Assuming your are in 60-hz land, I'd say it's very plausible. In a three-phase full bridge rectifier, there are 6 diode and each has one turn conducting per cycle. The dc output of the rectifier has 6 peaks per 60-hz cycle. Even when filtered there may be some 6*LF ripple remaining on the output dc and it may pass through to signals derived from that dc.
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Yes im in 60 hz -land
Thank you for your response, was wondering why I have an increse in amplitude at 300hz aswell as 6-900hz.
Still ripple? And does it matter for the equitment?
Im so sorry, im in 50 hz land off course.
And as you know its in 6times FL it occours.
But my question remains as to the increase of amplitude in this range.