Tutorial on Passive Filters, Data Transmission and Equalization

EyeDiagram

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 Virtex 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).

I would like to acknowledge my friend and colleague, Dr. Timo Pfau for his expertise in setting up and configuring the Xilinx FPGA board.

 

39 comments

  1. BillW says:

    Wow! I only meant to watch for a few minutes, but just couldn’t pull myself away. Outstanding! Thank you.

  2. Serhat says:

    Thank you very much indeed for all the videos very well educative

  3. Sylvain says:

    So when is the next one ? It’s been 3 month already :)

  4. nick says:

    Amazing videos. Could you do a video talking about square waves? I have questions about them and stuff taht i cant understand
    The frequency of the square wave vs the frequency of the harmonics… what real whats not?
    How can i say i have a square wave of X frequency if there are other frequencies inside, and also very high ones?

    • Shahriar says:

      All the frequencies are real. The frequency with the largest power is the fundamental frequency of the square wave. If the other frequencies were not there, you would not get a square wave, you would get a sine wave!

  5. Holger says:

    Really really coooool. Thank you. Blew my head off.

  6. Dave C says:

    Shahriar, I’m very glad you are back to posting videos. You are doing the world a great service by creating this type of high quality of content and offering them for free. I love the mix of theory and experiments in your approach. I’m a mechanical engineer by formal training. I am anxious to learn more about electrical engineering. This is particularly true now since I got a job at a company that designs and manufactures a lot of RF equipment. I just wanted to express my deep gratitude to you and to encourage you to continue. Thanks!

  7. Daniel says:

    Awesome videos. Khan Academy, MIT OCW, Stanford, your videos. I love the internet! Education for everyone.

    Thank you!

  8. Alex says:

    Well done. How is it that the backplane traces (conductors) degrade the signal so much but those inter-connecting cables do not? Why not simply construct the backplane using the same principles/properties of those white wires you use to hook things up with? Also, about how much does one of those active probes go for? Any links to so-called “ghetto” active probes?

    • Shahriar says:

      The backplane traces are Copper lines over a ground plane and in between is the board dielectric. This structure forms a transmission line called a microstrip. Due to the losses of the Copper line, the type of dielectric material and the exposure of the line to space, the microstrip’s high frequency performance is limited. The white SMA cables you see comprise an inner Copper conductor, surrounded by a dielectric (in this case teflon) and then surrounded again by the ground. Teflon is a much better dilectric, and the whole coax cable structure has better frequency response. You can get coax cables that work up to 110GHz. SMA is good up to about 18GHz before the connectors’ other modes of propagation kicks in. You can’t make PCB traces using coax cables. (You can, but then it will not be a PCB!)

      Active probe can cost thousands. But a reasonable one can be had for a couple of hundred.

  9. Slag7 says:

    Great job, as always! I look forward to trying out a couple of the experiments.
    I’d like to see more about that Rigol DM3068 DMM. I’m in the market for a 6.5 digit, and the price seems ok, if it performs well.
    My DP1308A has been nice, your review led me to that purchase.
    Looking forward to your next video.

  10. robrenz says:

    Excellent mix of theory and hands on. Getting me to see the backplane and other things as a form of filter was a light bulb moment.

  11. jbremnant says:

    Awesome post, just like others preceding it. I also want to encourage you to keep posting your tutorials / experiments. I am learning tons, just by watching your videos.

    Also, would it be possible for you to recommend reading materials or online resources for topics you cover? It will help for folks that want to dig into further details on the material.

    Thanks!

  12. Stephen Henderson says:

    At my age and point in my career, I doubt that I’d ever have a chance to be exposed to these amazing topics and such a genuinely wonderful treatment of the topics if not for your videos. I don’t know if the world is a better place with these videos, but my world sure is. This perspective shift from time-domain to frequency-domain is just mind-blowing. I want to start placing all data in my life into a spectrum analyzer. Please continue this work. Finally: I think that the equipment review & applications method is a great foundation for structuring these videos. Amazing work here, thank you.

  13. Ed says:

    I am fantastically pleased you’re back. These videos are among the best pedagogic content on the web. Please keep them coming!

  14. madengr says:

    Wow! Great stuff. Now I have to watch the other videos. Was the Tyco backplane differential pairs? Looks like the Xilinx board was.

  15. [...] The real focus of the video is high speed data communications, getting up into the GHz per second range. [Shahriar] covers filtering techniques from simple RC low pass filters to pretty complex microwave filters. Explaining frequency and time domain measurements of a 1.5Gbps signal through a low bandwidth channel. He also shows how equalization can be used to overcome low bandwidth limitations. [...]

  16. Dale says:

    Your Videos are Fantastic, please keep them coming.

  17. Tony says:

    I’m a EE and I think your videos are fantastic! Do you own all of the equipment?

  18. [...] The real focus of the video is high speed data communications, getting up into the GHz per second range. [Shahriar] covers filtering techniques from simple RC low pass filters to pretty complex microwave filters. Explaining frequency and time domain measurements of a 1.5Gbps signal through a low bandwidth channel. He also shows how equalization can be used to overcome low bandwidth limitations. [...]

  19. Georg says:

    Great stuff, long time ago since i studied this, but given your really good explainations (which teachers would have done this back then) all comes back to mind pretty quickly. hope to see more of that kind of videos ! ;)

  20. raul says:

    Wonderful!!! I learnt so many things with your video! Keep them coming!

  21. Alex says:

    You’re the best. Thank you, this was well worth the wait. What I especially like is that you go into the fine details of theory, which very few people do. Typically you either see entry-level tutorials or, at the opposite extreme, heavy loads of theory. Your tutorials are pretty well balanced, although I wouldn’t mind seeing some extra math in your videos occasionally. Thanks, watching your videos is a great educational pleasure.

  22. MikeK says:

    He’s back! Thanks, Shahriar.

  23. [...] The real focus of the video is high speed data communications, getting up into the GHz per second range. [Shahriar] covers filtering techniques from simple RC low pass filters to pretty complex microwave filters. Explaining frequency and time domain measurements of a 1.5Gbps signal through a low bandwidth channel. He also shows how equalization can be used to overcome low bandwidth limitations. [...]

  24. Bartek says:

    Really nice tutorial, interesting topic, I would like see more of them. And if you could recommend some sort of a further reading materials for every of your videos for people who would like to expand their knowledge on the subject, I think about some application notes maybe, that would be very helpful.

  25. Chet says:

    If a 15MHz square wave was passed through the filter wouldn’t it just show a 15MHz sine wave i.e. the first term of the fourier series?

    • Shahriar says:

      Yes, it would! What if it were 7.5MHz?

      • AlexB says:

        An attenuated sine wave at 15MHz, i.e. the second harmonic?

        • Shahriar says:

          Are you sure? It is a trick question! What does the spectrum of an ideal 7.5MHz square wave look like?

          • AlexB says:

            Ah, then nothing, since a square wave has only odd harmonics.

          • mechatrommer says:

            based on fourier its constituted of DC component (if it exists) + 7.5MHz fundamental sine + 15MHz 1st sine harmonics + 7.5 x N MHz sine where constitutes infinitum of (N – 1) harmonics with different amplitude (even numbered harmonics = zero amplitude). but well its subjective, it can be represented with composition of cosine waves instead of sine. fourier made it simpler with sine. too much theory in you? :D a big salute to Mr Shariar. i agree with others, well worth a wait! cant i register in your forum yet? last time i cant :(

          • Shahriar says:

            Yup!

  26. Ondre says:

    One word: Awesome!

    I really, really liked the video! Anyhow, it makes me a little sad that I didn’t learn so much about all of this at my university.

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