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

[Bob Camp]

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> wrote:
> 
> A good filter in the cable is highly recommended, 5 KOhm  & 1000  uF cleans 
> many things
> 
> 
> In a message dated 8/1/2016 11:12:51 A.M. Eastern Daylight Time,  
> kb8tq at n1k.org writes:
> 
> Hi
> 
> It’s just very standard FM modulation math. The only  gotcha is the 
> (often unknown) bandwidth of the EFC port. Even on a  precision 
> OCXO, it might be <10 Hz, it might be over a KHz …. The trap  many
> fall into is the “small angle” restriction. You can get into  modulation 
> indexes that will get the second and third order terms  contributing. 
> It’s more common to see on vibration, but it can happen on a  noisy
> EFC.
> 
> Bob
> 
> 
>> On Aug 1, 2016, at 9:46 AM, Attila  Kinali <attila at kinali.ch> wrote:
>> 
>> Moin,
>> 
>> I need some formulas that relate EFC noise to the (added) phase noise  of
>> an OCXO. It shouldn't be too difficult to come up with something.  But
>> before I make some stupid mistakes, i wanted to ask whether  someone
>> has already done this or has any references to papers? My  google-foo
>> was not strong enough to find something.
>> 
>> Attila Kinali
>> 
>> --  
>> It is upon moral qualities that a society is ultimately founded. All  
>> the prosperity and technological sophistication in the world is of no  
>> use without that foundation.
>>        -- Miss Matheson, The Diamond Age, Neil  Stephenson
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