Steve Rooke wrote: > 2008/10/12 Myers, Charlie <[EMAIL PROTECTED]>: > >> Hello to the Time Nuts, >> >> I have been reading the mail on this topic for the last week or so with >> great interest. Lots of interesting ideas have been put forth for >> measuring frequency to a high degree of precision and for comparing a 10 >> MHz clock's frequency to a highly accurate 10Mhz frequency "standard". >> >> The way I measure the frequency of a 10 MHz clock is to compare it to a >> second 10MHz clock of known accuracy and stability, not only with a >> frequency counter but also with a phase meter. >> >> I have several GPS disciplined OCXO's, one GPS disciplined Rubidium >> oscillator, and several free running rubidium oscillators. I measure >> the frequency of an unknown 10 MHz clock using a 2 step process. First >> I measure the unknown 10 MHz clock using an HP 5384A reciprocal counter >> that employs my known 10 MHz clock as its external timebase. I set the >> gate time to 10 seconds and the counter delivers a frequency measurement >> with a resolution of less than 3 mhz (3 millihertz). So, assuming my >> known timebase is "bang on", I know the frequency of the unknown 10 MHz >> source to an accuracy of roughly 3e-10 or 3 parts in 10 billion. >> >> To get a more precise measurement of the frequency difference between >> the two 10 MHz clocks, I supply the known 10 MHz clock to the Channel A >> input of an HP 3575A Gain-Phase meter and the unknown 10 MHz clock to >> the channel B input of the Gain-Phase meter. I measure the change in >> the phase angle between the 2 input clocks over some convenient time >> interval (e.g., 10, 100, or 1,000 seconds) and compute the frequency >> difference using the formula: >> >> >> Frequency Difference = [Change in Phase Angle (in degrees) / Measurement >> Duration (in seconds)] X [1 / 360] >> >> The frequency difference can then be converted to frequency accuracy >> using the formula: >> >> Accuracy = Frequency Difference / 1e7 >> >> >> This seems like a pretty straight forward technique. Am I missing >> something? >> > > So what's time nutty about this method... > > :) > Using ~40 year old boat anchor rackmount equipment to measure something that can be done more accurately with a handful Of ICs.
Bruce _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
