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

[Bob Camp]

Hi

The answer is (as always) "no free lunch". An op amp simulating a capacitor is always going to be performance limited. Different limitations come in with different circuits, but they all suffer from noise / drift / leakage. Put another way - for a long time constant, you are just as good off using the op amp as a buffer and dropping in a giant resistor.  

Bob


On Jan 1, 2012, at 4:22 PM, David wrote:

> Gyrators are usually used to create impractical inductances or
> frequency dependant negative resistances but I suppose you could.  I
> do not think you would gain anything though since you would be trading
> one set of non-ideal behaviors for a different set.  This is
> especially the case since the non-ideal behavior of inductors is
> almost always worse than the non-ideal behavior of capacitors.
> 
> For example, you can sometimes avoid large feedback or input
> resistances by substituting a T-network but offset voltages and
> voltage noise will be multiplied accordingly.
> 
> On Sun, 1 Jan 2012 13:52:27 -0700 (MST), "Don Latham"
> <djl at montana.com> wrote:
> 
>> Aren't there op-amp circuits that create a large capacitance? The gyrator?
>> 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.
>>> 
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>> 
>>> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.