[time-nuts] Effect of EFC noise on phase noise
kb8tq at n1k.org
Mon Aug 1 17:10:55 EDT 2016
Broadband is not where you run into the trouble on any of these circuits. It’s
always what happens within a decade or two past cutoff or inside the pass band.
> On Aug 1, 2016, at 4:50 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> The broadband thermal noise at a circuit point with a cap is always kT/c
> On Monday, 1 August 2016, Bob Camp <kb8tq at n1k.org> wrote:
>> 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
>> and stray pickup.
>>> 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 <
>>>>> A good filter in the cable is highly recommended, 5 KOhm & 1000 uF
>>>>> many things
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