Category: eye diagram

• Eye Diagram Using LFSR

  Heya,

  Unfortunately I am back to school again, but I only have one hard class (designing op amp), and 2 garbage classes, so I probably will continue working on this.

  Last time, I managed to transmit signals over 1000 foot UTP cable. However, to check whether it was garbage signals or not we need to generate eye-diagram to analyze our signals. I had no luck creating appropriate eye-diagram due to the code I uploaded to the Arduino. I needed something that have separate clock and something random.

  Talking about random, I thought about linear–feedback shift register (LFSR). LFSR is a shift register and with a feedback of two or more “taps”. Based on the circuit schematic shown in Picture 1, I tapped bit 0 and 2. You can of course tap more than two, however, there are certain of pattern you need to follow (refer to Picture 2) that can be referred to the length of the loop.

  

  

  Moreover, LFSR would spit out pseudo-random numbers that I need for my eye-diagram testing. The previous set up with Arduino was bad because it spit out constant pattern that of course you can’t analyze whether your signal is bad or not.

  So, I built LFSR with d-flip flop (using TI SN74LS273N), and XOR gate ( HD74LS86 )

  

  Using the circuit set up from Picture 1 (a), I use function generator as a clock for the D-flip flop, and output it from pin #6 of the d-flip flop (since I’m only using 2 taps).

  anddddd there she is, the not so pretty eye-diagram (lol). But it’s a progress!

  

  

  

  October 6, 2019

  Circuit Analysis, Electrical Engineering, LFSR, RS485, Signal Integrity, XOR Gate

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• Fixed schematic and Circuit Board

  

  

  The difference between the new one and the previous schematic would be I added 4 120 ohm resistors (R1, R2, R3, R4) from the outgoing side (PIN#9 and #10) of transceiver and incoming side (PIN#12 and #11) of the transceiver.

  So far it looked like I managed to transmit 1Mbps over 1000 foot UTP cables, however at this point I can’t confirm whether the signal I transmitted was smooth or mostly jitters. I still need to figure out how to set up the eye-diagram with external trigger. Will keep you posted!.

  September 14, 2019

  Electrical Engineering, RS485, Signal Integrity, Transceiver

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• Eye Diagram on Oscilloscope

  Heya people!

  It’s been a quiet week, but I managed to review and ordered few things that I needed to get for my on going project. By next week I should be getting my DC-DC converter, Digital Isolator, RS-485 transceiver, and some passive components. I might post the detail on why or how to pick these components to make it compatible to your microcontroller (MCU).

  Further, why did I mention eye diagram on the topic? What is eye diagram?

  Since Signal Integrity is the main topic of my project, an eye-diagram, which is an oscilloscope display (literally it look like an eye), could tell us the quality of serial bit stream and it can be used to debug signal integrity issues. According to Sciencedirect, amplitude noise and timing jitter could impact our system’s performance. However, using simple Bit Error Rate (BER) measurement alone can’t tell the difference between amplitude noise and timing jitter. Eye diagram, on the other hand, provides more information regarding both issues.

  

  We can take high-bandwidth instantaneous samples of high speed digital signal and the sum of the samples (superimposing ‘0’ and ‘1’) are the eye diagram! To make it more clear, just remember those timing diagram you did in your System Verilog class. The eye diagram would look EXACTLY like one of those state diagrams!

  I was playing with my oscilloscope earlier to display the eye diagram ( I own Tektronix TBS1102B), and unlike the advanced version, I had to manually set up the closest eye diagram I can get from it (see short video below) and I was using high baud rate (2Mbps) to demonstrate the best display of eye diagram.

  void setup() {
    Serial.begin(2000000);
    pinMode(4, INPUT);
    digitalWrite(4, LOW);
   }
  
  void loop() {
      Serial.write(0xAA);
  }

  From this short clip, we can see clearly the transition state from High to Low and so on.

  If you are using the same type of Oscilloscope, this can be accomplished by running code above (connect it with your Arduino, of course), Autoset it, then pick sample (by adjusting the horizontal knob) to the non-static rising edge, then select utility > display > enable persist (to as long as you want).

  ps: I found this artwork (or maybe enlarged screenshot) on Instagram and I want one for my office! lol

  https://www.instagram.com/p/BSE7K2eg8Yu/?utm_source=ig_web_button_share_sheet

  August 16, 2019

  Electrical Engineering, eye diagram, jitter, noise, Signal Integrity, timing jitter

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