The power supplies in the I/O chassis are magnetically noisy and can couple to internal boards and the signal and power cables and connectors that are near the I/O chassis (here). CDS set up a test stand with a BNC AA chassis connected to an I/O box for testing a new type of power supply.  Figure 1 is a photograph of the setup with the old supply (box in the back corner) and new supply (card with the gloved magnetometer on it). I was able to switch back and forth between the supplies. Two of the channels passing through the I/O box were used, one to carry the magnetometer signal, and a blank channel that was terminated at the AA chassis input. Figure 2 shows the spectra of the magnetometer channel for each power supply (the magnetometer was moved back and forth to sit on the active supply), and the coherence between the magnetometer channel and the blank channel.  The magnetic field from the old supply can be orders of magnitude larger than that of the new supply over broad regions. The old supply also displays the drifting features of beating high frequency oscillators.  The new supply only showed coherence at harmonics of 60 Hz while the old one impressed several lines and a region of increased coherence onto the blank channel. When the new supply was used, the blank channel level was a little lower, and did not have the drifting features most likely produced by beating high frequency oscillators in the old power supply (Figure 3). 

The second problem with the I/O chassis is that the fans at the front produce peaks in the channels that pass through it by power supply ripple (here - the peaks go away when a separate supply is used for the fans). Figure 3 shows that these fan peaks are present with both the old and the new power supply. One possible configuration to test is to power the fans directly off of the power supply card instead of off of the main board.

Robert, Richard, Dave, Jim