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

[Don Collie]

Can`t it be done with just 3 resistors?
Cheers,.............................................Don C.


----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: <SAIDJACK at aol.com>; "Discussion of precise time and frequency 
measurement" <time-nuts at febo.com>
Sent: Saturday, November 03, 2007 10:19 PM
Subject: Re: [time-nuts] Fury Interface Board: How about TI OPA277?


> 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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>>
>
>
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