Hi 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….
No free lunch. Bob On Dec 31, 2011, at 11:54 PM, Chris Albertson wrote: > On Sat, Dec 31, 2011 at 5:56 PM, Hal Murray <[email protected]> wrote: > >> >>> As soon as you say "Software" the device is no longer simple. Even a >>> microprocessor is a very complex device and so is its development system. >>> The software inside the uP is not simple either if you count the number >> of >>> possible paths through the code (2 raided to the power of the number of >>> branches.) >> >> Yes and no... >> >> Software doesn't have to be big, bloated, ugly, and complicated. (But I >> agree that it often is.) >> > > If you have eight "if" statements you have 2^8 = 256 possible paths through > the code. For a hobby application I goes you'd not bother to write up and > run 256 test cases. > >> >> This looks like fun to me, but I like writing that sort of code. Note that >> it doesn't need an OS or even any libraries. >> >> >> The context for "simple" wasn't well specified. >> >> Does simple refer to design or construction? > > > I think "simple" means you can explain how it works in a few sentences. > And if software is used you have to explain every calculation and decision > point. > > With software design and construction is the same thing if you only build > one unit. > >> >> How good does the GPSDO have to be? (After all, this is time nuts.) What >> sort of adev at what sort of time scale? >> >> >> 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. > > What's the input impedance of a VCXO or Rb unit? I assume we will need an >> op-amp to buffer the filter. >> > > I suspect you are right. > > >> >> The ugly problem in this area is that time constant to filter out phase >> detector noise overlaps the time constant needed to let environmental >> changes through. That doesn't matter if the filter is analog or digital. >> >> If the osc is stable (Rb) filter time constants of 1000s of seconds might >> make sense. That might help take care of some of the hanging bridges. >> > > The new $38 Rb units can only be adjust by RS-232 commands. So you need a > digital controller. No choice there. > The best oscillator for an analog controller would have to be a high > quality ovenized crystal. > > > About the time constants. If you are doing this in software then you can > track performance inside the controller and adjust. Seems you shouod be > able to tell the controller the "tau" you need and it should be able to > optimize. > > Once you have a uP then more features are easy to do, like maybe using > multiple GPS receivers or maybe fault detection and switching to holdover > mode > >> >> For those who aren't familiar with this trick, it's easy to make a low >> pass filter in software: >> X = X*(1-k) + k*new > > > Designing filters seems like an art. What is the frequency response of the > above for different values of k? I tend to like FIR filters because I > think I understand them better. I think yours is an IIR. > >> >> >> Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
