Jim Williams did this in one of his designs for measuring low frequency reference noise. The large value low leakage wet tantalum capacitor he used was like $400 and it took 24 hours for the dielectric absorption to settle:
http://www.linear.com/docs/28585 You can get the necessary time constant using a good 1uF film capacitor with good design and construction in this case. On Sun, 1 Jan 2012 15:11:04 -0500, Bob Camp <li...@rtty.us> wrote: >Any real world capacitor will have a dielecric with an associated insulation >resistance. It's a "more money gets better performance" sort of thing, but >there are indeed limits. A 1000 uF cap that has a "good" insulation resistance >number might cost you more than some new carsÂ…. > >On Dec 31, 2011, at 11:54 PM, Chris Albertson wrote: > >> On Sat, Dec 31, 2011 at 5:56 PM, Hal Murray <hmur...@megapathdsl.net> wrote: >> >>> I think the main problem in this area is building a low pass filter with a >>> long time constant. >>> >>> The time constant of the filter has to be: >>> long relative to the noise from the phase detector >>> short relative to aging of the oscillator >>> short relative to environmental changes >>> (so the osc can track temperature and voltage >>> those changes may be in the PLL system rather than the osc) >>> >>> If we are starting with PPS (rather than 10KHz), the filter time constant >>> needs to be 10s or 100s of seconds. How do I build an analog filter with a >>> time constant that long? >>> >> >> Time constant is just R*C. If you have a 1000uF cap and a 1K resistor you >> have 1 second. In theory you could build 100s just by using a 100K >> resistor but I think real world components are not perfect enough. _______________________________________________ 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.