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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Nov 3 09:19:27 UTC 2007


SAIDJACK at aol.com wrote:
>  
> 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
>  
>  
>   
Said

Another consideration is that for 10544A's and similar oscillators which
are only intended to drive high impedance loads( >= 1K) a simple 2
transistor cascode buffer (or maybe a 10811A style common emitter stage
with series feedback in the emitter circuit) may be required to allow
them to drive a 50 ohm load or an RF transformer satisfactorily.  A
cascode buffer has higher isolation than a common emitter stage.

Bruce
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