Oscillator and audio harmonic filtering...

[J.Shrum]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

Any thoughts? Any help would be appreciated.

Thanks as always
J.Shrum
[email protected]

 

[Luhan]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

Better to use a sinewave generating IC. Intersil 8038 as I recall;
maybe some others available today.

Luhan

 

[[email protected]]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.

I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics,

You dont need a notch filter just a low pass RC filter. If you can
ensure you have a square wave output the even harmonics are supressed
anyway.

 

[Tim Wescott]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

Any thoughts? Any help would be appreciated.

Thanks as always
J.Shrum
[email protected]

Google on "Wien Bridge Oscillator". Built right one of those would have
very low harmonics so you could really pack the tones in, and a
diode-stabilized version, while not having the very lowest harmonics,
would be smaller than the harmonic filter you'd need to put after a 555.

--

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Joerg]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

Any thoughts? Any help would be appreciated.

You either need to generate a sine wave to start with or use the output
of your square wave oscillator and filter it right at the source. Either
with LC or with an active filter around opamps. I don't think a
microphone/speaker combo will cut it, plus it would likely consume more
space than a real filter and pick up surrounding audio noises.

 

[Mochuelo]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

Any thoughts? Any help would be appreciated.

Thanks as always
J.Shrum
[email protected]

Using square waves for that is one of the best ways to make your life
harder unnecessarily. I would use sine waves generated and added
digitally (with an MCU or a DSP). You said "audible" frequencies, so
you should be able to also include some noise shaping, in case you
need higher resolution. You may not even need a DAC. Just a PWM output
and an external filter could work.

However, I think the whole strategy is bad. If the only information
that your tones carry is their frequency, why don't you just store,
for each tone, a value that is proportional to its frequency? Your
file will be much smaller.

What do your sensors provide? A variable voltage? A wave with variable
frequency? In the first case, you should digitize the voltage and
store it. In the second case, you should measure the frequency (easy
with an MCU) and store that measurement. Forget about storing the
whole wave.

Best,

 

[Joerg]

Hello Tim,

Google on "Wien Bridge Oscillator". Built right one of those would have
very low harmonics so you could really pack the tones in, and a
diode-stabilized version, while not having the very lowest harmonics,
would be smaller than the harmonic filter you'd need to put after a 555.

Such as this one here:
http://www.analog.com/UploadedFiles/Application_Notes/46855865AB111.pdf

 

[[email protected]]

Take a look on XR2206 , generate a sinewave with low distortion.

Eduardo.

 

[John O'Flaherty]

Greetings.
I'm currently working on a project that requires recording multiple audible
frequencies to the same audio file. Basically I'm recording the tone and
monitoring the freq spectrum for freq changes over a long period of time.
I'm using a 555 to produce the tone, since its frequency changes as the
condition of what I'm monitoring changes.
My issue is, when I view the spectrum, I end up w/ all of the harmonics. I
am wanting to remove these harmonics so I can use that bandwidth for other
tones simultaneously. Therefore I'd be able to monitor several sensors at
once... each sensor consisting of the same 555 circuit of course.
I realize I could use a notch filter on every oscillator, but I'm wanting to
see if there is a different way of filtering the harmonics, or even a way to
produce the tones w/out even creating the harmonics.
I was considering using an electret mic element coupled w/ a transducer
assuming that the harmonics wouldn't be broadcasted to the mic. But I don't
know if this will work.

With a square wave, the first harmonic is the third. Depending on the
magnitude of the expected frequency deviation, you might be able to put
the frequencies of several sensors in the space before the third
harmonic of the lowest frequency one.

 

[J.Shrum]

Excellent advice everyone. I'm going to carefully review all of the
suggestions and decide what is most feasible.

To describe my exact useage:
I am using thermistors to vary the tone freq of the 555. I then record that
over a period of hours. Then I can go back and view the spectral readout of
the wav/mp3, and look for any temperature dips, and spikes. So far its
worked very well, ... just gotta get rid of the harmonics so I could monitor
more locations w/ the use of the same recorder.

 

[J.Shrum]

Greg,
If I understand you correctly, are you suggesting use a circuit to alternate
between each sensor and record a 1-2 second bit of each one? I assume if
that was done, I would be able to identify the current sample, because it
would be the current lowest frequency in that particular time space. Did I
understand this right?