Hi all, I am beginner in digital filter design. Currently, I am designing a bandpass filter cutoff at 500hz and 1.8khz, with sampling frequency 6k Hz. I use butterworth filter with order 6 according to the specification. However, when I try to plot the signal, there is passband gain between 3k and 6k Hz in addition to the band before 3k Hz. The passband gain between 3k and 6k Hz is unwanted. Is there some way to remove the gain of this part or should I redesign the filter? Thank you for your help. Best Regards Henry 

Problem in Bandpass filter, passband reappear after half sampling frequency
Started by ●September 18, 2002
Reply by ●September 19, 200220020919
There is always two identical, like in mirror, spectras between 0 and
2*pi (corresponding to sampling freq.). Check out "sampling theorem". If you don't want that band your sampling rate should be at least 12kHz. Goran  Original Message  From: Henry Chang <> To: <> Sent: Wednesday, September 18, 2002 1:01 PM Subject: [matlab] Problem in Bandpass filter, passband reappear after half sampling frequency > Hi all, > > I am beginner in digital filter design. Currently, I am designing a > bandpass filter cutoff at 500hz and 1.8khz, with sampling frequency > 6k Hz. I use butterworth filter with order 6 according to the > specification. However, when I try to plot the signal, there is > passband gain between 3k and 6k Hz in addition to the band before 3k > Hz. The passband gain between 3k and 6k Hz is unwanted. Is there some > way to remove the gain of this part or should I redesign the filter? > > Thank you for your help. > > Best Regards > Henry > > > _____________________________________ > Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer. You need to do a "reply all" if you want your answer to be distributed to the entire group. > > _____________________________________ > About this discussion group: > > To Join: > > To Post: > > To Leave: > > Archives: http://www.yahoogroups.com/group/matlab > > More DSPRelated Groups: http://www.dsprelated.com/groups.php3 > > ">http://docs.yahoo.com/info/terms/ 

Reply by ●September 20, 200220020920
This problem is inherent in any real signal. Only a complex signal can have
a single sided spectra. One more way of avoiding this band without increasing the sampling frequency is to use the hilbert pair of your filter and construct a complex signal. This will then have only the desired frequencies between 5001800 Hz. say x is the input, h1 your filter and h2 the hilbert pair of h1 then: y1 = filter(h1,1,x) y2 = filter(h2,1,x) y = y1 + j*y2; If u have used 'remez' for designing the filter 'h1' then the same can be used for designing 'h2'. All u have to do is to add one extra argument at the end ('hilbert'): h1 = remez(..,..,..,.........,); h2 = remez(..,..,..,.........,'hilbert'); priyank  Goran Stepanovic <> wrote: > There is always two identical, like in mirror, spectras between 0 and 2*pi > (corresponding to sampling freq.). Check out "sampling theorem". If you > don't want that band your sampling rate should be at least 12kHz. > Goran > > > Hi all, > > > > I am beginner in digital filter design. Currently, I am designing a > > bandpass filter cutoff at 500hz and 1.8khz, with sampling frequency > > 6k Hz. I use butterworth filter with order 6 according to the > > specification. However, when I try to plot the signal, there is > > passband gain between 3k and 6k Hz in addition to the band before 3k > > Hz. The passband gain between 3k and 6k Hz is unwanted. Is there some > > way to remove the gain of this part or should I redesign the filter? > > > > Thank you for your help. > > > > Best Regards > > Henry > > > > > > > > > > _____________________________________ > > Note: If you do a simple "reply" with your email client, only the author > of this message will receive your answer. You need to do a "reply all" if > you want your answer to be distributed to the entire group. > > > > _____________________________________ > > About this discussion group: > > > > To Join: > > > > To Post: > > > > To Leave: > > > > Archives: http://www.yahoogroups.com/group/matlab > > > > More DSPRelated Groups: http://www.dsprelated.com/groups.php3 > > > > ">http://docs.yahoo.com/info/terms/ > > > > __________________________________________________ 

Reply by ●October 11, 200220021011
if you sample a signal at 6khz this implies that your input signal is banlimited to 3khz. there fore there will be no problem if there is an extra band at 3khz6khz.this duplication is due to the nyquist rate sampling.( reflection about fs/2). if your input signal bandwidth is more then sample the signal accordingly (twice). nitthilan  Priyank Saxena <> wrote: > This problem is inherent in any real signal. Only a > complex signal can have a > single sided spectra. > One more way of avoiding this band without > increasing the sampling frequency is > to use the hilbert pair of your filter and construct > a complex signal. This > will then have only the desired frequencies between > 5001800 Hz. > > say x is the input, h1 your filter and h2 the > hilbert pair of h1 then: > y1 = filter(h1,1,x) > y2 = filter(h2,1,x) > y = y1 + j*y2; > > If u have used 'remez' for designing the filter 'h1' > then the same can be used > for designing 'h2'. All u have to do is to add one > extra argument at the end > ('hilbert'): > h1 = remez(..,..,..,.........,); > h2 = remez(..,..,..,.........,'hilbert'); > > priyank >  Goran Stepanovic <> wrote: > > There is always two identical, like in mirror, > spectras between 0 and 2*pi > > (corresponding to sampling freq.). Check out > "sampling theorem". If you > > don't want that band your sampling rate should be > at least 12kHz. > > Goran > > > > > Hi all, > > > > > > I am beginner in digital filter design. > Currently, I am designing a > > > bandpass filter cutoff at 500hz and 1.8khz, > with sampling frequency > > > 6k Hz. I use butterworth filter with order 6 > according to the > > > specification. However, when I try to plot the > signal, there is > > > passband gain between 3k and 6k Hz in addition > to the band before 3k > > > Hz. The passband gain between 3k and 6k Hz is > unwanted. Is there some > > > way to remove the gain of this part or should I > redesign the filter? > > > > > > Thank you for your help. > > > > > > Best Regards > > > Henry > > > > > > > > > > > > > > > _____________________________________ > > > Note: If you do a simple "reply" with your email > client, only the author > > of this message will receive your answer. You > need to do a "reply all" if > > you want your answer to be distributed to the > entire group. > > > > > > _____________________________________ > > > About this discussion group: > > > > > > To Join: > > > > > > To Post: > > > > > > To Leave: > > > > > > Archives: > http://www.yahoogroups.com/group/matlab > > > > > > More DSPRelated Groups: > http://www.dsprelated.com/groups.php3 > > > > > > ">http://docs.yahoo.com/info/terms/ > > > > > > > > > > > > __________________________________________________ __________________________________________________ 