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

EWKehren at aol.com EWKehren at aol.com
Sat Jun 28 08:03:16 EDT 2014

Because the 96K attachment needs moderator approval I am sending it again  
without attachment. It is the one I posted before and shows clearly the  
frequency jumps. So here is the rest of it.
Sorry but I disagree with your statement. First and foremost we have to  
accept that these devices are not intended for time nuts (metrology). Already 
my  first 1985 FRK monitors the cell current and adjusts the C field  
accordingly. The HP 5065 does not since it is not intended for the same market,  
it relies on its temperature control of the total A12 assembly. A closer look 
at  FE specs you will notice the following statement " including frequency 
over  or undershoot at any fast or slow temperature slew rate". How do you 
think they  do it. In the case of units that have a DDS in the control loop 
they do it with  the DDS. How else do you explain the attached plot. Again my 
apologies for not  remembering who posted it. I noticed the jumps and 
mentioned them when I first  took a look at the 5680 I did use temperature 
control in a crude way using what  I call the Bang Bang fan control on an ATT FRS 
heatsink. My YSI did not register  any changes but the 5680 noticed and was 
visible using my Tracor 527 E.
The 5680 is not a unit that we will use it turned in to a distraction but  
we did the controller for time nuts. Our focus when it comes to temperature  
control is on the FRK that is why Juerg who focused on the 5680 only uses a 
heat  sink. Working out of a basement that is next to a garage data in the 
winter  looks a lot more stable than now. We monitor the tuning word which 
has a  resolution of 6.8 E-13 and when you see no change over long time 
periods in  January and changes now exceeding E-12 now to start asking why. The 
differences  are garage door is open more often and the sun shines part of 
the time directly  in to his basement lab.
When the loop is in the long time constant mode to take advantage of the  
GPS accuracy resolution which increases with time,  jumps by the 5680  DDS 
are no help. It all depends what ultimate accuracy time nuts want out of the  
5680. A good fan control along with a fixed setting the temperature ADC 
input  used for frequency control will yield best results. Work by time nuts  
will help we are not going to do it.
The reason is we are totally tied up with work on using the FE 405 B. I  
stumbled by accident on to it and bought some for testing. Initial tests show  
for me unbelievable performance specially when it comes to ADEV. It is 
perfect  for GPSDO applications since it is all digital control with a step 
resolution of  6 e-15. How ever it is also not all perfect since I did detected 
jumps that I  could not explain. Since our testing capabilities are limited 
I did make Tom  aware of the unit and he has now caught the bug.  
http://leapsecond.com/pages/fe405/  . We  think we have traced the problem to again 
frequency control using oven current  have disabled it and hope to have better 
data to prove that the 3 E-12 jumps are  due to the current monitor. Stay 
tuned. Hope it does not change the ADEV. Not  many of you have OCXO's with 
that low ADEV.
We are also using the controller for this device and results look very  
That is why we call it the Universal Controller. In the future it will  
control many other devices.
But for now please those of you that have the equpment and the expertize  
focus on the  FE 5680 A to make it a viable low cost house reference for a  
large # of time nuts.
Bert Kehren

In a message dated 6/28/2014 3:21:48 A.M. Eastern Daylight Time,  
magnus at rubidium.dyndns.org writes:


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.


<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

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