SMPS output noise: Why is it amplified further away from the output capacitor?
Consider your scope probe + its GND lead as a magnetic receiver. Use the formula $$Vinduce = MU0 * MUr * Area/(2 * Pi * Distance) * dI/dT$$
Let area be 4&quot; by 4&quot;. Let distance between the probe/gnd and the SMPS be 4&quot;. Accept 4&quot; is 0.1 meter.
Use dI/dT of 0.1 amp in 10 nanoseconds (the FET on and off time).
Evaluate the Vinduce (at this point, I cannot predict how small or big this will be, so we just do the math).
Vinduce = 2e-7 * Area/Distance * dI/dT
Vinduce = 2e-7 Henry/meter * 0.1 meter * 1amp/100nanoSec
Vinduce = 2e-7 * 0.1 * 1e+7 = 2e-8 * 1e+7 = 0.2 volts.
For fast edges, the planes (all your planes) will provide some attenuation; Skin Freq for 35micron foil (the standard thickness, 1.4mils, 1 ounce/foot^2) is 4MHz or 125 nanoseconds, thus a 10nS edge gets lots of attenuation.
If your scope probe + GND lead is up in the air, you'll have little or no attenuation from the planes.
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This formula comes from combining Faraday law of induction with Biot-Savart law.
We then assume source of magnetic field is a long straight wire that is coplanar with the Area, and that the ratio of minumum distance to maximum distance is close enough to One we can ignoe some minor natural-log terms.