Intermodulation distortion of VGA output

In order to scale the linearity of a Variable Gain Amplifier, usually use the C/IM3 (the ratio of carrier power to 3rd intermodulation distortion) while the output is kept on 1Vpp (Peak-to-peak voltage) .
While measuring the harmonic distortion (also keep output 1Vpp), and assuming that amplifier gain is Av, we need an input signal with Peak-to-peak voltage 1Vpp/Av. To simulate or measure C/IM3, two tones with same amplitude X (also peak-to-peak), frequency f1 and f2, are needed. Should I use X=1Vpp/Av, or use X=1Vpp/2Av? The performance of C/IM3 of these two conditions differs about 12dB, which one is correct? thank you!

Hi

The tone amplitude should be 0.25/Av to have 1V peak to peak output:

cos(ω1t)+cos(ω2t) = 2*cos((ω1-ω2)t/2)?cos((ω1+ω2)t/2)

Regards

You just mean second choice, input amplitude 0.25/Av, i.e, peak-to-to 0.5/Av. I also prefer to this choice, but I am not very sure about it, even I also knew the equation you mentioned above. Did you have any experience in designing such block?

Hi

I have litle exprience: designed cmos gilbert based vga cell

Regards