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'[PIC:]18F452 100khz sampling frequency'
2004\01\13@040813 by td

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in discussion with filtering:
can i use PIC18F452/10bit ADC/ to detect 50Khz sound with sampling frequency 100kHz?
brgds Krasimir

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2004\01\13@042924 by Jinx

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> can i use PIC18F452/10bit ADC/ to detect 50Khz sound
> with sampling frequency 100kHz?

I think even under the best conditions that's pretty much on or
past the limit (according to Table 17.1 of the 18F452 manual)

If you're wanting to just detect the presence of 50kHz rather
than measure frequency, how about something like an LM567
or a similar notch filter to produce a there/not there signal for
the PIC ?

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2004\01\13@083322 by Roy J. Gromlich

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This is just a guess, but I think he wants to digitize the 50KHz signal and do some processing on it.  Unfortunately, the rule of thumb for signal processing says that you need to digitize at least 2X your highest frequency of interest (100K samples/sec). But if you are trying to do DSP on a 50KHz signal you would need to digitize faster -- 3X or even 5X. The A/D converter is not going to run at 250K samples/sec. --- you need an external chip to do that. But that's not a big deal:
12-bit 1MegaSamples/sec chips are readily available and are pretty cheap, even.

Roy J. Gromlich

Jinx <spam_OUTjoecolquittTakeThisOuTspamCLEAR.NET.NZ> wrote :

{Quote hidden}

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2004\01\13@083943 by hael Rigby-Jones

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>-----Original Message-----
>From: Roy J. Gromlich [.....rgromlichKILLspamspam@spam@PA.NET]
>Sent: 13 January 2004 13:31
>To: PICLISTspamKILLspamMITVMA.MIT.EDU
>Subject: Re: [PIC:]18F452 100khz sampling frequency
>
>
>This is just a guess, but I think he wants to digitize the
>50KHz signal and do some processing on it.  Unfortunately, the
>rule of thumb for signal processing says that you need to
>digitize at least 2X your highest frequency of interest (100K
>samples/sec). But if you are trying to do DSP on a 50KHz
>signal you would need to digitize faster -- 3X or even 5X. The
>A/D converter is not going to run at 250K samples/sec. --- you
>need an external chip to do that. But that's not a big deal:
>12-bit 1MegaSamples/sec chips are readily available and are
>pretty cheap, even.

Getting an 18 series PIC to keep up with that data rate and actually perform
some useful processing may be a little harder however!

Mike




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2004\01\13@092048 by Olin Lathrop

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Roy J. Gromlich wrote:
> This is just a guess, but I think he wants to digitize the 50KHz
> signal and do some processing on it.  Unfortunately, the rule of
> thumb for signal processing says that you need to digitize at least
> 2X your highest frequency of interest (100K samples/sec).

This is not just a "rule of thumb", but a hard theorectical limitation.  The
2x frequency is also called the "Nyquist rate".  In practise, you *need* to
sample at least a little faster than the Nyquist rate.  This is because in
reality there is no filter that has a totally sharp cutoff.  Most of the
time you will need to sample at least 4x the maximum frequency of interest,
although even that will require a reasonably decent digital filter.

> The A/D converter is not going to run at
> 250K samples/sec. --- you need an external chip to do that. But
> that's not a big deal:
> 12-bit 1MegaSamples/sec chips are readily available and are pretty
> cheap, even.

Or use a dsPIC with 12 bit 500K sample/sec A/D.  Check out the 30F2010,
which is also the first dsPIC to become available.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2004\01\13@141454 by Jinx

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> or a similar notch filter to produce a there/not there signal

Sorry ooops, band-pass of course

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2004\01\13@142940 by Jinx

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> >This is just a guess, but I think he wants to digitize the
> >50KHz signal and do some processing on it

I was assuming that this was some known tone from a HF
source (eg bat, ultrasonic transducer, dolphin, Barry Gibb),
in which case it's enough to know that the signal is there or
not. Perhaps there's some measurement like phase shift
that needs doing, and which could be done digitally, rather
than trying to determine the analogue quality of the wave.

Maybe the OP can fill us in a little

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2004\01\13@153415 by Mike Hawkshaw

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Jinx wrote:
> not. Perhaps there's some measurement like phase shift
> that needs doing, and which could be done digitally, rather

If it is a fairly narrowband signal, centered on 50kHz, maybe you could mix
it down to a lower frequency, say 1kHz, then do the processing, in the same
way that a radio mixes the RF down to an IF where the gain/filtering stages
are easier. As long as the "IF" is high enough that any side bands are
preserved you should be OK.

HP network analysers work like this, because the amplitude and phase
information is not destroyed by the mixing process. TV nature programmes
also use this sort of technique to give the viewer some idea of how, for
example, a bat might sound to another bat, by mixing the battie sounds down
to the human audio range.

All the best...Mike.

{Original Message removed}

2004\01\13@154659 by Jack Smith

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>If it is a fairly narrowband signal, centered on 50kHz, maybe you could mix
>it down to a lower frequency, say 1kHz, then do the processing, in the same
>way that a radio mixes the RF down to an IF where the gain/filtering stages
>are easier. As long as the "IF" is high enough that any side bands are
>preserved you should be OK.

This is an area in which I know enough to be dangerous, so no warranty on
the comments ...

If the signal is band limited, can't you simply subsample at a divisor of
the IF rate, thereby doing the mixing and A/D conversion in one step.


Jack

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2004\01\13@164528 by Richard.Prosser

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IIRC the Nyquist criteria strictly relates to the bandwidth of the signal,
not the maximum frequency (effectively you can use an alias if you know
enough about it).
Therefore you can sample at "sub Nyquist" rates provided your filters and
signal knowledge are sufficient.
RP



>If it is a fairly narrowband signal, centered on 50kHz, maybe you could
mix
>it down to a lower frequency, say 1kHz, then do the processing, in the
same
>way that a radio mixes the RF down to an IF where the gain/filtering
stages
>are easier. As long as the "IF" is high enough that any side bands are
>preserved you should be OK.

This is an area in which I know enough to be dangerous, so no warranty on
the comments ...

If the signal is band limited, can't you simply subsample at a divisor of
the IF rate, thereby doing the mixing and A/D conversion in one step.


Jack

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2004\01\13@170205 by Daniel Serpell

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El Tue, Jan 13, 2004 at 03:46:02PM -0500, Jack Smith escribio:
> >If it is a fairly narrowband signal, centered on 50kHz, maybe you could mix
> >it down to a lower frequency, say 1kHz, then do the processing, in the same
> >way that a radio mixes the RF down to an IF where the gain/filtering stages
> >are easier. As long as the "IF" is high enough that any side bands are
> >preserved you should be OK.
>
> This is an area in which I know enough to be dangerous, so no warranty on
> the comments ...
>
> If the signal is band limited, can't you simply subsample at a divisor of
> the IF rate, thereby doing the mixing and A/D conversion in one step.


It's possible, but you'll need [1] a good sample & hold, with enought
input BW (at least HF+IF) and a short and stable window width (stable
to much better than 1/(2*HF) ), and a good analog band-pass filter
centered at HF and with an atenuation better than the S/N that you could
allow at HF+-SF.

   Daniel.


Notes:
[1] BW = Bandwidth, HF = the "Carrier" frequency, in this example 50KHz,
IF = the "intermediate" frequency, can be the same as the side-band's
frequency plus some headroom, SF = sampling frequency.

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2004\01\13@181336 by Olin Lathrop

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Jack Smith wrote:
> If the signal is band limited, can't you simply subsample at a
> divisor of the IF rate, thereby doing the mixing and A/D conversion
> in one step.

Yes, that's actually a valid technique for signals known not to contain
frequencies below a specific limit.  This is how some sampling oscilloscopes
can display GHz signals.

One way to look at it is you're sampling a repetative signal (no frequencies
below the fundamental) but instead of sampling every .1 cycles, you sample
every 1.1 cycles.  If the signal is truly repetative, you get exactly the
same answer.  Note that the sample and hold still has to work at the full
bandwidth.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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