High-Q switched-capacitor BandPass Filter?

Is there such a thing as a High-Q switched-capacitor BandPass Filter
whose center frequency can be "tuned" +- 20% synchronously with its
switching clock?

I need something that would take in a ~45Hz periodic signal with lots of 
higher odd-order harmonics, and spit out only the fundamental minus the
harmonics, with no or predictable phase shift. The Q needs to be high 
enough to attenuate the second and subsequent harmonics by more than 40db.

I think that this function could also be done by just a low pass, but
I'm worried about the phase shift that a low pass might introduce.

Extra points if the filter could select the secon or third harmonics
on command, relative to the same switching clock.

The clock could be something like 32X to 256X the fundamental.

Is a power of two for the clock preferable? (vs e.g. 50X)

MikeM

"mikem" <mikem@bogus.adr> wrote in message
news:c7dmcu$qv9$1@coward.ks.cc.utah.edu...
> I think that this function could also be done by just a low pass, but
> I'm worried about the phase shift that a low pass might introduce.

Are you suggesting that bandpass filters don't introduce phase shift, or are
you suggesting that switched-capacitor filter don't introduce phase shift?
I'm no expert, but I don't think either suggestion is correct.

Walter Harley wrote:
> "mikem" <mikem@bogus.adr> wrote in message
> news:c7dmcu$qv9$1@coward.ks.cc.utah.edu...
> 
>>I think that this function could also be done by just a low pass, but
>>I'm worried about the phase shift that a low pass might introduce.
> 
> 
> Are you suggesting that bandpass filters don't introduce phase shift, or are
> you suggesting that switched-capacitor filter don't introduce phase shift?
> I'm no expert, but I don't think either suggestion is correct.


Say that I'm clocking a 100X 45Hz switched capacitor bandpass filter at 4500Hz.
Is not the frequency at which the magnitude is max and the phase shift is zero
45Hz? (I plan to make the filter clock an exact integer multiple of the freq
that I'm trying to filter....

MikeM

On Thu, 06 May 2004 09:42:21 -0600, mikem <mikem@bogus.adr> wrote:

>Is there such a thing as a High-Q switched-capacitor BandPass Filter
>whose center frequency can be "tuned" +- 20% synchronously with its
>switching clock?

Yes. This is easily done with an MF10/LMF100-type general-purpose
filter chip and a few resistors... see the National app notes. The
LMF100 is a more modern poly-gate version of the MF10, I think.

>I need something that would take in a ~45Hz periodic signal with lots of 
>higher odd-order harmonics, and spit out only the fundamental minus the
>harmonics, with no or predictable phase shift. The Q needs to be high 
>enough to attenuate the second and subsequent harmonics by more than 40db.

Sounds feasible. Make sure you have a simple RC or Sallen-Key lowpass
ahead of the filter to prevent aliasing, and another after to filter
out clock glitches.

>I think that this function could also be done by just a low pass, but
>I'm worried about the phase shift that a low pass might introduce.
>
>Extra points if the filter could select the secon or third harmonics
>on command, relative to the same switching clock.

Just rubber-band the clock.

>The clock could be something like 32X to 256X the fundamental.
>
>Is a power of two for the clock preferable? (vs e.g. 50X)

Not really. Doesn't matter much, but higher is better as regards
aliasing. I think the MF10 bandpass design allows arbitrary clock-cf
ratios.

Note that SCFs are a bit noisy, and are *very* sensitive to noise on
the power rails. They will cheerfully alias high-frequency noise back
into the passband.

John



>
>MikeM
>
>

John Larkin wrote:

> On Thu, 06 May 2004 09:42:21 -0600, mikem <mikem@bogus.adr> wrote:

>>Extra points if the filter could select the secon or third harmonics
>>on command, relative to the same switching clock.
> 
> Just rubber-band the clock.

You got me on that one???

MikeM

hmm,.
  well i am not 100% sure what your doing how ever.
i can tell you that i once made an inductive tuned filter
using a secondary field to be shunted which effects the
primary field over all effect on the reasonant point with out
coming into direct contact with the primary signal.
    another way is if you want to deal with caps only you can
  use a diode switch current path to tailer the cap via some
DC current.
  that would be 2 diodes back to back with current flowing in both 
directions.
  or to really do it rite, take a PIC or Atmel 8051 type mirco with
lots of memory and a ADC  converter using a FFT (fast fourier 
Transform)! you will have a DSP filter.


mikem wrote:

> Is there such a thing as a High-Q switched-capacitor BandPass Filter
> whose center frequency can be "tuned" +- 20% synchronously with its
> switching clock?
> 
> I need something that would take in a ~45Hz periodic signal with lots of 
> higher odd-order harmonics, and spit out only the fundamental minus the
> harmonics, with no or predictable phase shift. The Q needs to be high 
> enough to attenuate the second and subsequent harmonics by more than 40db.
> 
> I think that this function could also be done by just a low pass, but
> I'm worried about the phase shift that a low pass might introduce.
> 
> Extra points if the filter could select the secon or third harmonics
> on command, relative to the same switching clock.
> 
> The clock could be something like 32X to 256X the fundamental.
> 
> Is a power of two for the clock preferable? (vs e.g. 50X)
> 
> MikeM
> 
> 
> 

"MikeM" <trashcan@yahoo.com> wrote in message news:c7e3hc$637$1@coward.ks.cc.utah.edu...
> John Larkin wrote:
>
> > On Thu, 06 May 2004 09:42:21 -0600, mikem <mikem@bogus.adr> wrote:
>
> >>Extra points if the filter could select the secon or third harmonics
> >>on command, relative to the same switching clock.
> >
> > Just rubber-band the clock.
>
> You got me on that one???
>
> MikeM

Duct tape?

SioL

"MikeM" <trashcan@yahoo.com> wrote in message
news:c7e1lu$52b$1@coward.ks.cc.utah.edu...
> Walter Harley wrote:
> > "mikem" <mikem@bogus.adr> wrote in message
> > news:c7dmcu$qv9$1@coward.ks.cc.utah.edu...
> >
> >>I think that this function could also be done by just a low pass, but
> >>I'm worried about the phase shift that a low pass might introduce.
> >
> >
> > Are you suggesting that bandpass filters don't introduce phase shift, or
are
> > you suggesting that switched-capacitor filter don't introduce phase
shift?
> > I'm no expert, but I don't think either suggestion is correct.
>
>
> Say that I'm clocking a 100X 45Hz switched capacitor bandpass filter at
4500Hz.
> Is not the frequency at which the magnitude is max and the phase shift is
zero
> 45Hz? (I plan to make the filter clock an exact integer multiple of the
freq
> that I'm trying to filter....

Here's my understanding; as I said, I'm no filter expert, so perhaps someone
who knows what they're talking about will correct me.

A switched-capacitor filter is just a filter design that uses switched
capacitors to implement the integrators that are part of an active filter
design such as biquad or state-variable or whatever.  There are two reasons
to do that: one is that the integrator gain (and thus the filter frequency)
is controlled by the switching frequency; the other is that it's easier to
make precision capacitor ratios on silicon than it is to make precision
capacitors and resistors.  And for low frequencies, you don't need
high-value components, you just lower the switching frequency.

But either way, you've still got the same biquad or state-variable or
whatever filter, it's just that it happens to be implemented using switched
capacitors.  You still have to decide on a filter characteristic: Bessel,
Chebyshev, whatever.  That characteristic, combined with the number of
poles, determines the phase shift.  And so far as I know, all filters have
nonzero phase shift; they have to, because it takes finite time to determine
frequency.

Walter Harley wrote:

> "MikeM" <trashcan@yahoo.com> wrote in message
> news:c7e1lu$52b$1@coward.ks.cc.utah.edu...
> 
>>Walter Harley wrote:
>>
>>>"mikem" <mikem@bogus.adr> wrote in message
>>>news:c7dmcu$qv9$1@coward.ks.cc.utah.edu...
>>>
>>>
>>>>I think that this function could also be done by just a low pass, but
>>>>I'm worried about the phase shift that a low pass might introduce.
>>>
>>>
>>>Are you suggesting that bandpass filters don't introduce phase shift, or
> 
> are
> 
>>>you suggesting that switched-capacitor filter don't introduce phase
> 
> shift?
> 
>>>I'm no expert, but I don't think either suggestion is correct.
>>
>>
>>Say that I'm clocking a 100X 45Hz switched capacitor bandpass filter at
> 
> 4500Hz.
> 
>>Is not the frequency at which the magnitude is max and the phase shift is
> 
> zero
> 
>>45Hz? (I plan to make the filter clock an exact integer multiple of the
> 
> freq
> 
>>that I'm trying to filter....
> 
> 
> Here's my understanding; as I said, I'm no filter expert, so perhaps someone
> who knows what they're talking about will correct me.
> 
> A switched-capacitor filter is just a filter design that uses switched
> capacitors to implement the integrators that are part of an active filter
> design such as biquad or state-variable or whatever.  There are two reasons
> to do that: one is that the integrator gain (and thus the filter frequency)
> is controlled by the switching frequency; the other is that it's easier to
> make precision capacitor ratios on silicon than it is to make precision
> capacitors and resistors.  And for low frequencies, you don't need
> high-value components, you just lower the switching frequency.
> 
> But either way, you've still got the same biquad or state-variable or
> whatever filter, it's just that it happens to be implemented using switched
> capacitors.  You still have to decide on a filter characteristic: Bessel,
> Chebyshev, whatever.  That characteristic, combined with the number of
> poles, determines the phase shift.  And so far as I know, all filters have
> nonzero phase shift; they have to, because it takes finite time to determine
> frequency.
> 
> 

If you're picking out a carrier a bandpass filter will have zero phase 
error at the carrier, because the filter is "anticipating" the carrier 
as much as it is "sensing" it.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Walter Harley wrote:
> 
> "MikeM" <trashcan@yahoo.com> wrote in message
> news:c7e1lu$52b$1@coward.ks.cc.utah.edu...
> > Walter Harley wrote:
> > > "mikem" <mikem@bogus.adr> wrote in message
> > > news:c7dmcu$qv9$1@coward.ks.cc.utah.edu...
> > >
> > >>I think that this function could also be done by just a low pass, but
> > >>I'm worried about the phase shift that a low pass might introduce.
> > >
> > >
> > > Are you suggesting that bandpass filters don't introduce phase shift, or
> are
> > > you suggesting that switched-capacitor filter don't introduce phase
> shift?
> > > I'm no expert, but I don't think either suggestion is correct.
> >
> >
> > Say that I'm clocking a 100X 45Hz switched capacitor bandpass filter at
> 4500Hz.
> > Is not the frequency at which the magnitude is max and the phase shift is
> zero
> > 45Hz? (I plan to make the filter clock an exact integer multiple of the
> freq
> > that I'm trying to filter....
> 
> Here's my understanding; as I said, I'm no filter expert, so perhaps someone
> who knows what they're talking about will correct me.
> 
> A switched-capacitor filter is just a filter design that uses switched
> capacitors to implement the integrators that are part of an active filter
> design such as biquad or state-variable or whatever.  There are two reasons
> to do that: one is that the integrator gain (and thus the filter frequency)
> is controlled by the switching frequency; the other is that it's easier to
> make precision capacitor ratios on silicon than it is to make precision
> capacitors and resistors.  And for low frequencies, you don't need
> high-value components, you just lower the switching frequency.
> 
> But either way, you've still got the same biquad or state-variable or
> whatever filter, it's just that it happens to be implemented using switched
> capacitors.  You still have to decide on a filter characteristic: Bessel,
> Chebyshev, whatever.  That characteristic, combined with the number of
> poles, determines the phase shift.  And so far as I know, all filters have
> nonzero phase shift; they have to, because it takes finite time to determine
> frequency.

I believe you are confusing group delay with steady state phase
shift.  I think the OP is generally correct that most band pass
filters have nearly zero phase shift (or an inversion) at center band,
and I think this also applies to switched capacitor implementations.

-- 
John Popelish