[time-nuts] Inside a CTS 1960017 OCXO
Bill Byrom
time at radio.sent.com
Sun Feb 28 17:49:21 EST 2016
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 ?
>
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