Hi Bruce and Henk, Your point about Bandwidth is a valid one, and as you say, it depends on the application. The 400H has corner frequencies of about 10Hz, and 4Mhz, so the noise BW is a bit wider than this [Y/N?]. This setup can`t measure noise at very low Frequencies, or Frequencies much greater than the [nominal] 4Mhz, because the noise frequency components simply can`t get through. For my purposes the setup I described is adequate - I mainly want to compare one regulator with another [although it would be nice to be able to do so over the frequencies 0Hz to infinity]. I suggest that if a regulator proves itself quieter over a restricted BW such as I described, it is *probably* quieter at other frequencies too. Take that Bruce! ;-)..............................Don.
----- Original Message ----- From: "Bruce Griffiths" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Wednesday, January 09, 2008 9:26 AM Subject: Re: [time-nuts] How to measure regulator noise? > 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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.