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

"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

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

"jaac" <jaime.aranguren@ieee.org> wrote in message 
news:1104711708.340093.186300@f14g2000cwb.googlegroups.com...
> Hi Tim,
>
> Thanks for prompt reply.
>
> To be precise, my project is related to the measurement of the
> phenomena named "flicker", which consists of amplitude variations on
> the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
> precisely in the envelope of the carrier, which here is 60 Hz (possibly
> with fluctuations, of course).
>
> As stated on the IEC standard, the demodulation is done by squaring,
> then some filtering is performed, to eliminate DC component and
> components higher than Fs/2. And then further filtering and analysis of
> the data, and storage.
If I understand this, you are digitizing some data at 8KHz, then squaring 
it.  The data is a 60Hz signal with some modulation on it.
When you square the signal, you will get:
a DC term (level depends on carrier strength)
a low frequency term due to modulation (I think this is what you are 
interested in)
the original 60 Hz signal with it's modulation
a signal at 120 Hz with various modulation signals

If the Fs you mention above is the original 8 KHz, then you will have the 60 
Hz and 120 Hz signals.  If the Fs you mention is the 200 Hz, then the 
signals around 120 Hz will be aliased to around 20Hz and may fall into the 
passband of your filter

   Best wishes,
   --Phil Martel
>
> Your idea of upsampling before demodulation, excuse me, is not a good
> one. For my case, it would be better to do all of the downsampling
> after the demodulation. I know that it is the almost the same as you
> suggested, it is just locating the sample rate conversion to the place
> where it is more benefical. I bet that's what you had in your mind.
>
> The idea of synchronous AM demodulation seems interesting. Could you
> please elaborate on the subject, or point me to information about it?
> Kindest regards,
>
> JaaC
>

jaac wrote:

> Hi Tim,
> 
> Thanks for prompt reply.
> 
> To be precise, my project is related to the measurement of the
> phenomena named "flicker", which consists of amplitude variations on
> the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
> precisely in the envelope of the carrier, which here is 60 Hz (possibly
> with fluctuations, of course).
> 
> As stated on the IEC standard, the demodulation is done by squaring,
> then some filtering is performed, to eliminate DC component and
> components higher than Fs/2. And then further filtering and analysis of
> the data, and storage.
> 
> Your idea of upsampling before demodulation, excuse me, is not a good
> one. For my case, it would be better to do all of the downsampling
> after the demodulation. I know that it is the almost the same as you
> suggested, it is just locating the sample rate conversion to the place
> where it is more benefical. I bet that's what you had in your mind.
> 
> The idea of synchronous AM demodulation seems interesting. Could you
> please elaborate on the subject, or point me to information about it?
> Kindest regards,
> 
> JaaC
> 
Sorry, I was assuming that you were forced into downsampling first -- if 
you can, doing your demodulation first would be best, of course.

Google on "synchronous AM" to get appropriate information -- it's a 
well-known method for high quality AM demodulation.

Given that you're working off of a specification it would probably be 
easiest to do just that, unless you're prepared to justify how your 
method is exactly equivalent.

-- 

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

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC