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

[Bruce Griffiths]

Bruce Griffiths wrote:
> 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
>>  
>>  
>>   
>>     
> Why restrict operation to just the -2.5V to +2.5V portion of the
> 10811A/10544A EFC range, isnt the 0 to +5V portion just as useful?
> This requires no additional opamps or references or even a negative supply.
> The only drawback is that one has to manually adjust the oscillator
> somewhat more often than if the full EFC range were used.
>
> Another option is use a simple opamp reference inverter circuit using no
> precision resistors to subtract 2.5V from the Fury RFC output.
> The reference inverter can include a lowpass filter to reduce the
> reference noise.
>
> The simpler we can make the interface with acceptable performance the
> better.
> Limiting the number of power supplies required is also useful,
> particularly if one wishes to be able to run the system from batteries
> in the event of the loss of a mains supply.
>
> Of course if one has an OCXO with an EFC range of 0 to -10V, or 0 to -5V
> etc then using an opamp or 2 is unavoidable.
>
> If the interface is limited to OCXOs that have an EFC range that
> includes 0 to +5V then additional (negative) supplies and opamps can be
> avoided.
>
> Maybe we need to compile a list of suitable OCXOs complete with datasheets.
> The list includes:
> 10544A
> 10544B
> 10544C
> 10811A
> E1938A
> Symmetricom 1000B
> SRS SC10
> ...............
> For which circuits and/or datasheets are available.
> Tabulating their EFC ranges would also be useful.
>
> Bruce
>
>