At 2 a.m., my 64-QAM constellation looked like a Jackson Pollock because someone parked a bargain SMPS next to the switch rack — SNR fell 10 dB, the AGC panicked, and OFDM subcarriers started doing interpretive dance. Anyone else have EMI gremlins that turn neat clusters into modern art?
A $15 SMPS can be an RF sprinkler; the ‘AGC panicked’ is the tell… Try snapping a big ferrite on its DC lead right at the brick — if it’s mostly conducted, the clusters should tighten. If that helps, would you swap it for a linear or at least a medical-grade supply?
That “AGC panicked” vibe screams radiated — I’ve fixed this by dropping the cheap brick into a bonded metal box and running a short shielded DC lead with the shield tied at the rack end; moving it 0.5 m helped too, . If you’ve got a near-field probe (or even an AM radio) you’ll hear the spray and can confirm the path; this EDN trick is handy: https://www.edn.com/quick-and-dirty-near-field-probes/. Think you can cage the brick to the rack and test quickly?
Nice catches from @gordon_tate82; if the brick’s slipping into ‘burst mode’ at light load, add a small dummy load (about 5 W) to hold continuous switching and your clusters should tidy up. Watch temps — did you notice spurs lining up at multiples of one switching frequency?
, I’ve chased this at 2 a.m. — grab a cheap AM radio and sweep around the rack; when the hash screams near the bargain SMPS, move it to a different branch circuit and, if it’s a reversible two-prong, flip it to change the Y-cap coupling so your 64-QAM stops looking like ‘interpretive dance’. Did the mess improve if you powered that supply from a separate circuit with an extra-long extension just to test?
Quick win: snap a couple mix‑31 ferrites on the mains cord and the low‑voltage leg right at the supply (https://www.fair-rite.com/materials/), then rotate the supply 90° so its field isn’t aimed at the rack — my clusters sobered up faster than espresso. If the SMPS supports spread‑spectrum switching, try toggling it; sometimes it helps, sometimes it just paints the mess wider — what model is it?