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'[EE]: Amplify then filter ?'
2001\08\02@190745 by Pedro Drummond

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Sorry for the missing tag lead. Here it goes again:

-----------

Hello, Gods' oracle.

One question:

I will be using the 567 filter IC to separate 12kHz signals from regular
audio. The problem is that the 12kHz has a much lower amplitude. Should I
amplify and then insert the full audio mixture to the 567, or will it be
able to detect a much lower signal, at the right frequency ?

Thanks in advance.


Pedro.

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2001\08\02@205946 by Jim

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Seeing no other replies - I'll jump in here with
my limited experience with the LM567 some
18 to 20 years ago ...

My expereince, in the use of LM567's to do TouchTone
decoding, was that the more you can do to get the tone
(or tone range) of interest as the stonger signal - the
better, the cleaner, the fewer false outputs the 567
will show.

Seeings how you're working to detect a 12 KHz tone (or
signal) - why not high-pass (starting at, say, 10 KHz) or
bandpass in the 12 KHz area?

My solution to TT decoding was to precede each bank
of 567's (two tone ranges) with an appropriate low or
high pass op-amp filter. The 567's would then actually
'work' whereas before operation could not in any way
be guaranteed ...

Jim

{Original Message removed}

2001\08\02@211836 by Ned Seith

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Pedro & Jim,

Sounds good and works for me.

If the 12KHz band actually has the lowest amplitude of any of the bands, I
would invert the signal with an op amp configured as an inverting amplifier
and then apply it to the LM567s. This would then give the 12KHz band the
highest amplitude, otherwise, I would proceed with Jim's recommendations.

Sincerely,
Ned Seith
Nedtronics
59 3rd Street
Gilroy, CA 95020
(408) 842-0858

At 08:00 PM 8/2/01 -0500, you wrote:
{Quote hidden}

>{Original Message removed}

2001\08\02@212909 by Barry Gershenfeld

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Same thought, (nobody responded); same time frame, (20
years ago).  I think it's like FM--it's going to try to
capture the strongest of the signals.  If you try to
amplify it to the point where the louder signal is
clipping, you just make the "capture effect" worse.

So filter it until your 12KHz signal is louder than
the rest.   Or, maybe the 567 isn't going to be
your friend on this project.

Barry

{Quote hidden}

>{Original Message removed}

2001\08\02@213121 by adastra

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>
> If the 12KHz band actually has the lowest amplitude of any of the bands, I
> would invert the signal with an op amp configured as an inverting
> amplifier
> and then apply it to the LM567s. This would then give the 12KHz band the
> highest amplitude, otherwise, I would proceed with Jim's recommendations.
>
>

You're kidding... Right?

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2001\08\03@115823 by Pedro Drummond

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> >My solution to TT decoding was to precede each bank
> >of 567's (two tone ranges) with an appropriate low or
> >high pass op-amp filter. The 567's would then actually
> >'work' whereas before operation could not in any way
> >be guaranteed ...

If memory serves me, on 567's data sheet they present the DTMF decoding
solution without this preamplifier.

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2001\08\03@123600 by Jim

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  "567's data sheet they present the DTMF
    decoding solution without this preamplifier."

Yeah yeah yeah ... try using that bare-boned approach
in the *real world* where the tone levels from each of  the
two TT freq groups aren't *quite* the same level ... as can
be seen in the amatuer radio world where TT levels (on TT
mics) varies from rig to rig ...

NOW convice me that LM555's are just as stable (across
temperature, time) as the rock-based TT encoders. Heathkit
found out the hard way AFTER product had been shipped
into the field ...

Jim

{Original Message removed}

2001\08\03@123615 by Harold M Hallikainen

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On Thu, 2 Aug 2001 19:17:54 -0600 adastra <.....fosterKILLspamspam@spam@ADASTRAN.COM> writes:
> >
> > If the 12KHz band actually has the lowest amplitude of any of the
> bands, I
> > would invert the signal with an op amp configured as an inverting
> > amplifier
> > and then apply it to the LM567s. This would then give the 12KHz
> band the
> > highest amplitude, otherwise, I would proceed with Jim's
> recommendations.
> >
> >
>
> You're kidding... Right?
>

       Just imagine what happens at frequencies where the amplitude is zero!

Harold

PS - I used LM567's (I think they were originally NE567's from Signetics)
when they first came out. They were clever chips, but I had a lot of
trouble with false detection of tones and chattering outputs.


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2001\08\03@160930 by David VanHorn

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At 11:14 AM 8/3/01 -0500, Jim wrote:
>    "567's data sheet they present the DTMF
>      decoding solution without this preamplifier."
>
>Yeah yeah yeah ... try using that bare-boned approach
>in the *real world* where the tone levels from each of  the
>two TT freq groups aren't *quite* the same level ... as can
>be seen in the amatuer radio world where TT levels (on TT
>mics) varies from rig to rig ...

The data sheet circuit tells you that the 567 can do this job, under ideal
conditions.
In real life, you have to deal with a huge range of amplitude (-9 to -40dBm
IIRC), several dB of twist in either direction, and noise.

The 567 is ok up to a point, but it doesn't work miracles.
Stick a filter in front, and you'll be much happier.


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2001\08\03@162818 by George Tyler

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I once used a pll to respond to sonar signals in noise, it worked with a
signal 6db lower than the noise. on a scope you could not even see the
signal in the noise, and this was with no extra filtering.

{Original Message removed}

2001\08\04@044324 by Peter L. Peres

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> I once used a pll to respond to sonar signals in noise, it worked with a
> signal 6db lower than the noise. on a scope you could not even see the
> signal in the noise, and this was with no extra filtering.

The signal was 6 dB lower than the noise floor, but not in the capture
band of the PLL, otherwise it would not lock ? Afair the magic 'getting
signals from below noise level' done by PLLs is due to their capture
bandwidth being a sort of filter. This correspondingly improves the S/N
ratio for the wanted signal. A scope will not show the singal in the noise
but I am almost certain that a scope with a prefilter with the same
bandwidth as the PLL capture loop will show it.

Peter

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2001\08\04@115706 by Olin Lathrop

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> If the 12KHz band actually has the lowest amplitude of any of the bands, I
> would invert the signal with an op amp configured as an inverting
amplifier
> and then apply it to the LM567s. This would then give the 12KHz band the
> highest amplitude, otherwise, I would proceed with Jim's recommendations.

Huh!!??  You're not really serious, I hope!

Just in case someone out there might get confused, multiplying the signal
by a constant of -1 does not change its frequency content or the relative
amplitudes of different frequencies in any way.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\08\04@115753 by Mike Kendall

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There is some very interesting amateur radio work (and computer programs)
with super weak signals for VLF on the internet.  A key to it's success is
accurate frequency resolution.  From what I've read, even 1hz off can make a
difference sometimes.
Regards,
Mike
{Original Message removed}

2001\08\05@081519 by Martin McCormick

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       I am glad to read the other questioning responses.  I
was beginning to wonder if I was missing something.  The only way
one can invert frequencies is with some sort of product
detector/chopper/mixer, etc.

       I built a primitive broadcast automation system back in
1975 using a 567 as the tone decoder chip for the cuing system
at 25 HZ with no low-pass filter and it false-tripped all the
time.  The 567 will respond to frequencies that are 3 times the
oscillator frequency with the effect being that the output may
chatter but usually not fully go low.

       I fixed that problem with a circuit using a hex inverter
or a couple of stages of 7400 logic plus a NE555 timer chip.  I
think you could chuck both those chips and use a 12C508 today
with the idea being that your program must look for the 567 to go
low and stay there for a prescribed amount of time.  If it goes
back up, then the whole process starts again.

       My old cuing system worked perfectly after the extra
timing circuit was added.  I suspect I would have also had great
luck with a low-pass filter and maybe better selection of the
low-pass filter and timing components on the 567, itself.

       If you are going to use a NE567, be prepared for a bit of
cut and try time to really get it right.

       Also, if you use a 12C508 to handle the timing and
shaping fixes I was discussing, it may be possible to become more
creative with the programming as your 12C508
has enough program memory to more than handle what it needs to do
to emulate the RC timer circuitry using the NE555 I originally
described.

       I don't know if the 567 is considered to be obsolete as I
have not tried to buy any in many years, but I think the 12C508
signal shaper routine I described could turn a flaky and somewhat
marginal system in to a much better one.  Oh yes, the whole thing
can fit in to a 16-pin DIP socket with the only external
components being the resistors and capacitors for the 567 and a
bypass cap for the 12C508.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group

Olin Lathrop writes:
>Huh!!??  You're not really serious, I hope!
>
>Just in case someone out there might get confused, multiplying the signal
>by a constant of -1 does not change its frequency content or the relative
>amplitudes of different frequencies in any way.

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