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

EWKehren at aol.com EWKehren at aol.com
Sat Jun 28 12:31:41 EDT 2014


I can only answer some of your questions. The tuning word is modified by  
the temperature but if it came back to the same oven current it will again 
have  the same tuning word to the DDS.
Since we have not fount a temperature sensor they most likely use oven  
current that Bob mentioned.
If you do not plan to use it with a controller I would not recommend  
disconnecting it but if you have good temperature control you should not see a  
difference.
The controller can not selectively enable or disable the temperature sensor 
 and it would not make sense since the controller has no info as to 
temperature  and enabling the frequency control do to temperature would most likely 
cause a  frequency step.. The answer is use a laptop heat pipe it will do a 
great job  when portable and takes little power, Just received some more 
recharable lithium  12 V 9.8 A batteries. Light weight and low cost 
Bert Kehren
 
In a message dated 6/27/2014 11:04:31 P.M. Eastern Daylight Time,  
albertson.chris at gmail.com writes:

So the  FE5680A will actually change the DDS tuning word based on an
internal  temperature sensor?

I could see why you might ant to disable this or  maybe not depending
on how it works.  Does the FE5680 first read the  user programmed word,
apply a delta then write it back or does it ignore  user settings.   I
can't believe it would overwrite a user  programmed running word.

But this also means there is a sensitive  temperature sensor inside the
FE5680.  Is there any way to read this  sensor via the serial port?

What software are you using to communicate  with the FE5680.  I'd like
to try doe experiments.   1) see  if we can take advantage some how of
the existence of the temp sensor and  2) possibly use the analog input
(a faked temp sensor) to control the  FE5680.


The problem with disconnecting the temperature sensor is  much worse
performance when the GPS is not available in "hold over"  mode.   GPS
rarely fails but I'm sure some people disconnect the  GPS to use the Rb
as a portable frequency reference.   It would  be good if it were
temperature compensated while in holdover.

One  experiment comes to mind:  If the resister is removed, can there
GPSDO  controller selectively enable and disable  temperature
compensation?

What software are you using to support  your testing?



On Fri, Jun 27, 2014 at 6:50 PM, Scott Newell  <newell+timenuts at n5tnl.com> 
wrote:
> 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
>
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-- 

Chris Albertson
Redondo  Beach,  California
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