Possible to use difference math of DSO as distortion analyzer? - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th January 2011, 03:51 PM   #1
diyAudio Member
 
Join Date: Dec 2010
Default Possible to use difference math of DSO as distortion analyzer?

Warning, this may be considered a somewhat newbie question.

My understanding of what a distortion analyzer does is it basically notches out the original sine wave signal, so that what's left is just the distortion.

Could a modern DSO (e.g. Rigol 1052e or Instek 1062a) that has math functions be used to do the same thing without purchasing a distortion analyzer? I'm thinking take a low noise audio generator, and split the signal to feed the circuit being tested and one input channel of the DSO. Then feed the output of the circuit being tested into the other channel of the DSO, set it to subtract the two signals, and voila we can look at the distortion by itself, and even do FFT analysis on it.

Thinking about this a little more, I suppose there might be issues with the 8-bit sampling in the scope introducing quantization errors (distortion). And there might be a need to precision-adjust the levels so that we don't end up measuring level mismatches.

Has anyone tried this?
  Reply With Quote
Old 18th January 2011, 04:41 PM   #2
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
It's a good question actually. Unfortunately it is not that simple.

One problem is phase shift. If you introduce just 1 degree of phase shift between input and output, then you will end up with a residual signal after subtracting, at a level of about -40 dB (1%). I suppose you could try to insert a phase-shifting circuit, but then that circuit could introduce even more distortion and noise, and you would need to constantly tune the phase shift. And you can forget about using anything other than a sine wave input since the phase shift will be different at different frequencies and you would not be able to fix that. Very old distortion analyzers with a manually tuned notch had a similar problem... the notch would drift out of tune while in the process of taking the measurement (a multi-step process) which would lead to a significant source of error.

This still might be useful to look at the spectrum of the distortion. You can remove most of the fundamental in the subtraction, and do an FFT on the residual. Ignoring the spike at the fundamental frequency, you could then observe the relative levels of the harmonics.
  Reply With Quote
Old 18th January 2011, 05:08 PM   #3
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
for audio you can use better quality soundcards - although most cheap motherboad chipsets today are good to much better than -80 dB noise and distortion

Audio DiffMaker is free sw for very accurate .wav audio differencing with time, fr, sample rate compensation
Audio DiffMaker

RMAA is another free audio test sw which is good for quick loopback testing of your PC/soundcard to see its limitations
  Reply With Quote
Old 18th January 2011, 05:10 PM   #4
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
To measure distortion, there's two fundamental approaches.

1) Notch out the fundamental and measure the residual. This gives you THD+N.
2) Perform an FFT on the measured signal. This will give you the THD (no +N).

Most digital scopes use an FFT. You can get software that'll use a computer soundcard to do the same.

If you want to go the first route, I suggest looking at state-variable tracking filters. That's a bit of a research project, but in the end all you'll need for test equipment is a true RMS voltmeter. You can look at some of the existing designs. Something like the HP 8903A or 8903B audio analyzer would be a good place to start. You can get the service manual with full schematics on Agilent's website.

~Tom
  Reply With Quote
Old 18th January 2011, 05:29 PM   #5
diyAudio Member
 
Join Date: Dec 2010
Thanks for the very enlightening responses. I've been up late last night researching standalone distortion analyzers (#1 from the last post), and it looks like buying a reliable one even used gets up towards $1000 (not to say there aren't bargains that people luck into here and there). That's way out of my just-starting-out hobbyist budget.

I actually don't have a need for a distortion analyzer right now, but I just thought while I'm in the process of buying a scope and associated gear I might want to find one that comes along with something that could be used for distortion analysis later on down the road when I actually need it.

The Instek 1062a scope that I just ordered sounds like it won't do it well, but I suppose if I find a situation where I really need distortion analysis I can beg or borrow someone's PC based scope with built in distortion analysis.
  Reply With Quote
Old 19th January 2011, 11:46 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
read about Cordell's distortion magnifier.
__________________
regards Andrew T.
  Reply With Quote
Old 21st January 2011, 12:41 AM   #7
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
I've actually made a bit of a hobby out of buying broken distortion analyzers and fixing them. I run them through the full adjustment procedure and performance testing to verify the repair. I sell them for around $600. I don't have one right now, but I'll post them in the Swap Meet forum when I do.

~Tom
  Reply With Quote
Old 28th March 2011, 02:11 AM   #8
diyAudio Member
 
Join Date: Oct 2010
Location: Pampanga, Philippines
Quote:
Originally Posted by perfknee View Post
Could a modern DSO (e.g. Rigol 1052e or Instek 1062a)
So you decided to go for the Instek? I think Instek 1062A is a good basic scope for hobbyist but most forum members at EEVblog EEVblog Electronics Community Forum - Index choose Rigol over the Instek, maybe there is something on the 1052E that is missing on 1062A?
  Reply With Quote
Old 28th March 2011, 05:57 AM   #9
benb is offline benb  United States
diyAudio Member
 
Join Date: Apr 2010
These DSO's only use 8 bit A/D's (at least one of these, this one shows the spec here Instek GDS-1062A Oscilloscope Digital Storage 60MHz Instek GDS-1062A GDS1062A GDS 1062A), so with the FFT you're not really going to see anything (such as a distortion harmonic) below circa 48dB of the fullscale input. A modern 24-bit computer interface with free software will beat that bigtime.
  Reply With Quote
Old 29th March 2011, 06:49 PM   #10
cliffyk is offline cliffyk  United States
diyAudio Member
 
cliffyk's Avatar
 
Join Date: Nov 2009
^+1, the 8-bit ADCs of most DSOs is a very limiting factor in doing distortion or any sort of signal level analysis on audio signals.

At very best the level of harmonics and noise would have to be above -48dB to be visible at all, making the measurable THD+N floor a rather high 0.4%. In practice 0.8% would be a more practical limit.

OTH a caution re: using a sound card is that the sample frequencies are rather low, and that many consumer level cards/USB devices have 20-22kHz brick wall low pass filters on the inputs. These limit the useful fundamental test frequencies to 10kHz or less, as even at 10kHz only 1 harmonic will make it through the input filter. At frequencies >10kHz no harmonics would get past the brick wall.

Some high-end/pro level cards and USB devices do have switchable filters, at a bit less than the Nyquist frequency, depending on the selected sample rate. These can extend the useful fundamental frequency range.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Distortion Analyzer - Again Zero Cool Equipment & Tools 4 29th June 2013 11:14 PM
distortion analyzer recomendations? audiorob Pass Labs 438 1st June 2011 08:12 PM
Recommendation on Distortion Analyzer Mark245 Solid State 2 19th December 2009 10:06 AM
Distortion analyzer recommandations? tschrama Equipment & Tools 3 10th June 2008 07:47 AM


New To Site? Need Help?

All times are GMT. The time now is 05:38 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2