Dave,
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If you want to study the effectiveness of adding filters to reduce SMPS noise, you could use a B-source with rand(x), random(x), or white(x) to mimic a "typical" SMPS.? When simulated correctly, the noise spectrum from those semi-random sources is fairly flat, up to a point.? Then you could use LTspice's FFT feature and examine the before-and-after spectrum from adding a filter.
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For most users, the differences between rand(x), random(x), and white(x) are rather minor except for the DC offset.? I recommend white(x).
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Andy
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