"Abdul" <abdul_araheem@yahoo.com> wrote in message
news:ct85c8$454$1@rzsun03.rrz.uni-hamburg.de...
> Hi guys,
>
>                   It is a known fact that LFSR generate
flat spectrum, I
> choosed LFSR to test my 3rd order polyphase comb
decimation filter with
> decimation factor of 256. I am wondering what would be the
lenght of LFSR ?
> it should be 4 bit or 8 bit or 10 bit ?? All generate flat
spectrum. What
> characteristics help me in choosing the lenght of LFSR ??.
>
>   I have read few technical papers, In which LFSR is being
used in Build In
> Self Test(BIST) to test Sigma delta modulator and no where
specified the
> lenght of LFSR.

First of all, you probably want at least 31 bits unless you
really don't care about the periodicity introduced by the
LFSR.  Configurations with taps at (31,3,0), (31,6,0), and
(31,13,0) will produce maximal periods.  Second, the output
of a simple LFSR is highly predictable using the Massey
Berlekamp algorithm.  If you must to use an LFSR, I suggest
you use a configuration like the Bilateral Stop-and-Go
generator, or something similar.


>
>   I have one idea to select lenght.
>
>  1) Its depends on the how many pattern we would like to
generate for
> example 8 bit LFSR gives 2^8 -1 and 10 bit generate
2^10-1. More pattern
> would give us better results.
>
> 2) The sequence should be longer than the delay response
of filter.
>
> I am wondering whether this two points would justify in
choosing the lenght
> of LFSR.  if I can reduce lengh of LFSR by a factor of one
would help in
> reducing the area and power on ASIC chip.
>
> It would be nice if you guys forward some hints or ideas
to choose the
> lenght of LFSR.
>
> warm regards,
>
> Abdul
>
>

Hi guys,

                  It is a known fact that LFSR generate flat spectrum, I
choosed LFSR to test my 3rd order polyphase comb decimation filter with
decimation factor of 256. I am wondering what would be the lenght of LFSR ?
it should be 4 bit or 8 bit or 10 bit ?? All generate flat spectrum. What
characteristics help me in choosing the lenght of LFSR ??.

  I have read few technical papers, In which LFSR is being used in Build In
Self Test(BIST) to test Sigma delta modulator and no where specified the
lenght of LFSR.

  I have one idea to select lenght.

 1) Its depends on the how many pattern we would like to generate for
example 8 bit LFSR gives 2^8 -1 and 10 bit generate 2^10-1. More pattern
would give us better results.

2) The sequence should be longer than the delay response of filter.

I am wondering whether this two points would justify in choosing the lenght
of LFSR.  if I can reduce lengh of LFSR by a factor of one would help in
reducing the area and power on ASIC chip.

It would be nice if you guys forward some hints or ideas to choose the
lenght of LFSR.

warm regards,

Abdul