In the world of time nuts, little things matter. Nanoseconds are an eternity. We rip our hair out (if we have any left) over 1E-11 errors, weep over phase noise and shriek about Allen deviations, modified or otherwise.
The question is: just where are the little things? If you are new at this, I hope my experience helps you. If you are an old hand, you need not read on. Or you can just smile at someone else's foibles. I have been working on a disciplined, double oven crystal standard. I put the resistors controlling the gain of the VCXO in the outer oven along with the DAC that generates the EFT voltage so that I could tweak the loop gain by selecting the values during calibration without disturbing the main oscillator. The trade was to minimize any noise out of the DAC while ensuring the embedded processor could steer the VCXO with the necessary precision. I made the measurements, pulled some resistors out of my collection and was on my way to months of insanity while I tried to figure out why my lovely oscillator wandered like a over-sugared kid a petting zoo. I checked everything. The power supplies, the precision DACs; everything had been selected to be wonderfully stable. And it was in an oven, after all. After eliminating all the other suspects, I tested resistors of the same type I had installed in the EFT circuit. Using a precision, 8 digit ohmmeter, I put the resistors through a temperature excursion of about 20 deg C by external heating. The results are summarized below. I had used resistors of the first two types in the list. carbon composition 5% -550 ppm/deg C cheap 5 % metal film -240 ppm/deg C 1% metal film axial lead <-1 ppm/deg C 1% thick film surface mount <-1 ppm/deg C My resistors were mounted near the walls of the outer oven, about 2 cm from one of the heaters. A change of about 0.1 deg C would more than explain what I had been seeing. Also the carbon composition resistor drifted with time at elevated temperature. This may explain some of the long term trends I had seen. The moral of this tale of woe is to know thy components. Know them well. _______________________________________________ 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.
