How do I measure - 140 dB noise levels?

[gentlevoice]

Hello,

I'm looking for a cheap and straightforward way of measuring noise levels down to about -140 dBs (ref. 2 VDC). Any of you who knows how this can be done? A soundcard would be prefered but I haven't seen any with this kind of specs at a reasonable price .....

Regards,

Jesper

[richie00boy]

-140dB is the realm of high quality test equipment, i.e. not reasonable price.

[wakibaki]

Originally posted by gentlevoice
-140 dBs (ref. 2 VDC)

You mean 200nV? Ultra low noise opamp ~2.5nV/ Hz^-2 so you're going to see this much noise in the circuit with a bandwidth of 6400Hz already.

Or somebody correct my numbers...

w

[ostripper]

I think Nelson Pass "Passlabs" is/was working on a ultra low
noise precision amp to allow for testing sub 110DB noise.
Do a search in "passlabs" you might find it.

My soundcard (M-audio/envy 24) is -122 at 20khz -126 at 1khz
I use FFT and can see this, but it is good enough.(can see
junction noise in transistors... yikes)

BTW .. Why??? -110-120 noise is spectacular ,if it is below that
why the fuss?
OS

[gentlevoice]

Hi again,

- and thanks for your replies - they made me think further into what my needs are.

I would like to measure very, very low noise levels, e.g. the noise of batteries, however, would need to do so also at very low frequencies, i.e. down to <1 Hz. At the same time I will be measuring noise (at a high resolution) up to ~ 100 kHz.

I guess this could be done using a very low noise amplifier feeding into a quality (192 kHz/24 bit) soundcard, possibly with the exception of the ability to measure down to 1 Hz (I haven't seen any capable of measuring at these frequencies..?). The soundcard needs to be able to connect to my PC via USB or firewire. Also it should be able to record and playback at the same time as I intend to use it with the Praxis software (http://www.libinst.com/).

@ Ostripper: BTW I did a search on Nelson Pass' ultra low noise amp but did not find anything ... nor on M-audio/Envy ... ??

Suggestions are appreciated ...

Best regards,

Jesper

[kristleifur]

This one may work, I'm not sure:

Tangentsoft LNMP (Low Noise Measurement Preamp)
http://tangentsoft.net/elec/lnmp/

[gentlevoice]

@kristleifur

Thanks - it would be an interesting design for me, however, I don't have a source for the AD797 - which by the way seems to be very low noise...

Do you know where to source it?

Regards,

Jesper

[tschrama]

maybe this is an option.. with some mods for the low freq ability.. less then 0.6nV/sqrtHz.. gives 60nV at 10KHz bandwidth, or -150dB ref 2V

Ultra low noise measurement amp

[kristleifur]

Originally posted by gentlevoice
Thanks - it would be an interesting design for me, however, I don't have a source for the AD797 - which by the way seems to be very low noise...

Do you know where to source it?

Sorry, don't know, but you may want to check with Tangent himself, and at the Headwize.com DIY headphone amp forums. I think they were discussing these amps there a while ago.

[cuibono]

The Emu 0404 usb and Asus Xonar D2 have very very low noise and distortion levels...