In article <UNGdnfi04MejUZ3YnZ2dnUVZ_u2dnZ2d@rcn.net>, Jerry Avins <jya@ieee.org> wrote:

>pwaiaung wrote:
>> Hello everyone,
>>
>> I want to design a real time DSP system.Band of interest extends from o to
>> 4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
>>
>> What I want to know is how I can estimate the minimum stopband attenuation
>> for the anti-aliasing filter, minimum sampling frequency and The level of
>> the aliasing error relative to signal level in the passband for the
>> estimated Amin and Fs.
>
>The sample rate needs to be known in order to calculate those data. What
>is the Butterworth filter for? is it the anti-alias filter? If so, I
>find it odd that its order is specified without reference to the rest of
>the system.
>
>Jerry

Something else, the Butterworth response in monotonic so if you know at what
frequency you need, the minimum stopband attentuation will be whatever it is
at that frequency. When you get well above curoff (Fc where the response is
down three dB), the attentuation approaches 6 dB per octave per pole so at
most you get 18 dB per octave. In the octave right above cutoff, you might
get 3 dB per pole so you might be 9 dB down at 2 Fc.
Are you doing this filtering digitally?

Reply by robert bristow-johnson●September 9, 20062006-09-09

listen:
first, for the specs you mentioned, you can buy a cheap (or maybe
they're not so cheap anymore, i dunno) little DSP eval board from a
manufacturer like Analog or TI or someone else, or a board from a guy
like Al here. if the computer is old enough, if you can find an old
Turtle Beach Multisound or Tahiti or Fiji card, that would easily
satisfy your spec (IIRC they go down to DC). if you want a 4 kHz
bandwidth and a Butterworth rolloff, you can program that into the DSP.
but i don't understand the need to build this from scratch using a 12
bit A/D and analog anti-aliasing filter.
r b-j
pwaiaung wrote:

> >pwaiaung wrote:
> >> Hello everyone,
> >>
> >> I want to design a real time DSP system.Band of interest extends from o
> to
> >> 4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
> >>
> >> What I want to know is how I can estimate the minimum stopband
> attenuation
> >> for the anti-aliasing filter, minimum sampling frequency and The level
> of
> >> the aliasing error relative to signal level in the passband for the
> >> estimated Amin and Fs.
> >
> >The sample rate needs to be known in order to calculate those data. What
>
> >is the Butterworth filter for? is it the anti-alias filter? If so, I
> >find it odd that its order is specified without reference to the rest of
>
> >the system.
> >
> >Jerry
> >--
> >Engineering is the art of making what you want from things you can get.
> >=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=

> >
> The 3rd order butterworth filter is the anti-aliasing filter.
> we can estimate the sampling frequency as the twice of passband frequency.
> So smapling frequency should be at least 2*4kHz=3D8kHz. Am I right?
>
> To estimate minimum stopband attenuation, we need to assume that the
> maximum amplitude of analog signal is 1V.
> Quantization error is between +/- q/2.
> Since we use 12bit ADC, quantization step size to quantize the given
> analog signal is 1/2(pow)12=3D244.14 micro volt.
>
> So we need to attenuate the analog signal to be less than this
> quantization error at stopband frequency Fs/2.
> So the minimum stopband attenuation should be -72.25dB. Am I right?
>
> I don't know how to find the level of aliasing error relative to signal
> level in passband for the estimated Amin and Fs min.
>=20
> Who can help me?

Reply by Jerry Avins●September 8, 20062006-09-08

pwaiaung wrote:

>> pwaiaung wrote:
>>> Hello everyone,
>>>
>>> I want to design a real time DSP system.Band of interest extends from o
> to
>>> 4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
>>>
>>> What I want to know is how I can estimate the minimum stopband
> attenuation
>>> for the anti-aliasing filter, minimum sampling frequency and The level
> of
>>> the aliasing error relative to signal level in the passband for the
>>> estimated Amin and Fs.
>> The sample rate needs to be known in order to calculate those data. What
>
>> is the Butterworth filter for? is it the anti-alias filter? If so, I
>> find it odd that its order is specified without reference to the rest of
>
>> the system.
>>
>> Jerry
>> --
>> Engineering is the art of making what you want from things you can get.
>> �����������������������������������������������������������������������
>>
> The 3rd order butterworth filter is the anti-aliasing filter.

This must be an academic problem. Why else would the anti-alias filter
be specified without reference to the rest of the problem?

> we can estimate the sampling frequency as the twice of passband frequency.
> So smapling frequency should be at least 2*4kHz=8kHz. Am I right?

You can estimate the sampling frequency as twice the bandwidth, but you
can't build a system that way. With a 9th-order Chebychef filter, you
might go as low as 2.1 (just guessing).

> To estimate minimum stopband attenuation, we need to assume that the
> maximum amplitude of analog signal is 1V.

How is that related? Suppose it were 100v? (Hint: it's all relative to
full scale.)

> Quantization error is between +/- q/2.
> Since we use 12bit ADC, quantization step size to quantize the given
> analog signal is 1/2(pow)12=244.14 micro volt.

I see that you want to keep the alias components below the quantization
level. That's a useful but not universal criterion.

> So we need to attenuate the analog signal to be less than this
> quantization error at stopband frequency Fs/2.

If the stopband starts at Fs/2, where does the passband end? Fs/3?
That's why you need to sample faster than your estimate.

> So the minimum stopband attenuation should be -72.25dB. Am I right?
>
> I don't know how to find the level of aliasing error relative to signal
> level in passband for the estimated Amin and Fs min.
>
> Who can help me?

Finding the right number for the wrong idea isn't your problem here.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Reply by pwaiaung●September 8, 20062006-09-08

>pwaiaung wrote:
>> Hello everyone,
>>
>> I want to design a real time DSP system.Band of interest extends from o

to

>> 4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
>>
>> What I want to know is how I can estimate the minimum stopband

attenuation

>> for the anti-aliasing filter, minimum sampling frequency and The level

of

>> the aliasing error relative to signal level in the passband for the
>> estimated Amin and Fs.
>
>The sample rate needs to be known in order to calculate those data. What

>is the Butterworth filter for? is it the anti-alias filter? If so, I
>find it odd that its order is specified without reference to the rest of

>the system.
>
>Jerry
>--
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������
>

The 3rd order butterworth filter is the anti-aliasing filter.
we can estimate the sampling frequency as the twice of passband frequency.
So smapling frequency should be at least 2*4kHz=8kHz. Am I right?
To estimate minimum stopband attenuation, we need to assume that the
maximum amplitude of analog signal is 1V.
Quantization error is between +/- q/2.
Since we use 12bit ADC, quantization step size to quantize the given
analog signal is 1/2(pow)12=244.14 micro volt.
So we need to attenuate the analog signal to be less than this
quantization error at stopband frequency Fs/2.
So the minimum stopband attenuation should be -72.25dB. Am I right?
I don't know how to find the level of aliasing error relative to signal
level in passband for the estimated Amin and Fs min.
Who can help me?

Reply by Jerry Avins●September 7, 20062006-09-07

pwaiaung wrote:

> Hello everyone,
>
> I want to design a real time DSP system.Band of interest extends from o to
> 4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
>
> What I want to know is how I can estimate the minimum stopband attenuation
> for the anti-aliasing filter, minimum sampling frequency and The level of
> the aliasing error relative to signal level in the passband for the
> estimated Amin and Fs.

The sample rate needs to be known in order to calculate those data. What
is the Butterworth filter for? is it the anti-alias filter? If so, I
find it odd that its order is specified without reference to the rest of
the system.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Reply by pwaiaung●September 7, 20062006-09-07

Hello everyone,
I want to design a real time DSP system.Band of interest extends from o to
4kHz.I will use 12 bit ADC and 3rd order butterworth lowpass filter.
What I want to know is how I can estimate the minimum stopband attenuation
for the anti-aliasing filter, minimum sampling frequency and The level of
the aliasing error relative to signal level in the passband for the
estimated Amin and Fs.