[time-nuts] Building a DMTD/phase noise set in the 21st century

[Bruce Griffiths]

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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