Auto attentuator for input

[Norm Lucas]

I have been using my laptop computer in the field as an audio
bandwidth oscilloscope. This is done by way of appropriate shareware
and a attenuation box to the line input. The latter allows an external
signal to be manually reduced in amplitude to not exceed the maximum
level acceptable to sound cards, about 2Vpp.

Is there a way, perhaps using a micro-based circuit, to sense the
input voltage level and then control a digital pot to automatically
attentuate it to the 2Vpp?

If a signal is detected of less than about 0.5V, would it also be
feasible to automaticaly activate the appropriate one of several
stages of amplification?

Best regards,

Norm Lucas

 

[Jan Panteltje]

I have been using my laptop computer in the field as an audio
bandwidth oscilloscope. This is done by way of appropriate shareware
and a attenuation box to the line input. The latter allows an external
signal to be manually reduced in amplitude to not exceed the maximum
level acceptable to sound cards, about 2Vpp.

Is there a way, perhaps using a micro-based circuit, to sense the
input voltage level and then control a digital pot to automatically
attentuate it to the 2Vpp?

Sure, it is called 'auto ranging'.
You could switch in attenuators or use some sort of AGC.

If a signal is detected of less than about 0.5V, would it also be
feasible to automaticaly activate the appropriate one of several
stages of amplification?

Yes.
Peak detector, time constant (attack / decay) for reaction time.
Question then is: What are you measuring, what is the min voltage, what is
the max voltage, what is the dynamic behaviour?

 

[Al]

I have been using my laptop computer in the field as an audio
bandwidth oscilloscope. This is done by way of appropriate shareware
and a attenuation box to the line input. The latter allows an external
signal to be manually reduced in amplitude to not exceed the maximum
level acceptable to sound cards, about 2Vpp.

Is there a way, perhaps using a micro-based circuit, to sense the
input voltage level and then control a digital pot to automatically
attentuate it to the 2Vpp?

If a signal is detected of less than about 0.5V, would it also be
feasible to automaticaly activate the appropriate one of several
stages of amplification?

If you don't care about the amplitude, maybe a log amp. would do the trick.

Al

 

[Don Bowey]

I have been using my laptop computer in the field as an audio
bandwidth oscilloscope. This is done by way of appropriate shareware
and a attenuation box to the line input. The latter allows an external
signal to be manually reduced in amplitude to not exceed the maximum
level acceptable to sound cards, about 2Vpp.

Is there a way, perhaps using a micro-based circuit, to sense the
input voltage level and then control a digital pot to automatically
attentuate it to the 2Vpp?

If a signal is detected of less than about 0.5V, would it also be
feasible to automaticaly activate the appropriate one of several
stages of amplification?

Best regards,

Norm Lucas

It wouldn't need a micro, but to attempt a complete reply, I'd need to know
the nature of the signal(s).

 

[Martin Griffith]

It wouldn't need a micro, but to attempt a complete reply, I'd need to know
the nature of the signal(s).

A crude system could have the Vu circuit from the LM3914/5/6/7/8/9/10
datasheet with the topish o/p decreasing a counter and a bottom o/p
increasing the counter which alters some cmos switches that fiddlydum
the gain.

Added bonus flashing lights <aka WOPR>


Martin

 

[Norm Lucas]

It wouldn't need a micro, but to attempt a complete reply, I'd need to know

the nature of the signal(s).

The input signals would be from any audio frequency source, such as
one might encounter repairing audio gear. The PC software previously
mentioned simply provides an oscilloscope-like display of the
waveform.

Someone suggested an AGC, but my impression is that this would distort
the input signal, including the amplitude. The response needs to be
proportionate.

I already have a manaully activated attentuator and signal amp. The
idea is to make it automatic, and in a single functional unit, so
continual adjustment is not required with each different amplitude of
input signal.

Any suggestions along these lines would be appreciated.

Norm Lucas

 

[Martin Griffith]

The input signals would be from any audio frequency source, such as
one might encounter repairing audio gear. The PC software previously
mentioned simply provides an oscilloscope-like display of the
waveform.

Someone suggested an AGC, but my impression is that this would distort
the input signal, including the amplitude. The response needs to be
proportionate.

I already have a manaully activated attentuator and signal amp. The
idea is to make it automatic, and in a single functional unit, so
continual adjustment is not required with each different amplitude of
input signal.

Any suggestions along these lines would be appreciated.

Norm Lucas

Quick question.

What dynamic range do you have with the source signal?
might help with the discussion


Martin

 

[Norm Lucas]

What dynamic range do you have with the source signal?
might help with the discussion


Martin

10dB would suffice. Thanks for asking.

Norm

 

[Jan Panteltje]

10dB would suffice. Thanks for asking.

Norm

http://panteltje.com/panteltje/xpequ/index.html
You'd have to install Linux, the AGC works over much more then 10dB.
AGC does not really distort the signal, it very slowly (about 30 seconds)
increases gain until the peak amplitude is at some set level.
10dB is not even 4x.

 

[Fred Bloggs]

10dB would suffice. Thanks for asking.

Norm

I wouldn't bother with automating anything for this application. There
are simple low parts count configurations for amplifiers with precision
clamping. You can use that and if your displayed acquisition shows
clipping then you would switch in a lesser gain via a wafer switch until
it goes away. Automating the thing will never be more than a hunch, not
to mention a pain in the butt, because of the random nature of the
signals and the wide frequency range and mix.

 

[Don Bowey]

The input signals would be from any audio frequency source, such as
one might encounter repairing audio gear. The PC software previously
mentioned simply provides an oscilloscope-like display of the
waveform.

To me that means it may be speech, music, noise, steady tone, or whatever.
Is that what you are saying?

Someone suggested an AGC, but my impression is that this would distort
the input signal, including the amplitude. The response needs to be
proportionate.

Proportionate to what? Any means of adjusting the signal will introduce
distortion during periods of change. A misadjusted or poorly designed AGC
can cause severe distortion, but a correctly operating one should have only
"acceptable" distortion.

I already have a manaully activated attentuator and signal amp. The
idea is to make it automatic, and in a single functional unit, so
continual adjustment is not required with each different amplitude of
input signal.

Any suggestions along these lines would be appreciated.

A good AGC/amp *could* do the job if the control attack and decay times were
made manually variable to meet your changing needs. Mine was used to assure
conformance to a telco tariff for signals at a network interface; it has a
control range of over 40 dB.