# s/n measures

brad sanders brad.sanders at circellar.com
Sat Mar 22 02:17:47 CET 1997

```Paul, that's fine if you NEED an FFT - but c'mon... S/N measures have
been made accurately for decades now - without FFTs!

To check noise output of a circuit, what you need is a good mV meter
(or amp) and a rectifier/averager circuit. If you wanna get really
fancy, an A/C-weighting filter would be nice.

If you can build a synth, you can build all these. Use a very low
noise opamp (even, say, a 5534) in a 40db gain config, and connect to
your VOM. Measure the voltage with a shorted input, then connect your
DUT and note the voltage again. Remembering to correct for
A^2+B^2=C^2, you can determine the noise voltage output of your
circuit. For best results be careful with grounds, and stick the whole
circuit in a shielded metal box with a battery.

You can't determine the *spectral* density of the noise like this, but
you can determine the overall noise output. A pair of headphones
attached to the output of your noise amp might allow you to make a
good estimation of the (audible) noise spectrum.

Hey, Paul - what about this? Build a good noise amp, tweak a PC
soundcard (I've done it; you CAN improve these things) and use
commonly available PC software to build a low-end Model1? You can even
use the same soundcard to generate test signals....

>If I'm trying to match the performance (or at least get close to) of a
>CEM3330 VCA chip (100 db S/N), how can I measure just what the noise floor
>is???

>There really are only about 3 instruments made capable of accurately =
>making these measurements. The most widely used (by far) is an Audio =
>Precision Model 1. It is a true 18bit (100dB) device. This generates all =
>of the SNR plots you see in magazines like Stereophile. There is a new =
>Model 2 out that is 22bit (128db). The downside is that these are \$10K =
>boxes. I'm trying to find a used Model 1 for \$4500.

```

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