IM3 with imbalanced sideband

[rfcn]

To test the inter-modulation distortion of a PA, a two-tone test is done. The two input tones (f1, f2) have same power. Because of the nonlinearity of the PA side bands (IM3) are generated, 2f1-f2 and 2f2-f1. But the two side bands have different power level and the left one is larger than the right one. What causes this problem?

Regards,
rfcn

 

[vfone]

The cause is "memory effect", and/or poor AM-to-PM response of the amplifier.
Usually careful design of the bias decoupling network at baseband/video frequencies (or f2-f1) solve somehow this issue.
Generally if the sidebands even they are asymmetrical, if they doesn't overtake the spec limits, they are not harmful for the amplifier.

 

[rfcn]

Thank vfone!

My WiFi can't pass spectrum mask specification because the one sideband is higher than the other one. I think this is caused by the "memory effect". Wish your method can help me. But I can't modify the bias decoupling in the IC. What I can do is to add more decoupling capacitor to the power supply. Wish it works. Thanks!

Regards,
rfcn

 

[VSWR]

What happens if you change the distance between f1 and f2? Will this change the level of 2f1-f2 and 2f2-f1?

 

[rfcn]

How to measure the IP3 if the PA has memory effect?