[time-nuts] HP 10811 40 Hz low.

Bob Camp kb8tq at n1k.org
Fri Dec 16 15:02:35 EST 2016


The “outer oven” on the Z3801 10811’s is simply a warmup heater to improve performance when the unit
starts at -40C. When operated as designed, it is effectively “gone” in normal operation. The 10811’s normal
oven circuit (“inner oven”) on the modified 10811 is still what does all of the stabilization of the crystal. The 
temperature stability of the Z3801 version is not significantly different than the standard 10811 when measured
over a normal ambient range. Since our basements are rarely at -40C there is not a lot of value to the "outer
oven" in the standard configuration. A proper outer oven / inner oven combination would be designed and
configured very differently than what you see in the Z3801. The “boost heater” approach does show up in 
other gear, very little of it is in TimeNuts stuff. 

Why do it this way? The 10811 as built simply did not have enough power to warm up properly from -40. At
the time, there was some thought that GPSDO’s would be deployed in un-heated enclosures (that never happened). 
To get the unit working, a number of “quick and dirty” hacks were employed to hook the existing OCXO into
the Z3801 design. They were of a “fast time to market” nature rather than “figure it out and do it right”.


> On Dec 16, 2016, at 12:44 PM, ed breya <eb at telight.com> wrote:
> I had a reference problem on my Z3801A years ago, and ultimately found that the opamp that controls the 10811 oven temperature was bad. I think it turned out there was a bad batch of certain date codes. Replacing the IC with an equivalent type fixed it right up, with no other changes or adjustments.
> Also note that the the ovens are sequenced to avoid drawing too much power from cold-start. As I recall, the inner one has to get up near operating temperature to get things running ASAP, then its status signal enables the outer oven driver (a switched-mode converter on the Z3801A power supply board) to start - it takes longer to stabilize. I think the inner oven should take maybe 20-30 minutes tops, to reach the spec range, and the outer one gradually catches up over maybe an hour or two.
> The crystal is very sensitive to temperature until it gets to turnover, so the frequency can seem way out of whack compared to its normal operating point. The oven temperature doesn't have to be very far off to cause this, so a failure in the inner oven control that throws it out of range could cause problems.
> Ed
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