[time-nuts] Low noise voltage regulators

[Bruce Griffiths]

Bob Camp wrote:
> Hi
>
> Since the LPRO has a "noisy" 3 terminal regulator inside it, making the
> outside voltage quiet (as in noise density) probably will not help much.
>    
I've seen people insist on a low noise regulator ahead of a fluxgate 
magnetometer that used an LM7805 regulator.
The plastic enclosure was sealed with acetoxy grade silicon and buried 
in the ground at the end of 50m of cable with the predictable result.
> Keeping the voltage *stable* will indeed help things. I think you need a
> high stability linear regulator rather than a low noise one.
>
> One other thing to think about is line isolation at both audio and RF. Most
> regulators have poor isolation above a few 10's of KHz.
>    
At least the line isolation can be improved substantially at audio 
frequencies by adding active filtering (eg Wenzel style albeit with a 
few extra parts to prevent zenering transistor junctions or excessive 
base current on startup) at the regulator input. Cascade 2 or more if 
you need higher rejection.
RF rejection requires passive filtering.

Bruce
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Neville Michie
> Sent: Wednesday, February 24, 2010 4:57 PM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] Low noise voltage regulators
>
> Hi,
> I remember a reference, probably by Bruce, that LEDS provide a low
> noise voltage reference.
> I am proposing to build a voltage regulator for a thermally
> controlled LPRO rubidium oscillator,
> with the voltage regulator being mounted on the 0.5 inch thick
> aluminium heat sink plate.
> The LEDS would also be mounted on the plate, which has controlled
> temperature.
> The LPRO has internal voltage regulation, and by running it at ~40C
> and 18Volts, the thermal
> flux within the unit is minimised as is the power demand.
> What I want to know is if a LM317 running on a stack of LEDs driven
> by the LM317 output
> would provide a low noise power source? What would be better?
>
> cheers, Neville Michie
>
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