[time-nuts] re low noise regulators

SAIDJACK at aol.com SAIDJACK at aol.com
Fri Dec 14 17:03:18 EST 2007


 
In a message dated 12/14/2007 13:51:47 Pacific Standard Time,  
bruce.griffiths at xtra.co.nz writes:


>If we assume that in both cases the noise spectrum is flat (i.e  white)
>in the 100Hz - 10KHz band
>If we also assume that the  noise bandwidth is 10KHz (difficult to know
>more precisely as the  details of the band limiting filters used arent  
given)



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.
 
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.
 
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.
 
I may be mistaken though... BTW: ceramic caps (especially Y5V types) have  
pretty bad microphony, so they should be avoided if possible.
 
bye,
Said



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