# Digital Filter Test with LFSR

Started by January 26, 2005
 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 to 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, Thomas.
 Thomas- > 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 to 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. Expected system impulse response length is one good criteria. As one example, some acoustic measurement systems use a MLS method (maximum length sequence) which is similar to LFSR. The length of non-repeating pseudo-random sequence is chosen based on the expected measurement situation. For instance, in measuring the response of a concert hall with lots of reflections and echos it's necessary to choose a long sequence -- 18 bit or more (equivalent to 5 sec at 48 kHz sampling rate). -Jeff
 Thomas From your description, it looks like you want to test a 3rd order comb (decimation??- why decimation?) filter using a test signal with flat spectrum. If you want to test a filter, why would you choose a signal which would have the characteristics as that of noise. I am unable to understand. Normally a sweep signal is chosen to observe the frequency response of a filter. Please give exact context of the problem. Do let me know. Regards arun -----Original Message----- From: Jeff Brower [mailto:] Sent: Wednesday, January 26, 2005 7:26 PM To: Thomas Patrick Cc: Subject: Re: [matlab] Digital Filter Test with LFSR Thomas- > 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 to 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. Expected system impulse response length is one good criteria. As one example, some acoustic measurement systems use a MLS method (maximum length sequence) which is similar to LFSR. The length of non-repeating pseudo-random sequence is chosen based on the expected measurement situation. For instance, in measuring the response of a concert hall with lots of reflections and echos it's necessary to choose a long sequence -- 18 bit or more (equivalent to 5 sec at 48 kHz sampling rate). -Jeff