[time-nuts] DIY FE-5680A lobotomy (disable temp compensation)

[EWKehren at aol.com]

Thanks Bob for putting it in perspective, 0.05 C is very doable, looks like 
 it is also used in some OCXO's.
Bert Kehren
PS  on a related subject I just pulled PROCEEDINGS OF THE ANNUAL  SYMPOSIUM 
ON FREQUENCY CONLTROL ATLANTIC CITY in the late 70's did attend a few  
because of my involvement in GPS but then did not understand half of it and now  
want to check what I have learned since. Very interesting reading.
 
 
In a message dated 6/28/2014 9:36:32 A.M. Eastern Daylight Time,  
kb8tq at n1k.org writes:

Hi

Most of these lightweight Rb’s do the same thing. They  watch the oven 
current on one or the other section and try to guess the  external temperature. 
Based on that guess they do a simple temperature  correction on the unit. 
The older analog units feed a DC signal into the EFC.  The newer digital units 
feed a signal into the DDS.

In both cases  (analog and digital), the ADEV of the units can be improved 
by disabling this  “feature”. That of course assumes you are at a constant 
(as in very constant)  abient temperature. In the case of the analog part, it
’s the noise on the  heater current that gets you. In the case of the 
digital approach, it’s the  tuning granularity of the DDS that messes things up 
(and possibly heater  current noise as well). 

How constant is “very constant”? That depends  on the Rb you have. A good 
bet is that your device runs better than 2 to 4 ppb  over a 100C range 
without the compensation turned on. That gives you 20 to 40  ppt per degree C. To 
hit 1 ppt you would need to control the device to better  than 0.05 C. If 
you simply want to hit the 0.1 ppb temperature spec, then you  only need a 
two degree control. If you look at the temperature compensation  data words 
(ddd steps), some Rb’s in a batch are much better than others, so  there is no 
easy way to be sure of the results ahead of time.  

Bob

On Jun 28, 2014, at 3:21 AM, Magnus Danielson  <magnus at rubidium.dyndns.org> 
wrote:

> Hi,
> 
> I  fail to see what the benefit is of removing this unless a better temp  
compensation scheme is used. It is not likely to interfer with the external 
 loop as it reduces the midterm noise that is systematic. It does add some  
higher rate noise but that is quantization errors of the systematics it  
reduces. I like to see measurement that support the claim and I am skeptic. As 
 I see it you give the external loop more systematic noise to dampen and 
the  tighter loop you make the more you will expose.
> 
> Cheers,  
> Magnus
> 
> <div>-------- Originalmeddelande  --------</div><div>Från: Scott Newell  
<newell+timenuts at n5tnl.com>  </div><div>Datum:28-06-2014  03:50  (GMT+01:00) 
 </div><div>Till: time-nuts at febo.com </div><div>Rubrik:  [time-nuts] DIY 
FE-5680A lobotomy (disable temp compensation)  </div><div>
> </div>Bert asked me to send an update on  the FE-5680 tempco mod progress.
> 
> It appears that the FE-5680A  temperature signal (or maybe it's really 
> a current sense signal?) can  be disabled by removing a single 10k 
> 0805 surface mount  resistor.
> 
> Using Elio Corbolante's terrific high-res scans,  I've noted the 
> resistor location:  http://www.n5tnl.com/time/fe-5680a/lobotomy.png
> 
> Why would you  want to disable temperature compensation? As we've 
> seen, the unit's  firmware will adjust the DDS frequency as the 
> temperature signal  changes. If you're using the '5680 inside a 
> control loop, it's likely  to conflict. By removing the resistor, that 
> channel of the 12 bit ADC  will be tied to ground through an existing 
> 2.21k resistor. The unit  will see a constant 0 counts from the ADC 
> and assume it's really  cold.
> 
> I modified one unit and monitored it for a few hours  over a range of 
> temps, running it nice and hot with no heatsink, then  blasting it 
> with a fan and placing it on an ice-cold heatsink. I  observed no 
> change in the DDS tuning words.
> 
> It's a  really easy mod--remove four screws, set aside the insulator 
> sheet,  and apply your hot leucotome/soldering iron.
> 
> 
> I've  also found a simple mod to replace the temperature signal with 
> the  output of the unused trimpot. This allows you to simulate any 
>  temperature you want. If there's any interest, I'll set up a test and 
>  monitor the DDS tuning words as the unit's firmware tries to adjust 
>  to the fake temp signal.
> 
> 
> -- 
> newell   N5TNL
> 
> _______________________________________________
>  time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to  
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the  instructions there.
>  _______________________________________________
> time-nuts mailing list  -- time-nuts at febo.com
> To unsubscribe, go to  
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the  instructions  there.

_______________________________________________
time-nuts  mailing list -- time-nuts at febo.com
To unsubscribe, go to  
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the  instructions there.