[time-nuts] Phase measurement of my GPSDO

[Tobias Pluess]

Hi Gerhard

ah yes I didn't know that only old OpAmps have the phase reversal problem.
At least in the data sheets for some newer types it is sometimes explicitly
mentioned - "no phase reversal" - but for others it is not, and so far I
never was enough interested in this problem to find out that it only
happens for the JFET ones, but good to know! unless it is mentioned
explicitly, I usually assume that it can happen because it once caused me
some serious troubles and headache in a circuit board which was not
designed by me, but which I had to debug. At that time I did not yet know
about this property of some amps.

Tobias

On Fri., 3 Apr. 2020, 23:42 Gerhard Hoffmann, <ghf at hoffmann-hochfrequenz.de>
wrote:

>
> Am 03.04.20 um 23:08 schrieb Bruce Griffiths:
> > One can merely add diodes to the opamp feedback network form a feedback
> limiter and maintain the opamp outputs within the range for which the opamp
> is well behaved whilst maintaining the increase in slew rate for the output.
>
> Has anybody here  ever tried the OPA698 / OPA699 limiting op amps?
>
> http://www.ti.com/lit/ds/symlink/opa698.pdf
>
> A lower 1/f corner would be appreciated, and slightly less noise.
>
> >> On 04 April 2020 at 04:26 Tobias Pluess <tpluess at ieee.org> wrote:
> >>
> >>
> >> Jup, some of them even have phase reversal when they are overloaded, so
> it
> >> is perhaps not a good idea in general, but I think there are opamps
> which
> >> are specified for this.
>
> That phase reversal thing is a misfeature of old JFET-OpAmps when
>
> overdriven at the input. It created weird behavior of feedback loops.
>
> Newer ones have that corrected.
>
>
> Cheers, Gerhard
>
>
>
>
>
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