[time-nuts] Inside a CTS 1960017 OCXO

[Bill Byrom]

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 ?
>