How to calculate intermodulation distortion ratio ?

pawelpl · 21st August 2011, 07:11 PM

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 ?

sycorax · 27th August 2011, 07:23 PM

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.

pawelpl · 30th August 2011, 12:19 PM

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.

ingenieus · 26th September 2011, 11:29 AM

Thanks. I've been looking for this answer myself.

I assume that the value for "AmplitudeOf()" is in Volt.

21st August 2011, 07:11 PM	#1
pawelpl 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 nf1+mf2 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
sycorax 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 CD2M-PRO -> AKM4397 DAC -> Aikido pre -> GM70PP mono -> Martin Logan Vantage ESL -> Me!

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30th August 2011, 12:19 PM	#3
pawelpl 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
ingenieus diyAudio Member Join Date: Apr 2011	Thanks. I've been looking for this answer myself. I assume that the value for "AmplitudeOf()" is in Volt.