[time-nuts] How to measure regulator noise?

[Bruce Griffiths]

Henk ten Pierick wrote:
> On Jan 8, 2008, at 12:50, Don Collie wrote:
>
>   
>> I`d just hang an AC millivoltmeter[or microvoltmeter] across the  
>> regulator`s
>> output.
>> I use my H/P 400H, which will give readings down to about 50uV. If  
>> your
>> regulator produces less noise than this [say a 723, with
>> 2uV], then you`ll need a more sensitive meter.
>>     
>
> Electronic voltmeters or microvoltmeters have a noise bandwidth which  
> is larger than the bandwidth on the front panel.  The front panel  
> bandwidth is related to accuracy and is not the -3dB bandwidth. The  
> noise bandwidth is nearly always range setting dependent and can vary  
> very much. I have seen a factor of four in noise bandwidth between  
> adjacent range settings. The only way to have a good indication on  
> noise is in a known and constant bandwidth. The noise bandwidth of a  
> filter is not the same as the 3dB bandwidth and dependent of the  
> filter order and shape. It is always more.
>
> Henk
>
>   
Thus if one is to make useful comparative measurements with an AC
voltmeter, an external filter which has a significantly narrower
bandpass than that of the AC voltmeter is useful.
A spectrum analyser with the capability of averaging cross power spectra
has the added advantage (over an AC voltmeter) of being able to make
meaningful measurements of noise below the noise of its input amplifiers
(or the noise of external preamplifiers). Since its difficult to build a
preamp with noise much below 10nV/rtHz whilst ensuring that the preamp
input will survive worst case transients etc when connected to a power
supply of 10V or more, such a capability is useful for measuring the
noise of ultra low noise regulators which may have high frequency noise
of 20nV/rtHz or less.

For example the preamp used in Linear technologies AN83 is virtually
guaranteed to be damaged by connecting its input to a powered up 20V supply.
Off course, connecting it to the power supply before powering up the
supply will (if the regulator output slew rate is sufficiently low)
allow it to be used to measure the noise of higher voltage supplies (at
least if higher voltage 330uF OSCON caps were available). However
accidents/mistakes do happen (as do faulty regulators) and eventually
the preamp input opamp will be destroyed.
Using standard electrolytics for the input coupling capacitor for
testing higher voltage supplies isnt particularly useful as they have
increased leakage current and associated noise. Schemes such as using a
relatively high resistance in series with the preamp input which is
shorted out after the coupling capacitor has charged are not foolproof
and eventually a mistake will lead to destruction of the preamp input stage.

For circuit schematics of preamps with input protection schemes that
allow power supplies with outputs greater than 5V to be tested see:
http://www.ko4bb.com/~bruce/LNPS.html
<http://www.ko4bb.com/%7Ebruce/LNPS.html>

Bruce