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21st August 2011, 08:11 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, 08:23 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.
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30th August 2011, 01:19 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, 12:29 PM  #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. 
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