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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Nov 2 19:42:45 EDT 2007


Bruce Griffiths wrote:
> kevin-usenet at horizon.com wrote:
>   
>>>> One trick I use to get better tracking is to build ratios out of multiples
>>>> of a single resistor value.
>>>>       
>>>>         
>>   
>>   
>>     
>>> Can you guarantee this works every time?
>>>     
>>>       
>> Define "guarantee".  For a reasonable risk ratio, yes.  Resistor
>> manufacturers don't go out of their way to package resistors from
>> different batches on the same reel, so you if you use adjacent parts from
>> the reel, the odds are overwhelming that you'll get a uniform material
>> composition, which leads to a consistent tempco.
>>
>> That's certainly good enough for me to build a product out of
>> the parts before doing the temperature testing.  (It's not
>> like parts always meet guaranteed specs, either.  Remember the
>> Signetics 25120 Sale price vs. AQL curve.)
>>
>> If you want it pre-tested, get a resistor network.  Either custom, or
>> again get multiple copies of the same resistance and connect them in
>> the ratios you want.
>>
>> For more on the subjet of resistor tempco tracking, including all
>> of the above suggestions echoed by a moderately experienced
>> engineer, see "Debugging Analog Circuits" by Robert A. Pease.
>> See chapter 2, page 28.
>>
>>   
>>     
> The Achilles' heel of this technique is the statistical temperature
> fluctuations experienced by individual resistors.
> If the resistors are not closely thermally coupled than their
> temperatures may not track sufficiently closely.
> This effect is exacerbated when the product of the resistor tempco and
> the temperature rise of the resistor is too large.
> The old resistor databooks used to include a section on the temperature
> fluctuations of resistor hot spots.
> These fluctuations are usually due to convective cooling fluctuations,
> however if the resistor element is small enough temperature fluctuations
> due to thermodynamic considerations can be significant.
> In other words the resistor tempco and hot spot temperature rise should
> be kept small for this technique to be most effective.
>
>   
>>> OCXO requires a supply voltage with better than 0.01% stability for time
>>> intervals less than the discipling loop response time.
>>>     
>>>       
>> Huh?  Where do you get that from?  Dont they internally subregulate?
>>   
>>     
> There is no internal Oven supply regulator in either the 10811A or the
> 10544A.
>   
>> The raw power is used only for the oven... oh.  Yes, and power is V^2/R,
>> so epsilon change in voltage causes 2*epsilon change in oven power.
>>
>>
>>   
>>     
> Except for extremely unlikely pathological cases, the response to small
> changes in heater voltage etc is linear.
> Thus one can with a fair degree of accuracy predict the various required
> voltage stability from  the 10544A datasheet specifications:
>
> 1) A 10% change in Oven voltage produces a frequency change of < 1E-10
> If one requires an instability of less than 1E-12 due to oven voltage
> fluctuations then an oven voltage short term stability of 0.1% or so may
> be necessary.
>
> 2) A 1% change in Oscillator supply voltage produces a frequency change
> of less than 5E-10.
> If one requires an instability of less than 1E-12 due to oscillator
> supply fluctuations then an oscillator supply voltage short term
> stability of better than 0.002% may be necessary.
>
> 3) A 1% change in Oven controller supply voltage produces a frequency
> change of less than 5E-10.
> If one requires an instability of less than 1E-12 due to oven controller
> supply fluctuations then an oscillator supply voltage short term
> stability of better than 0.002% may be necessary.
>
> The 10811A has somewhat improved specifications
>
> 1) A 10% change in Oven voltage produces a frequency change of < 1E-10
> If one requires an instability of less than 1E-12 due to oven voltage
> fluctuations then an oven voltage short term stability of 0.1% or so may
> be necessary.
>
> 2) A 1% change in Oscillator supply voltage produces a frequency change
> of less than 2E-10.
> If one requires an instability of less than 1E-12 due to oscillator
> supply fluctuations then an oscillator supply voltage short term
> stability of better than 0.005% may be necessary.
>
> The maximum recommended EFC voltage noise is 100uV rms or less.
>
> The short term EFC voltage drift should also be less than 100uV.
>
> Bruce
>
>   




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