[time-nuts] re low noise regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 14 17:58:48 EST 2007


Said
SAIDJACK at aol.com wrote:
> Hi  Bruce,
>  
> I do believe many regulators have most of their noise in the 1/f band below  
> 100Hz though... which also happens to be the hardest frequency-band to filter  
> out.
>   
Thats where using the average of several low noise reference as the
regulator reference is about all that you can do to reduce the noise
below that due to a single reference.
NIST favour using active (darlington emitter follower buffered RC low
pass filter) low pass filters with a cutoff frequency of 0.16Hz. The
additional output volatge due to the emitter follower is about 160 ppm/K
with a 24V supply. However this solution is only feasible when the load
current is relatively low ( up to 50mA or so) as in their isolation
amplifiers etc.
>  
> 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.
>  
> I may be mistaken though... BTW: ceramic caps (especially Y5V types) have  
> pretty bad microphony, so they should be avoided if possible.
>  
>   
Its best to avoid Y5V caps if at all possible especially if the
operating temperature range is wide.
NIST seem to favour using plastic dielectric capacitors where they are
suitable in their low noise designs..
> bye,
> Said
>   
Bruce



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