Aren't there op-amp circuits that create a large capacitance? The gyrator? Don
David > 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. > > -- "Neither the voice of authority nor the weight of reason and argument are as significant as experiment, for thence comes quiet to the mind." R. Bacon "If you don't know what it is, don't poke it." Ghost in the Shell Dr. Don Latham AJ7LL Six Mile Systems LLP 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com _______________________________________________ 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.