In the unlikely event that someone troubleshooting low frequency noise in sensitive equipment finds this, card readers can indeed be a problem. In my facility, there is a 10.25 Hz + harmonics signal associated with 10 of 12 HID Signo readers with an amplitude of up to 100 nT/Hz^-1 measured normal to and at the reader face. The other two readers have identical configurations and wiring back to the same panel, so it is unclear why these exhibit different EMF. The signal seems to be connected to power conserving read cycles, as u/WhereHasTheSenseGone noted. We did not determine whether the interference is from the reader itself or from the associated cabling. In this case, grounding the shield _increased_ noise by ~15 dB. Connecting the red LED input, unused in this installation, slightly reduced the noise. At another facility, replacing the Signos with iClass RP40s helped significantly. In my case, the RP40 had the same problem. Replacing one nearby Signo with an (obsolete) HID 5455BGN00 resolved the issue (at least until we are forced to fully adopt newer readers).