[time-nuts] Effect of EFC noise on phase noise

[Scott Stobbe]

Yep, it supports the big C (padded out with increasingly smaller caps) in
general wins. For two low pass filters, one with say 100nF and one with
10nF, same fc, the 100nF filter will have 10 times less noise power, or
sqrt(10) less rms noise. Near DC is another story.

On Mon, Aug 1, 2016 at 5:10 PM, Bob Camp <kb8tq at n1k.org> wrote:

> HI
>
> Broadband is not where you run into the trouble on any of these circuits.
> It’s
> always what happens within a decade or two past cutoff or inside the pass
> band.
>
> Bob
>
> > On Aug 1, 2016, at 4:50 PM, Scott Stobbe <scott.j.stobbe at gmail.com>
> wrote:
> >
> > The broadband thermal noise at a circuit point with a cap is always kT/c
> >
> > On Monday, 1 August 2016, Bob Camp <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> If you wire up all the possible circuits and check them all out … the
> >> answer is that big C / small R wins. Big R gets you into resistor noise
> >> issues
> >> and stray pickup.
> >>
> >> Bob
> >>
> >>> On Aug 1, 2016, at 4:16 PM, David <davidwhess at gmail.com
> <javascript:;>>
> >> wrote:
> >>>
> >>> This duplicates the problems encountered when trying to quantify low
> >>> frequency noise from a voltage reference; it is difficult to make an
> >>> low frequency high pass filter with lower noise than the lowest noise
> >>> references and the capacitor is the problem.
> >>>
> >>> In Linear Technology Application Note 124, Jim Williams discusses the
> >>> problems with electrolytic capacitors for this type of application.  I
> >>> have read that you *can* get away with aluminum electrolytics if you
> >>> grade them for low leakage and low noise.  The dielectric absorption
> >>> is also a problem unless you can wait hours for best performance.
> >>>
> >>> What about the alternative of buffering the signal with a low noise
> >>> low input bias current operational amplifier so that a large film
> >>> capacitor can be used instead?  Is the low frequency noise of a good
> >>> operational amplifier still too much?  What about a chopper stabilized
> >>> amplifier without suitable output filter?
> >>>
> >>> On Mon, 1 Aug 2016 11:46:51 -0400, you wrote:
> >>>
> >>>> Hi
> >>>>
> >>>>
> >>> .. until you discover that you picked the *wrong* capacitor
> manufacturer
> >> and you have
> >>>> more noise from leakage in the cap than you did to start out with :)
> >> In general “big C and
> >>>> small R” is the better solution than “big R and small C”.
> >>>>
> >>>> The pesky part is that with electrolytic caps, the whole “noise
> >> current” thing changes as
> >>>> the voltage moves around. You go to measure things and by the time the
> >> gear is set up,
> >>>> the noise has dropped. Turn it all off, come back the next day and
> it’s
> >> noisy again.
> >>>>
> >>>> An even more subtle issue can be capacitor temperature coefficient on
> >> really long Tau filters. If C
> >>>> changes (due to temperature fluxuations) faster than the settling time
> >> of the filter, you get noise. Charge
> >>>> is the same so delta C gives delta V.
> >>>>
> >>>> I *wish* I could tell you that was all purely theoretical.
> >> Unfortunately it’s based on empirical data
> >>>> collected in the “how could I be so stupid” fashion.
> >>>>
> >>>> Bob
> >>>>
> >>>>> On Aug 1, 2016, at 11:21 AM, KA2WEU--- via time-nuts <
> >> time-nuts at febo.com <javascript:;>> wrote:
> >>>>>
> >>>>> A good filter in the cable is highly recommended, 5 KOhm  & 1000  uF
> >> cleans
> >>>>> many things
> >>> _______________________________________________
> >>> time-nuts mailing list -- time-nuts at febo.com <javascript:;>
> >>> 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 at febo.com <javascript:;>
> >> 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 at 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 at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>