[time-nuts] Fury Interface Board: How about TI OPA277?

[SAIDJACK at aol.com]

In a message dated 11/2/2007 17:51:33 Pacific Daylight Time,  
kevin-usenet at horizon.com writes:

>>  H'm... and if you really want the full +/-10V range, fitting a  regulator
>> and op-amp into the 2V of available headroom requires an  LDO and a
>> rail-to-rail output op-amp.



Hi guys,
 
I've been following the discussion about increasing the Fury EFC range from  
0-5V to -10V to +10V with great interest!
 
I just had an idea on how to avoid all the issues  potentially introduced by 
using an Opamp circuit.
 
Let's take a step back and see how much EFC voltage deviation is  really 
required:
 
1) let's assume we use an HP 10811, so temperature stability is very  good 
and certainly requires less than +/-2.5V range to compensate for (on the  MTI 
double oven units we typically see less than 100uV deviation on the EFC due  to 
temperature!).
 
2) Now let's assume an aging of 5E-08 per year - certainly good OCXO's will  
be better than this. 5E-08 per year at 10MHz is about 0.00137Hz aging  per day.
 
3) For 10811's I have measured a range of 4Hz for a 5V EFC change, so let's  
assume it's EFC gain is 0.8Hz/Volt. This into 0.00137Hz/day means a voltage  
change of 0.00171V per day.
 
This means a -2.5V to +2.5V EFC range would be enough to compensate for  
about 8 years of aging on our well-aged theoretical OCXO, so going to +/-10V is  
probably much more than needed.
 
4) So why don't we just run the OCXO ground at +2.5V instead of 0V, and run  
the Fury ground at 0V?
 
This means the Fury's EFC output (0V to 5V) looks like a -2.5V to +2.5V  
range to the OCXO due to the OCXO's ground being offset by 2.5V.
 
The 10MHz output of the OCXO can be easily transformer-coupled into Fury as  
someone has said earlier, so no problem here.
 
Offsetting the OCXO ground by 2.5V should be possible by adding a -2.5V low  
noise regulator to the system. EFC current is very low, so a low noise  
negative voltage reference may be used to generate the -2.5V.
 
No need for opamps, complex bipolar voltage regulators, etc. Of course any  
noise or drift in the -2.5V regulator would show up in the EFC voltage as an  
error.
 
What do you think?
 
bye,
Said
 
 
 
 
 
 



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