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

[Scott Newell]

At 10:03 PM 6/27/2014, Chris Albertson wrote:
>So the FE5680A will actually change the DDS tuning word based on an
>internal temperature sensor?

It appears so. (Again, assuming it's actually a temperature sensor. I 
guess it could be measuring heater current?)


>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

I've not done extensive testing with a user programmed frequency, but 
it does appear to apply the tempco correction to the user frequency.


>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?

Yes. The temp signal goes to a 10k:2.21k divider, filtered by a cap, 
and then to pin 4 of the 4 channel 12 bit ADC. Command 0x22 returns 
the ADC readings as four 16 bit values. (There's also a VCXO related 
value, the trimpot, and what I thought was a rail or reference, but 
now I'm unsure on that one.)


>What software are you using to communicate with the FE5680.  I'd like

Just some C crap I tossed together. It's currently running under Cygwin.


>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.

Servo controlling a fan based on the temp signal or VCXO sounds fun. 
I'm not sure you'd be happy with the fake temp signal--the unit is 
probably using a custom lookup table to correct the frequency vs temp.

For example, I just spun the trimpot down from fullscale to 0. Here's 
the fake temp signal, the VCXO signal, and the two DDS tuning words 
as the unit tries to apply tempco correction:

4094    1870  1140962691 1140662043
4094    1859  1140962691 1140662043
2    1853  1140962974 1140662326
0    1845  1140962974 1140662326


-- 
newell N5TNL