Sample rate conversion doubt

"jaac" <jaime.aranguren@ieee.org> wrote in message 
news:1104773346.953976.218710@f14g2000cwb.googlegroups.com...
> Hi Phil,
>
> Thanks for the reply.
>
> You are right. I would get aliasing. Thank you for pointing this out.
>
> To be honest the 200Hz Fs comes from originally having the carrier at
> 50 Hz in the IEC document, but this won't be my case. If I stay without
> the sample rate conversion, and sampling directly at low frequency, I'd
> better change it to 240Hz, and I'll have to redesign my filters (I'm
> lazy to do that).
>
> More interesting, I'll go for the decimation after squaring at 8kHz.
> So, my bandpass filter, from 0.05Hz to 35Hz @ 200Hz will effectively
> reject the aliased signals, I won't modify the original signal's shape
> (important because my information is there), and more important, I'll
> only need one ADC per channel, instead of two (one sampling at 8kHz for
> spectral analysis, the other one sampling at 200Hz for waveshape
> fluctuation analysis).
>
> Thank you very much!
>
> JaaC
>
Glad to have been of help.  Won't you have to re-design your filter (0.05Hz 
to 35Hz @ 200 Hz) to work at 8KHz?  If you just feed the 8KHz samples in, I 
suspect it will filter from 0.4Hz to 280Hz

   Best wishes,
   --Phil 

Philip Martel wrote:

> Glad to have been of help.  Won't you have to re-design your filter
(0.05Hz
> to 35Hz @ 200 Hz) to work at 8KHz?  If you just feed the 8KHz samples
in, I
> suspect it will filter from 0.4Hz to 280Hz
>
>    Best wishes,
>    --Phil

Hi Phil,

No. The signal flow would be like this:

Samples @ 8kHz -> Squaring (AM demodulation) @ 8kHz -> Decimation by 40
(New Fs: 200 Hz) -> Bandpass filtering (0.05Hz - 35 Hz) @ 200Hz ->
Other processing @ 200 Hz. What's wrong here?

Regards,

JaaC

Philip Martel wrote:

> Glad to have been of help.  Won't you have to re-design your filter
(0.05Hz
> to 35Hz @ 200 Hz) to work at 8KHz?

The idea is to have them working at 200 Hz, not at 8kHz. Specifically,
to ease the design of the filters.

JaaC