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

[Chris Albertson]

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