); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Bruce Griffiths wrote: > [EMAIL PROTECTED] 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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