[time-nuts] re low noise regulators
Didier Juges
didier at cox.net
Sun Dec 16 16:35:07 EST 2007
I think the issue with the series resistor is that the oven current is
temperature dependant, so the drop through the 2 ohm resistor is temperature
dependant too, regardless of the tolerance or stability of the resistor
itself.
As long as the current/temperature curve of the oven is predictable, that
could be compensated, assuming the loop response of the oven has no
overshoot. I think it requires making too many assumptions for that scheme
to work though.
Didier KO4BB
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of SAIDJACK at aol.com
> Sent: Sunday, December 16, 2007 2:27 PM
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] re low noise regulators
>
> In a message dated 12/14/2007 15:00:00 Pacific Standard Time,
> bruce.griffiths at xtra.co.nz writes:
>
> >> A simple RC filter of say 2 Ohms into 4700uF has a -3dB cut-off at
> around
> >> 17Hz (4700uF caps are getting quite small these days). That would
> >> take
> care of
> >> most of the 100Hz to 10KHz noise.
> >
> >It still helps to have as low a noise as possible before using such
> >brute force filtering to get that extra few dB of noise reduction.
> >> Using a typical current of 0.16A at 12V for a Euro-can
> OCXO we would
> >> only
>
> >> have 0.32V voltage drop across the resistor.
> >>
> >However the resultant drop will depend on the ambient temperature.
> >In principle the regulator output could be given a suitable
> >temperature drift characteristic to compensate.
> >However doing this without degrading the noise at the filter output
> >may be challenging.
> Hi Bruce,
>
> the tempco effect of the series resistor may not be a factor
> at all for typical OCXO's.
>
> If we take a standard 1% resistor such as the Panasonic
> ERO-S2PHF2R00 (available on Digikey) with 100ppm/C tempco
> (that's a fairly bad tempco), then we
> get:
>
> 0.32V * 100ppm = 32 microvolts per Degree C change (actually it's
> +/-50ppm, so this example is worse than what we would see in
> real life).
>
> If we now take the Supply-Voltage-versus-Frequency
> characteristic of a typical standard single oven OCXO of
> 1E-08, at +/-5% variation for a 12V part we
> get:
>
> 1E-08 for 1.2V change, then we get: 8.33E-09/V * 32E-06V =
> 2.66E-013 change per degree C.
>
> For this particular OCXO with a stability of 1E-09 per degree
> C, the change in frequency due to the change in the 2 Ohm
> resistor value is about 3700x less than the temperature
> stability of the OCXO itself.
>
> In other words the effect of the resistor is inconsequential.
>
> For a good double-oven OCXO this is even less of a factor,
> for exampe the MTI double oven we use only has 8.3E-11 per
> volt change - two orders of magnitude less sensitivity than
> the example above.
>
> The capacitors' microphonic sensitivity may actuallyy be more
> of a factor here.
>
> >It still helps to have as low a noise as possible before using such
> >brute force filtering to get that extra few dB of noise reduction.
>
> It's not just a few dB, it's -3dB at 17Hz already, and that
> drops at ~-20dB per decade. At 170Hz it could theoretically
> give -23dB already, or in other words if the noise floor was
> entirely due to the supply voltage noise, then the
> difference would be between say -140dBc/Hz and about
> -160dBc/Hz at 170Hz - this could be a very significant improvement.
>
> You are absolutely right, it is very difficult to get low
> noise <10Hz.
>
> bye,
> Said
>
>
>
>
>
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