All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > General Electronics Chat

Notices

General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 11-12-2009, 12:45 PM
Bill_Marsden's Avatar
Bill_Marsden Bill_Marsden is offline
Super Moderator
 
Join Date: Mar 2008
Location: Dallas, TX (GMT-5 w/ DST)
Posts: 19,069
Blog Entries: 5
Default Sine Wave distortion measurements

Way back when I used a piece of HP equipment to measure distortion of a sine wave. It was fun to use, but long gone.

How would a hobbiest measure distortion of a sine wave? It can look good on a oscope and still be pretty bad.

One thought is to take the waveform in question, filter it though several low pass filters to create a reference (without phase shifting it), invert it, and null it with op amps. What is left is the distortion.

Anyone seen a home brew setup to do this? With all the audiophiles out there I suspect it exists.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
Reply With Quote
  #2  
Old 11-12-2009, 01:14 PM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,043
Default

I don't know how you could run a signal through several low pass filters and not get at least SOME phase shift, just due to propagation delays alone.

[eta]
Just a thought - wind yourself a broadband 1:1 isolation transformer using a quality ferrite toroid. Use the output of the transformer to drive the y-axis of your o-scope, and the input signal for the x-axis. If there is no distortion and no losses, you should get a perfect circle on the 'scope.
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.

Last edited by SgtWookie; 11-12-2009 at 01:40 PM.
Reply With Quote
  #3  
Old 11-12-2009, 01:57 PM
Bill_Marsden's Avatar
Bill_Marsden Bill_Marsden is offline
Super Moderator
 
Join Date: Mar 2008
Location: Dallas, TX (GMT-5 w/ DST)
Posts: 19,069
Blog Entries: 5
Default

Any imperfections in the circle being distortion?
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
Reply With Quote
  #4  
Old 11-12-2009, 01:58 PM
studiot's Avatar
studiot studiot is offline
E-book Developer
 
Join Date: Nov 2007
Location: Somerset UK
Posts: 4,014
Default

It is often overlooked that to get meaningful measurements on low value distortion you require a very low distortion signal.

The "poor man's" way is to use spot frequencies. Both the generator and the filter can be finely tuned, twinTEE being the frequency defining network of choice, although I have seen the Wein used as well.
__________________
Do I look old?
I don't feel old.
I don't feel anything till noon.
Then it's time for my nap.
Reply With Quote
  #5  
Old 11-12-2009, 02:04 PM
Bill_Marsden's Avatar
Bill_Marsden Bill_Marsden is offline
Super Moderator
 
Join Date: Mar 2008
Location: Dallas, TX (GMT-5 w/ DST)
Posts: 19,069
Blog Entries: 5
Default

The old HP equipment (don't remember it's model number) you adjusted for a in the meter, then flipped the switch to automatic, and it would tune itself to the waveform. My shop used it to check the quality of amplifiers in milspec equipment.

I think it was a precursor to a selective voltmeter.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
Reply With Quote
  #6  
Old 11-12-2009, 04:17 PM
KL7AJ KL7AJ is offline
E-book Developer
 
Join Date: Nov 2008
Location: North Pole, Alaska
Posts: 1,547
Default

Quote:
Originally Posted by Bill_Marsden View Post
Way back when I used a piece of HP equipment to measure distortion of a sine wave. It was fun to use, but long gone.

How would a hobbiest measure distortion of a sine wave? It can look good on a oscope and still be pretty bad.

One thought is to take the waveform in question, filter it though several low pass filters to create a reference (without phase shifting it), invert it, and null it with op amps. What is left is the distortion.

Anyone seen a home brew setup to do this? With all the audiophiles out there I suspect it exists.

Howdy!

The traditional distortion analyzer was pretty straightforward. It had a NULL circuit for notching out the fundamental frequency (a modified Wheatstone Bridge). You'd then measure alll the remaining power, which was all the harmonics, if any. Although you couldn't separate the diffferent harmonics, it was usually THD you were looking for.

With FFT analyzers, you can get a lot more detail, of course.


If you have an idea ahead of time as to what the harmonic content might be, you could also use a null-type analyzer on those particular harmonics and eliminate them, one at a time.


Eric
__________________
REAL men don't need voltmeters.
Reply With Quote
  #7  
Old 11-12-2009, 04:19 PM
KL7AJ KL7AJ is offline
E-book Developer
 
Join Date: Nov 2008
Location: North Pole, Alaska
Posts: 1,547
Default

I might add that with a trained eye and a good oscilloscope, you CAN see harmonic distortion down to about 1%.


eric
__________________
REAL men don't need voltmeters.
Reply With Quote
  #8  
Old 11-12-2009, 06:14 PM
Externet's Avatar
Externet Externet is online now
Senior Member
 
Join Date: Nov 2005
Location: Mideast USA
Posts: 480
Default

For a signal generator, I don't know. For an amplifier, try an oscilloscope channel A at input, channel B at output, invert B if not inverted output, and select A+B.
Adjust channel gain for minimum amplitude reading at maximum magnification. It may need adjusting delay setting. Will yield a qualitative waveform, not a % reading.
__________________
Abolish the deciBel !
Reply With Quote
  #9  
Old 11-12-2009, 06:54 PM
studiot's Avatar
studiot studiot is offline
E-book Developer
 
Join Date: Nov 2007
Location: Somerset UK
Posts: 4,014
Default

The thing to do to eyeball it in a scope is to use a dual channel.
Input the signal input to the amp into one channel
Take the output from the amp through a sharp filter to remove this frequency and amplify what's left 1000 or 10,000 times in the second channel.

You can see the distortion this way.
__________________
Do I look old?
I don't feel old.
I don't feel anything till noon.
Then it's time for my nap.
Reply With Quote
  #10  
Old 11-12-2009, 06:58 PM
Bill_Marsden's Avatar
Bill_Marsden Bill_Marsden is offline
Super Moderator
 
Join Date: Mar 2008
Location: Dallas, TX (GMT-5 w/ DST)
Posts: 19,069
Blog Entries: 5
Default

Sorry, no dual channels here. X and Y maybe, but have I mentioned how old my oscope is?

Another thought occurs, the PC sound card oscope can also be used as a spectrum analyzer, so I've heard.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
Reply With Quote
Reply   Post New Thread

Tags
, , ,


Related Site Pages
Section Title
Worksheet Opamp oscillator circuits
Worksheet Design Project: audio media-based signal generator
Worksheet Performance-based assessments for AC circuit competencies
Worksheet Advanced electromagnetism and electromagnetic induction
Worksheet Passive filter circuits
Worksheet Mixed-frequency signals
Textbook Power quality measurement : Ac Metering Circuits
Textbook AC voltmeters and ammeters : Ac Metering Circuits
Textbook More on spectrum analysis : Mixed-frequency Ac Signals
Textbook Square wave signals : Mixed-frequency Ac Signals


Similar Threads
Thread Thread Starter Forum Replies Last Post
AC sine wave to DC square wave, newb, saw a similar thread SpeedEuphoria General Electronics Chat 11 05-01-2010 12:28 AM
triangular to sine wave kahafeez The Projects Forum 13 12-16-2008 07:27 PM
changing a modified sine wave to true sine wave using simulink hyde6666 The Projects Forum 0 07-15-2008 06:35 PM
Harmonic Distortion of Sine Wave bipin General Electronics Chat 4 05-07-2004 09:13 AM
Harmonic Distortion of Sine Wave bipin General Electronics Chat 1 04-29-2004 04:03 PM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 08:57 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.