabled. Eye diagram is based on an acquisition length of 250 kS (which translates into just about 72 kUI).Of course you are severely bandwidth limited. The generally accepted rule of thumb is that you need to capture 5 harmonics. Here is a screenshot of the same signal as above, this time acquired with a 4 GHz scope at 40 GS/s, again there are about 72 kUI in the eye diagram.Thanks again for the great tutorial, please keep them coming!ChrisQuote from: Hugoneus on February 13, 2012, 01:39:18 pm

author: Jad.z
author_rank: Regular Contributor
timestamp: February 14, 2012, 06:33:24 pm
content: Hi ShahriarGreat VPM (value per minute), I really enjoyed this one.This is one of your best tutorials yet.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 14, 2012, 06:38:15 pm
content: Thank you Chris for your contribution.As you know spectrum analyzers offer much greater dynamic range that an oscilloscope would. And in this case, we are able to see signals up to 3GHz which I would not be able to do on my 1GHz, 5GS/s scope.The tone your are referring to could come from a few places.1) It may be due to some square-law non-linearity at the transmitter.2) Most likely it is due to clock feed through from the full-rate retimers inside the FPGA.3) Sometime a tone is used to aid the CDR on the receiver (although, not in this case).Something else for your consideration, below is an eye diagram of a 75Gb/s CMOS TIALA Retimer that I published a few years ago while I was a PhD student. The blue eye is the input and the pink eye is the output. I think it is still the fastest retimer ever reported.S. Shahramian, A.C. Carusone, S.P. Voinigescu, “An 81Gb/s, 1.2V TIALA-Retimer in Standard 65nm CMOS”, IEEE Compound Semiconductor Integrated Circuits Symposium, Technical Digest, pp. 151 - 154, Nov. 2008.

author: olsenn
author_rank: Frequent Contributor
timestamp: February 14, 2012, 06:55:13 pm
content: QuoteSuper! I will buy the Rigol DSA815 SA then... now that I know it can handle at least a modestly high input voltage, I think it should work very well for what I need. I look forward to your future videos!

author: kodon
author_rank: Contributor
timestamp: February 14, 2012, 08:03:51 pm
content: Very nice video, I find it really good value!And very impressive lab! Also your earlier videos are great. Keep posting 'em

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 15, 2012, 12:41:36 am
content: Quote from: kodon on February 14, 2012, 08:03:51 pmThanks!

author: chscholz
author_rank: Regular Contributor
timestamp: February 15, 2012, 06:19:39 am
content: Impressive work on the retimer!!!Stay warm up there in NJ and thanks so much for your videos!Quote from: Hugoneus on February 14, 2012, 06:38:15 pm