[time-nuts] How to measure regulator noise?

Don Collie donmer at woosh.co.nz
Tue Jan 8 06:50:15 EST 2008

I`d just hang an AC millivoltmeter[or microvoltmeter] across the regulator`s 
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.
It might be wise to short the input wires of the meter, to check the 
inherent noise in the measurement setup - for meaningful results this should 
be, perhaps, no more than a tenth
of the expected reading [depending on how riggerus you want to be]. Bear in 
mind that most analogue meters are average responding, and calibrated for a 
sinewave - so to get the RMS value for white noise [or whatever] a 
correction factor should be applied [depending also on what sort of accuracy 
you need/want].
     If you want to *look* at the noise, I understand that one of the Tek 
7000 series plug-ins is able to display very small amplitudes of this order. 
Again, because you may be dealing with comparitively small voltages 
[compared with those which might be induced due to hum fields etc],
it would be wise to check the residual noise by shorting the probe[s] at the 
regulator. This is valid since the output impedance of the regulator is 
nearly zero.
Go to it!,..........................................Don C.

----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Tuesday, January 08, 2008 10:03 AM
Subject: Re: [time-nuts] How to measure regulator noise?

> Poul-Henning Kamp wrote:
>> In message <079101c8510f$af8c35a0$7900a8c0 at athlon1200>, "Dave Brown" 
>> writes:
>>> John
>>> You won't need much of a cap for DC blocking as inputs are hi-Z....too
>>> big and the chargeup may well trigger the overrange condition.
>> You need to put a shunt resistor after the capacitor if the input is hi-Z
>> but an even better way is to offset the gnd clip with a couple of fresh
>> batteries so that you can avoid the capacitor.
>> Batteries have very low, but not zero noise, so what you do is:
>> Put six fresh 1.5 volt batteries in series, so that three of them
>> is the "wrong way" and the resultant voltage is zero and then you
>> measure their noise.
>> Then you put three of them "the wrong way" on your 5v supply, so that
>> the output voltage is only .5V and then you measure the noise.
>> That worked fine for me using a HP6885B
> This technique only offsets the voltage to within 3/4V of zero when
> measuring an arbitrary voltage regulator output.
> With a discrete regulator it is usually easy enough for test purposes to
> adjust the regulator output so that it is within a few tens of
> millivolts of the battery stack voltage.
> A low noise preamp with perhaps 40-60dB gain is still required before
> the spectrum analyser.
> Even residual dc voltages of a few tens of millivolts may saturate such
> an amplifier.
> A simple dc servo can be used to remove the residual offset (<1V)
> without using electrolytic capacitors with their attendant leakage and
> noise.
> A low frequency cutoff well below 1Hz is possible without adding
> significant noise.
> Alternatively for measurements in the 10Hz to 100kHz range a preamp like
> that used in Linear Technology's AN83:
> http://www.linear.com/pc/downloadDocument.do?id=4172
> may be useful.
> It uses electrolytic coupling capacitors together with low value 
> resistors.
> For ultralow (milliHertz) frequency noise measurement AC coupling is
> best avoided altogether, if possible.
> Bruce
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