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

Dr. Ulrich Rohde ka2weu at aol.com
Mon Aug 1 21:56:50 UTC 2016


With my filter , I had good success and 5 K is not too high , Ulrich 

Sent from my iPhone

> On Aug 1, 2016, at 11:46 AM, Bob Camp <kb8tq at n1k.org> 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> 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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