
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Software Tools SPICE, PCB CAD, speaker design and measurement software, calculators 

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 

Thread Tools  Search this Thread 
21st August 2011, 07:11 PM  #1 
diyAudio Member
Join Date: Jun 2011

How to calculate intermodulation distortion ratio ?
As I understand IMD is measured by feeding two sine waves, f1 and f2, at different frequencies into the amplifier.
On the output of the amplifier we have signal with spectrum that contain peaks on n*f1+m*f2 frequences. So, my question is how does formula to calculate IMD look like ? And how much peaks we should take into account ? 
27th August 2011, 07:23 PM  #2 
diyAudio Member
Join Date: Sep 2006
Location: The North of The Netherlands

I assume you are using an audio spectrum analyzer to see the spectrum. You then simply take the difference between the wanted signal and the highest peak in the unwanted signal. When you measure voltages and like to have the result in dB you use the formula: Uwanted / Uunwanted * LOG10.
If you turn up the signal 1 dB and the peak you are measuring is going up 1 dB also the peak is second order distortion. If the peak is going up by 2 dB you are looking at third order distortion.
__________________
DIY CD2MPRO > AKM4397 DAC > Aikido pre > GM70PP mono > Martin Logan Vantage ESL > Me! 
30th August 2011, 12:19 PM  #3 
diyAudio Member
Join Date: Jun 2011

Sycorax, thanks for reply.
I do calculate IMD by formula below, but not sure if it is correct. IMD = sqrt( ... + AmplitudeOf(8000Hz)^2 + AmplitudeOf(9000Hz)^2 + AmplitudeOf(12000Hz)^2 + AmplitudeOf(13000Hz)^2 + ...) / sqrt(AmplitudeOf(10000Hz)^2 + AmplitudeOf(11000Hz)^2) In this example f1=10000Hz, f2=11000Hz. 
26th September 2011, 11:29 AM  #4 
diyAudio Member
Join Date: Apr 2011
Location: Pretoria

Thanks. I've been looking for this answer myself.
I assume that the value for "AmplitudeOf()" is in Volt. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Intermodulation Distortion Values below THD Values by SS Amps  is this possible ??  tiefbassuebertr  Solid State  28  30th May 2011 11:20 AM 
How to calculate S/N ratio  synthius  Chip Amps  3  28th April 2008 04:39 AM 
how to calculate the duty ratio of L4981???  davy_shi  Power Supplies  5  20th May 2005 09:59 AM 
Aliasing Intermodulation Distortion and filterless DACs  halcyon  Digital Source  68  20th May 2004 03:01 PM 
THD, Intermodulation, S/N  Diogo  Solid State  13  4th September 2002 12:29 PM 
New To Site?  Need Help? 