I think the issue with the series resistor is that the oven current is temperature dependant, so the drop through the 2 ohm resistor is temperature dependant too, regardless of the tolerance or stability of the resistor itself.
As long as the current/temperature curve of the oven is predictable, that could be compensated, assuming the loop response of the oven has no overshoot. I think it requires making too many assumptions for that scheme to work though. Didier KO4BB > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] > Sent: Sunday, December 16, 2007 2:27 PM > To: [email protected] > Subject: Re: [time-nuts] re low noise regulators > > In a message dated 12/14/2007 15:00:00 Pacific Standard Time, > [EMAIL PROTECTED] writes: > > >> A simple RC filter of say 2 Ohms into 4700uF has a -3dB cut-off at > around > >> 17Hz (4700uF caps are getting quite small these days). That would > >> take > care of > >> most of the 100Hz to 10KHz noise. > > > >It still helps to have as low a noise as possible before using such > >brute force filtering to get that extra few dB of noise reduction. > >> Using a typical current of 0.16A at 12V for a Euro-can > OCXO we would > >> only > > >> have 0.32V voltage drop across the resistor. > >> > >However the resultant drop will depend on the ambient temperature. > >In principle the regulator output could be given a suitable > >temperature drift characteristic to compensate. > >However doing this without degrading the noise at the filter output > >may be challenging. > Hi Bruce, > > the tempco effect of the series resistor may not be a factor > at all for typical OCXO's. > > If we take a standard 1% resistor such as the Panasonic > ERO-S2PHF2R00 (available on Digikey) with 100ppm/C tempco > (that's a fairly bad tempco), then we > get: > > 0.32V * 100ppm = 32 microvolts per Degree C change (actually it's > +/-50ppm, so this example is worse than what we would see in > real life). > > If we now take the Supply-Voltage-versus-Frequency > characteristic of a typical standard single oven OCXO of > 1E-08, at +/-5% variation for a 12V part we > get: > > 1E-08 for 1.2V change, then we get: 8.33E-09/V * 32E-06V = > 2.66E-013 change per degree C. > > For this particular OCXO with a stability of 1E-09 per degree > C, the change in frequency due to the change in the 2 Ohm > resistor value is about 3700x less than the temperature > stability of the OCXO itself. > > In other words the effect of the resistor is inconsequential. > > For a good double-oven OCXO this is even less of a factor, > for exampe the MTI double oven we use only has 8.3E-11 per > volt change - two orders of magnitude less sensitivity than > the example above. > > The capacitors' microphonic sensitivity may actuallyy be more > of a factor here. > > >It still helps to have as low a noise as possible before using such > >brute force filtering to get that extra few dB of noise reduction. > > It's not just a few dB, it's -3dB at 17Hz already, and that > drops at ~-20dB per decade. At 170Hz it could theoretically > give -23dB already, or in other words if the noise floor was > entirely due to the supply voltage noise, then the > difference would be between say -140dBc/Hz and about > -160dBc/Hz at 170Hz - this could be a very significant improvement. > > You are absolutely right, it is very difficult to get low > noise <10Hz. > > bye, > Said > > > > > > **************************************See AOL's top rated recipes > (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004) > _______________________________________________ > 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. _______________________________________________ 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.
