); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY [EMAIL PROTECTED] wrote: > Hi guys, > > here is a thought: why not use EFC parasitic ground current loops to cancel > out OCXO Tempco? > ___________________ > > Most single oven heaters will have a significant tempco. > > If that EFC tempco happens to be the inverse to the heater current, then it > should be possible to carefully select the heater-current-induced EFC-ground > voltage change to cancel out the OCXO tempco?! > > In other words, if heater current increases (due to falling temperatures), > then EFC voltage will increase due to the parasitic ground loop. > Not quite, the heater current induced ground system voltage drop subtracts from the EFC voltage so the effect depends on the sign of the frequency vs EFC voltage slope. However one could sense the OCXO current and add (or subtract) a voltage proportional to the current from the EFC voltage to compensate for internal and external ground bounce. Some allowance would perhaps have to be made for the ground wiring resistance tempco (complex because the temperature at one end is the oven temperature and at the other the PCB temperature. Either a pot could be used or a microprocessor can perform the calculation and adjust the EFC DAC ouput as appropriate. > > If one is lucky, then the OCXO has a negative tempco that requires a higher > EFC voltage at lower temps. > > Then, it should be possible to pass just the right amount of current through > the EFC ground connection to get a good tempco cancellation :) This current > could be scaled by selecting the length of the EFC ground cable for example. > > Dirty, but probably effective once calibrated. > With some ingenuity and an opamp or 2, either sign can easily be be accommodated. Using a length of PCB track for the current sense resistor may provide a degree of temperature compensation for the tempco of the internal ground wiring resistance, provided that the internal ground wiring resistance tempco tracks that of copper. However the finite thermal gain of a single oven OCXO will in most cases dominate the OCXO thermal drift so an adjustable tempco sensing resistor may be more useful. > > bye, > Said > Bruce
_______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
