[time-nuts] The 10811 double oven mystery

[Jarl Risum]

I have been puzzled by a mistake which has been published recently on this
list and elsewhere as well. It concerns the operation of the HP 10811
double oven TCXO used in the HP Z3801 GPSDO.

It is claimed that the outer oven is only in use during start up or during
extremely low ambient temperatures and is not operating during normal
conditions. This is not correct.

The 10811 outer oven circuit in the HP Z3801 is designed to maintainin a
stable temperature around the 10811-60158 OCXO somewhere in the interval
between 60 and 65 deg. C in order to reduce the 10811-60158 OCXO's
sensitivity to temperature changes which – without the outer oven - is
specified as 4,5 x 10 e-9 from 0 to 71 deg. C.

The HP Z3801 was made as a GPS disciplined timebase for synchronization of
CDMA Cellular Wireless Land Network base stations and was required to
maintain system syncronization for up to 24 hours in situations with
failure of GPS reception. In order to satisfy this requirement, the HP
Z3801's internal processor keep track of the gradual ageing of the 10811
during normal GPS-disciplined operation and applies a calculated, predicted
correction voltage to keep the OCXO on frequency during periods with no GPS
signal (HP's Smart Clock technology).

A diagram and detailed description of the 10811 outer oven circuit can be
found at KO4BB's manual collection under
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/Z3801.
Pictures of the outer oven can be found at
www.realhamradio.com/GPS_Frequency_Standard.htm.

In 2013 Warren Sarkison published an improved 10811 Outer Oven Controller
with improved temperature regulation. Description and numerous references
to his design can be found by Googling ”Warren S. Design 10811 Outer Oven
Controller”. This circuit - or something similar - is much to be
recommended if optimum operation of the dual oven 10811 outside of the HP
Z3801 is considered.

Jarl