If the spurs are at exactly 60 Hz multiples (not 120 Hz or 180 Hz) and you are in an area with 60 Hz line power you might have magnetic field coupling from nearby power transformers. Normal conductive shielding won't solve this issue, and Mu-Metal or other high permeability low frequency materials must be used. The voltage induced in ground loops by AC magnetic field depends on the area enclosed by the loop, so twisting wiring can help in some cases.
I suggest a test as follows: * If possible, move the DUT (device under test) to an area which is at least a couple of meters from AC mains power lines and transformers connected to AC mains power. Power the DUT from a battery based supply. Keep any galvanic connection (any conductive wires or metal cases touching metal benches) away from the DUT, except a single safety ground wire. * If that's not easy (and I don't that level of isolation is practical for most of us), then turn off all AC mains line powered equipment within a couple of meters of the DUT. Unplug all equipment nearby from the AC mains power line, then plug the power supply used by the DUT and the minimum test equipment required for the 60 Hz spur test into a single quality AC power strip. This single AC power strip (with only the DUT supply and critical test equipment) should be the only item plugged into the nearby AC mains power sockets. Using a single quality power strip will establish a safety wire ground which is nearly a single point ground. In some buildings there can be problems if the safety ground wires for different AC mains line sockets have different potentials due to connected equipment or water pipe or other external connections. * Keep any transformers which are plugged into the AC mains line power as far as possible from the DUT and the connections between the power supply, DUT, and any test equipment. * Be sure to turn off any nearby AC mains line powered lighting in the area and see if that changes the spurs. * Connect a heavy gauge (such as #10) copper wire between the metal chassis or other ground point on the test equipment, power supply, and DUT and see if the spur level changes. If it does, then you have ground loop problems. The effective source impedance of magnetically coupled currents is very low, so it can be difficult to eliminate them completely. Single point ground systems are usually a good idea. -- Bill Byrom N5BB On Sun, Feb 28, 2016, at 12:22 PM, Adrian Godwin wrote: > Could it be microphonic capacitors, or the crystal itself ? > Have you got a big old heavy power supply with a buzzing transformer on > your workbench ? > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.