[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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