[time-nuts] MTI 260-0624-D OCXO

Bob Camp kb8tq at n1k.org
Sat Mar 12 14:12:57 UTC 2016


Hi

Judging from your data, the pot takes the oven from at or above the inflection temperature of the crystal down to a point well below the crystal’s lower turn temperature. That’s about all it needs to do. 

Bob

> On Mar 11, 2016, at 11:19 PM, timenut at metachaos.net wrote:
> 
>> Hi
> 
>> Anything electrical (duty cycle/ harmonics / output level / etc) 
>> is designed in on a modern part done in high volume. 
> 
>> The only candidates for a pot on top of the oven are:
> 
>> Inner oven temperature
>> Outer oven temperature
>> EFC
> 
>> You have already ruled out EFC (it would be a coarse set so
>> visible on a normal counter). 
> 
>> That leaves inner oven and outer oven. 
> 
>> Why inner oven - to adjust for the data from your temperature run. The parts
>> are at best coarse set based on crystal temperature. 
> 
>> Why outer oven - same as above or to set for inner / outer separation
>> and clearance after the thermal stabilization in the sealed can. 
> 
>> If there is no apparent change in frequency (and from the location of
>> the pot) that suggests the outer oven. Regardless of which oven is being set, the
>> adjustment is likely a “move it 1.25 turns" sort of thing.  You then 
>> re-run the temperature test to see if it is in spec yet. 
> 
>> Bob
> 
> 
> Bob,
> 
> In a separate communication, Angus has stated...
> 
>> On the two that I tested, it appeared to adjust the temperature of
>> both of the ovens.
> 
>> One was a 5 MHz 260-0545-B. The outer oven was at 78.2 degC and at
>> least around that point the temp changed at about 1 degC per turn. The
>> EFC adjustment on this one was not quite enough to bring the frequency
>> to exactly 5 MHz, but the oven temp did look to be close to correct.
> 
>> The other was a 16.384 MHz 260-0546-G which was unused old stock, but
>> had overall poor stability. It turned out that the oven temp on it was
>> 1-3/8 turns from the turnover point, but when or how that happened I
>> don't know.
> 
>> It would be interesting to hear from somebody who knows how this type
>> of thing is actually adjusted in production.
> 
> I finally got my 260 (sort of) repaired and have done some measurements.
> 
> Generally, I can confirm what Angus was saying, to the limits of my ability to
> measure. One of the things that I can measure is the source voltage. I have a
> hacked together 12v supply. When hooked up to the 260, it initially reads just
> over 9v. As it warms up, it eventually tops out just under 10v. There appears
> to be roughly four different points, around 9.1v, 9.35v, 9.8v and 10v. The 260
> starts at around 9.1v, quickly moves up to 9.35v and after a while to 9.8v. It
> will then sometimes move briefly up to 10v and back down to 9.8v.
> 
> I interpret this as different heating stages - probably both ovens on, one
> oven on, some sort of maintenance heating point and no heaters on.
> 
> What I find as I adjust the pot (after letting the 260 warm up) is that
> turning the pot counter clockwise the supply voltage will go up to 10v and
> turning it clockwise it will drop to 9.35v or 9.1v. So, from that it would
> appear that you are right in that the temperature set points are being
> adjusted by the pot. Whether only one is adjusted or both are changed as Angus
> says isn't clear.
> 
> I also noticed that the 260 consistently has an adjustment range of +/-2e-7,
> regardless of where the frequency is set, with 0v applied to the adjustment
> pin giving a frequency 4Hz higher than when the 5.65v reference voltage is
> applied to the adjustment pin. It will adjust further if the reference pin is
> hooked up to the supply voltage.
> 
> So, I decided to get as much information as I could. I turned the pot
> clockwise until it "ticked" once each turn. That dropped the temperature to
> the minimum. I then turned it counter clockwise 1/2 turn at a time and
> recorded each temperature / frequency reading. I did this until it "ticked"
> once each turn. That took nearly three days. To obtain the temperature, I
> used Kapton tape to tape a temperature sensor for my multimeter to the casing
> of the outer oven. This is not necessarily the hottest spot, nor does it
> actually measure the temperature in either the inner or outer oven. Further,
> the multimeter's precision is only around 1 degree C. The accuracy is unknown,
> but it should be repeatable and roughly linear. I used my (uncalibrated)
> 2465BCT to measure the frequency. This is limited to 7 digits without a
> calibrated frequency standard and is probably slightly off. Still, that gave
> me a good bit of data. Here is the raw data.
> 
>   60    5.000001
>   61    5.000002
>   61    5.000003
>   62    5.000003
>   62    5.000004
>   63    5.000005
>   63    5.000006
>   64    5.000006
>   65    5.000007
>   65    5.000008
>   66    5.000008
>   66    5.000009
>   67    5.000009
>   67    5.000010
>   68    5.000010
>   69    5.000010
>   70    5.000011
>   71    5.000011
>   72    5.000012
>   73    5.000012
>   74    5.000012
>   75    5.000012
>   76    5.000012
>   76    5.000013
>   77    5.000013
>   78    5.000012
>   79    5.000012
>   80    5.000012
>   81    5.000012
> 
> Here is a rough graph. It shows the expected SC temperature / frequency type
> of curve. However, what is interesting is the range over which adjusting the
> temperature affects the frequency. This is a total of 12Hz! Also, it should be
> noted that the outer case was not in place, so the outer oven is exposed to
> ambient temperature. That probably limited the maximum reachable temperature.
> I could noticeably change the both the temperature and frequency by simply
> blowing on the 260.
> 
> 
>   5.000013                                                                  *   *
>   5.000012                                                  *   *   *   *   *       *   *   *   *
>   5.000011                                          *   *
>   5.000010                              *   *   *
>   5.000009                          *   *
>   5.000008                      *   *
>   5.000007                      *
>   5.000006              *   *
>   5.000005              *
>   5.000004          *
>   5.000003      *   *
>   5.000002      *
>   5.000001  *
> 
>            60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81
> 
> The vendor has refunded my money for the 260 (the one that rattles), so as
> soon as I get a chance I will try a new opening technique and see if it is
> possible to repair it. Having learning a lot about the construction from the
> first one, I think it is at least a possibility.
> 
> I will let everyone know what happens - with pictures - once I have done that.
> That may take me until next week. I am attending the Charlotte hamfest
> Saturday.
> 
> 
> Mike
> 
> 
> 
> 
> 
> 
> 
> 
> 
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