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

David davidwhess at gmail.com
Mon Aug 1 16:16:01 EDT 2016

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:

.. 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. 
>> On Aug 1, 2016, at 11:21 AM, KA2WEU--- via time-nuts <time-nuts at febo.com> wrote:
>> A good filter in the cable is highly recommended, 5 KOhm  & 1000  uF cleans 
>> many things

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