[time-nuts] Building a DMTD/phase noise set in the 21st century
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Aug 30 09:35:56 UTC 2019
Actually the leakage of at least some standard electrolytic capacitors can be quite low if one waits long enough. I've seen a leakage as small as a few nanoamps after several minutes at room temperature with randomly selected 100uF capacitors.
Bruce
> On 30 August 2019 at 18:53 ed breya <eb at telight.com> wrote:
>
>
> I think there may be some confusion over the "super-capacitor" term.
> Over the years, I've seen two types.
>
> The most commonly encountered are the ones in consumer gear, for storing
> charge to keep CMOS RAM alive during power outage and such, for a
> reasonable amount of time. These may be from 47 mF to a Farad or so, and
> have high ESR - they're not expected to dump huge charges, rather steady
> uA range flows for the CMOS.
>
> The other, more exotic kind are for bigger energy storage and power
> conversion - these have very low ESR, like batteries, and may have lots
> of Farads, but generally low working voltages, similar to a battery
> cell. Higher working voltages are attained by stacking, with the
> expected reduction of capacitance, of course.
>
> I've only played with the former, since the latter were unusual and
> expensive. One thing I can say is that the CMOS backup types are pretty
> crappy capacitors for any use beyond their normal role, and they don't
> last very long, in terms of service life - perhaps a decade or so. Most
> that I've salvaged were found to be nearly totally open, like regular
> old electrolytics after their juices have evaporated.
>
> There may be more types nowadays, that overlap a number of applications.
>
> In the situation I mentioned previously, I was planning on paralleling a
> bunch of references to average out the noise somewhat, with combining
> resistances around a hundred ohms. I had hoped to put a bunch of big
> OSCONs in for filtering at the summing junction, but was wary of their
> possible leakage currents lugging it down. I figured I could use
> reasonable-valued, low leakage Ta caps instead, with enough low noise
> amplifier gain to boost their effective values.
>
> Ed
>
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