John Ackermann N8UR wrote: > I'm experimenting with some low noise regulators, and want to do some > meaningful measurements to compare them (and hopefully compare to > published specs). > > I have an HP 3561A FFT analyzer, which can probably be beaten these days > by a sound card, but has the advantage of absolute calibration and GPIB > to dump data. I also have a very low noise 40dB audio amplifier that > covers <10 Hz to >100 kHz. > > I've tried just looking at the regulator output with the analyzer, with > and without the amp, but am not sure what I'm seeing. In particular, > even with a capacitor to block the DC, the analyzer indicates > "Overrange" at a reference level below -5 or so dBV. That puts most of > the noise spectrum below the bottom of the screen. > > Any suggestions on an appropriate test configuration, and on how to get > readings that directly relate to the root-Hertz value shown by regulator > data sheets? > > Thanks, > > John > > John
To convert the readings to dbV/rtHz, you have to know the system gain and the noise bandwidth of the analyser. One method of calibration is to use a white noise source of accurately known power spectral density. The simplest way to build such a noise source is to use a low noise FET input amplifier to amplify the Johnson noise of a 150k metal film resistor (50nV/rtHz) to the required level. You can achieve a gain of around 10X with a FET opamp with a 3dB bandwidth of 100kHz or so. Additional amplifiers can be used to increase the output noise level. The lower frequency (3dB increase in noise spectral density) limit occurs when the FET amplifier flicker noise is 50nV/rtHz. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
