Hi For some designs (like a Rb) the time constant may be in the "days" range….
Even for something simple, you can easily get out to several thousand seconds. You also need low noise past the cutoff time (for short times the cap may help you). That's going to get you into resistor noise and / or op amp noise. All of this will push up the capacitance required. Quick and dirty example: 1 pps comparison setup 10 ns jitter on the GPS ( 1x10^-8 at one second) 1x10^-11 as the desired GPSDO jitter at one second. … you need > 1,000X attenuation of the jitter at one second. With a 1 uf cap, that's gets you to pretty noisy resistors. Most TBolt's are quite happy doing the sort of thing in the example. Bob On Jan 1, 2012, at 3:43 PM, David wrote: > 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 cars…. >> >> 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. _______________________________________________ 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.
