[time-nuts] Bad batch of HP10811's

[Jeroen Bastemeijer]

Dear Time-nuts,

Just a quick experiment. I started with the cold 10811 OCXO (switched 
off for >24 hours). I connected it to a counter and noted the frequency 
every 30 seconds. (last two measurement each 5 minutes).

You can see the curve in the attached PDF. The offset at t=0 was 1706 Hz 
(Compared to 10.000000MHz).

After 10 minutes warm-up the total offset is 0.75Hz (compared to 
10.000000MHz) and offset is 0.12Hz from the "final" value. According to 
the spec the offset should be within 5*10^-9.

 From this I conclude the oscillator is not faulty. Sorry, Didier  ;-)

The possible reason for this offset is probably not the crystal, but the 
tempco of the rest of the circuit. The oven heats up till about 82 
degrees centigrade. A PN-juntcion at room temperature (22 degrees) will 
increase 60 degrees in temperrature, it's forward voltage will change by 
approx. 120 mV!!!!! This change will lead to a significant change in 
circuit properties (e.g. of the oscillator and the attached AGC circuit).

Probably this was the reason HP specfied it's oscillator first after 10 
mins of warm-up.

This makes me wonder: If an oscillator gives a big offset, the 
ovencontroller may be faulty... not working at all, or not getting warm 
enough.

Jeroen PE1RGE



The raw data:
Time Frequncy [MHz]  Offset [Hz]
0    9,998294    1706
30    9,998405    1595
60    9,998645    1355
90    9,998854    1146
120    9,999038    962
150    9,999197    803
180    9,999349    651
210    9,999466    534
240    9,999569    431
270    9,999666    334
300    9,99974    260
330    9,999804    196
360    9,999858    142
390    9,999906    94
420    9,99994027    59,73
450    9,99996696    33,04
480    9,99998633    13,67
510    9,99999979    0,21
540    10,00000062    -0,62
570    9,99999958    0,419999999
600    9,99999925    0,75
900    9,99999933    0,67
1200    9,99999937    0,630000001


Didier Juges wrote:

>That sounds like an awful lot of drift, absolutely inconsistent with a 
>crystal oscillator of any kind.
>
>Either this OCXO is really sick, and just happen to end up at the right 
>frequency by accident, or it is an OCO (Oven Controlled Oscillator :-)
>
>Didier KO4BB
>
>Jeroen Bastemeijer wrote:
>  
>
>>Dear Rick,
>>
>>Are you sure the 500Hz is too much for a cold oven? I checked my 5345A 
>>(recently acquired) counter, which was switched off for some time. 
>>Measuring the frequency directly after applying power and switching on, 
>>resulted in about 1640 Hz frequency offset. After warm up, the error is 
>>below 0.4 Hz. (The unit used here is a 10811a). The error drops pretty 
>>fast, allthough I didn't measure an accurate curve. Is the frequency 
>>difference caused by the tempco of the oscillator circuit?
>>
>>Cheers, 73s Jeroen PE1RGE
>>  
>>    
>>
>
>
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-- 
Ing. Jeroen Bastemeijer

Delft University of Technology
Department of Electrical Engineering
Electronic Instrumentation Laboratory
Mekelweg 4, Room 13.090
2628 CD Delft
The Netherlands

Phone: +31.15.27.86542
Fax: +31.15.27.85755
E-mail: J.Bastemeijer at TUDelft.nl

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