"While we have been discussing stuff that is about 10E-20 below what we can measure, is it possible that the limit of resolution of the measurement of time is determined by noise? Specifically, thermal or Johnson noise in those 50 ohm impedances that we use for our cabling."
This didn't provoke any response, so maybe you think I'm crazy. You could still be right, but let me explain. We are talking about the accuracy of clocks, which are basically digital counters that are mostly immune to noise. You also need an oscillator that generates something that a clock can count. Noise gets into it at the interface between the oscillator and the clock. Assuming electronics (because mechanical oscillators have too many sources of error) then something electronic has to detect the periodic oscillations and turn them into pulses for the clock's first counter/divider. Noise in the detector will cause jitter in the clock. The problem can be reduced by increasing the signal to noise ratio. Can amplifiers be made noise-free? Certainly not in radio receivers. We are talking about accuracy better than 10E-15. Johnson noise in 50 ohms is around 10E-8 volt at 100C with a one MHz bandwidth. You would need a signal of around 10E+7 volts to swamp the noise. Precision oscillators are cryogenic these days, which could eliminate thermal noise. Truly random noise can be reduced by filtering or averaging over time. The amount of time available depends on the use of the clock. A time-of-day clock should have no error from noise. Regards, Bill Hawkins _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
