[SI-LIST] Switch Mode PS Noise

Hi folks,

I'm currently investigating a power supply circuit based on an IC with
two switch-mode outputs and a small linear regulator.  The linear
regulator uses an external pnp transistor in a simple low dropout,
common emitter configuration, and is currently running out of
regulation, with an average DC level of 2.39V instead of the required
1.8V.

I believe that noise from one of the switcher outputs is coupling into
the base of this transistor, causing current pulses at the output
which are being averaged across the output capacitor to increase the
nominal DC level.  When I shut down the offending switch mode output,
the linear regulator works properly.  So to my question.......

I'm using a fairly cheap and cheerful oscilloscope and probe.  When I
probe the base of the pnp transistor, I see 600mV spikes
time-correlated with the switch-mode transitions from 0-12V.  I'd like
to understand how much of this noise is just pickup by the probe
leads, and how much is actually present on the base pin of the pnp.  I
wondered if the following was a valid approach:

1.  Place the ground lead of the probe at a fixed ground point on the
PCB, near the area of interest.
2.  Probe a ground via as close to the pnp base as I can find.
3.  Observe the noise on this measurement.
4.  Now probe the pnp base.
5.  Observe the noise on this measurement.  Subtract the noise
observed in 3 to approximate the "real" coupled noise at the pnp base.

Clearly this isn't going to be high precision, I know.  Any other
suggestions for an improved understanding of what's going on here
would be much appreciated.

Thanks. D.
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