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

Bob Camp kb8tq at n1k.org
Mon Aug 1 16:32:35 EDT 2016


If you wire up all the possible circuits and check them all out … the 
answer is that big C / small R wins. Big R gets you into resistor noise issues
and stray pickup. 


> On Aug 1, 2016, at 4:16 PM, David <davidwhess at gmail.com> wrote:
> 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:
>> 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
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