The output used 4u7 and 22nf capacitors ( bulk and decoupling) . By switching out two of the 22nf capacitors to 150pf ( can't remember exactly but one was a 0805 , the other an 0402 i believe. it's 8 year ago... the 22nf were x7r, the pf was c0g) i was able to make enough of a dent in the spectrum so it would pass without affecting the circuit. they used 4 bulk caps and 5 or 6 of the little guys.
Sometimes you do need to tweak the PDN. network analyzers can be very helpful for that.
Reminiscent of the olden days when they'd throw in a "trap" network as needed; most often, I think, this was something needed for signal filtering anyway, like to notch the carrier tone in a TV's IF strip (45-something odd MHz, whatever it usually was). But sometimes this applies to output filtering, or supply filtering. Up to and including making an LC filtered power supply ("choke input rectifier") resonant at twice mains, or other harmonics, to make use of a much smaller choke than would otherwise be needed.
Also "swinging" chokes, where the air gap was intentionally "too" small (and tilted or stepped, to give more gradual saturation), reducing the minimum load current for stable voltage regulation. (An LC filter delivers the average rectified voltage (Vout ~= 0.9 Vrms), but reverts to cap-input behavior (Vout ~= 1.4 Vrms) at light load. So the regulation is pretty shite unless a minimum load is provided.) Which again saves weight compared to a full size (linear) part.
Funny thing is, swinging chokes are just as easy to make these days (e.g. high-mu powdered iron types), but they're not too useful, as the inductance is part of the loop compensation in an SMPS controller, and variable inductance tends to just make the dynamics worse; consistent behavior is preferable.
Tim
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