[SI-LIST] Re: purpose of 8b/10b encoding

  • From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 13 Sep 2001 20:02:22 -0500


If you'd like to 'see' the difference between a 
truly-random and an 8b10-encoded bit stream, I
recommend using a pseudo-random pattern generator
and spectrum analyzer.  Note: the pattern generator
and spec-an can be either real test-lab hardware or
can be in software (via matlab or other).

* Looking at random digital signal

Generate a serial bit stream using a pseudo-random
generator with a long-length.  Most bit error rate
testers have 2^32-1 code (which allow up to 32 consecutive
1's or 0's), or you can generate one in code. 

Pump the bit stream into a spectrum analyzer.

* Looking at 8b10b-encoded signal

For comparison sake, use the same exact pseudo-random
pattern generator with the same settings.  Put that
into a deserializer (i.e., 1:8 demux), put that into
an 8b10-encoder, then put the 10 bits into a serializer
(i.e., 10:1 mux). 

Put the resulting serial bit stream into a spec-an.

If all goes well, you should be able to 'see' two
distinctive differences:

1) the fundamental frequency of the 8b10-encoded
stream should be 25% higher than the raw
pseudo-random stream (note the frequency locations
of the peaks/valleys); and

2) the magnitude of the spectrum at the lower frequencies
of the 8b10-encoded stream is lower.

Hope this helps.

Sorry, I do not have plots to share.  If you are
able to produce some through C/MatLab, I'd appreciate
getting a copy.


> Okay the que is:
> If I am to write a C/Matlab code for converting a *raw* data 
> to 8b10b encoded data what exactly I need to do? or in other 
> words, Pat how did you
> *look at* the spetrum of raw and encoded data. 
> How did you generate both the streams?
> I hope C/Matlab are sufficient to get a feel for it.
> or was is it back-of-the-envelope calculation.
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