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Problem in Bandpass filter, passband reappear after half sampling frequency

Started by Henry Chang September 18, 2002
Hi all,

I am beginner in digital filter design. Currently, I am designing a
bandpass filter cut-off 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



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 cut-off 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 >
>
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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 500-1800 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 cut-off 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 DSP-Related Groups: http://www.dsprelated.com/groups.php3
> >
> > ">http://docs.yahoo.com/info/terms/
> >
> >


__________________________________________________



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 3khz-6khz.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
> 500-1800 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 cut-off 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 DSP-Related Groups:
> http://www.dsprelated.com/groups.php3
> > >
> > > ">http://docs.yahoo.com/info/terms/
> > >
> > >
> >
> >
> > __________________________________________________


__________________________________________________