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Choosing XF Magnitude of supply when measuring supply noise

david73

I have read many pages on using pxf to measure supply noise, and I know to divide the output by the slope of the crossing point get the jitter transfer function

My question is, if the output of a pxf simulation is a voltage-to-voltage transfer function, does it even matter what XF magnitude of the voltage supply source is chosen in the simulation?

In the end, it seems that once the second/voltage transfer function is calculated, you just multiply by the expected amplitude on the supply noise, and it doesn't matter what was chosen as XF magnitude

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  • Andrew Beckett

    In general I'd leave the xf magnitude on the sources blank. It does affect the results (otherwise it would be a bit pointless) - in essence, the transfer function from the source to the xf/pxf output is multiplied by the value specified as the xf magnitude on the source; so if you specify 2, the transfer function will be double what it would have been otherwise. It's mainly there to allow you to compensate for (say) differential inputs (if needed). Personally I tend to just scale the result rather than setting xf magnitude, as otherwise it's easy to forget that you set the xf magnitude on a source when you come back to it 6 months later...

    Andrew.

Reply
  • Andrew Beckett

    In general I'd leave the xf magnitude on the sources blank. It does affect the results (otherwise it would be a bit pointless) - in essence, the transfer function from the source to the xf/pxf output is multiplied by the value specified as the xf magnitude on the source; so if you specify 2, the transfer function will be double what it would have been otherwise. It's mainly there to allow you to compensate for (say) differential inputs (if needed). Personally I tend to just scale the result rather than setting xf magnitude, as otherwise it's easy to forget that you set the xf magnitude on a source when you come back to it 6 months later...

    Andrew.

Children
  • david73
    in reply to Andrew Beckett

    Thanks! I knew it had to make a difference, just didn't know how.

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