[time-nuts] Thunderbolt? (re simple gpsdo.)

Bob Camp lists at rtty.us
Sun Jan 1 22:29:58 UTC 2012


Hi

If you do a quick Google search for "insulation resistance" you get to:

http://www.electrocube.com/support/insulation_resistance.asp

or a bunch of similar information. The capacitor it's self (no external resistance at all) has a time constant between it's capacitance and internal leakage. There's nothing the op amp can do to take care of this. Indeed the leakage of the op amp *adds* to the cap's leakage. Unless it's a very good op amp, the op amp leakage will have a noticable negative impact.

Bob

 
On Jan 1, 2012, at 3:56 PM, Don Latham wrote:

> http://www.falstad.com/circuit/e-capmult.html
> I did not take the time to analyze...
> Don
> 
> David
>> Jim Williams did this in one of his designs for measuring low
>> frequency reference noise.  The large value low leakage wet tantalum
>> capacitor he used was like $400 and it took 24 hours for the
>> dielectric absorption to settle:
>> 
>> http://www.linear.com/docs/28585
>> 
>> You can get the necessary time constant using a good 1uF film
>> capacitor with good design and construction in this case.
>> 
>> On Sun, 1 Jan 2012 15:11:04 -0500, Bob Camp <lists at rtty.us> wrote:
>> 
>>> Any real world capacitor will have a dielecric with an associated
>>> insulation resistance. It's a "more money gets better performance" sort
>>> of thing, but there are indeed limits. A 1000 uF cap that has a "good"
>>> insulation resistance number might cost you more than some new cars….
>>> 
>>> On Dec 31, 2011, at 11:54 PM, Chris Albertson wrote:
>>> 
>>>> On Sat, Dec 31, 2011 at 5:56 PM, Hal Murray <hmurray at megapathdsl.net>
>>>> wrote:
>>>> 
>>>>> I think the main problem in this area is building a low pass filter
>>>>> with a
>>>>> long time constant.
>>>>> 
>>>>> The time constant of the filter has to be:
>>>>> long relative to the noise from the phase detector
>>>>> short relative to aging of the oscillator
>>>>> short relative to environmental changes
>>>>>  (so the osc can track temperature and voltage
>>>>>    those changes may be in the PLL system rather than the osc)
>>>>> 
>>>>> If we are starting with PPS (rather than 10KHz), the filter time
>>>>> constant
>>>>> needs to be 10s or 100s of seconds.  How do I build an analog filter
>>>>> with a
>>>>> time constant that long?
>>>>> 
>>>> 
>>>> Time constant is just R*C.  If you have a 1000uF cap and a 1K
>>>> resistor you
>>>> have 1 second.  In theory you could build 100s just by using a 100K
>>>> resistor but I think real world components are not perfect enough.
>> 
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> 
> 
> -- 
> "Neither the voice of authority nor the weight of reason and argument
> are as significant as experiment, for thence comes quiet to the mind."
> R. Bacon
> "If you don't know what it is, don't poke it."
> Ghost in the Shell
> 
> 
> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
> 17850 Six Mile Road
> POB 134
> Huson, MT, 59846
> VOX 406-626-4304
> www.lightningforensics.com
> www.sixmilesystems.com
> 
> 
> 
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