[sdiy] VCA signal analysis

[cheater00 .]

On Mon, Dec 9, 2013 at 12:44 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Hi Rutger,
>
> CV bleed is relatively easy, you can do it with a basic signal
> generator and a scope - feed a square wave into the CV input and see
> how much of it appears on the output.  With a square wave you get both
> steady-state (for a low frequency square wave assuming settling time
> of the circuit is much less than the period of the stimulus), and nice
> sharp edges so you can see if there's any capacitive coupling going
> on.
>
> For noise and distortion I assume you're only interested in the audio
> band 20-20,000Hz?
>
> For noise alone then a decent quality external PC sound card would
> probably do.  Verify it is good enough by terminating its input with a
> low resistor (say 100 Ohms) and see what the measured input signal is.
>  I use Visual Analyzer:
>
> http://www.sillanumsoft.org/
>
> for this kind of thing.  Not the most intuitive UI, but good enough for "free".
>
> Distortion requires a low-distortion signal generator, ideally 10x
> better than you're trying to measure so as not to introduce too much
> distortion itself.  Again, a sound card might do, and definitely if
> you're working in well-controlled environment it'd probably be good
> enough.  Ideally you would also use a sharp notch filter to remove the
> fundamental so you get the full dynamic range of your analyser to look
> at the distortion artifacts.
>
> If you believe the specs then this might be good enough:
> http://www.creative.com/emu/products/product.aspx?pid=15185
>
> Or something similar.
>
> But definitively a notch filter.  And not that hard to make:
> http://www.moorepage.net/Twin-T.html

How do you measure the distortion introduced by the self-made notch filter?

> Or, as you  say, just get an audio analyser and be done with it:
> http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=8#analysers
>
> Yes, it is bigger, and not quite as good as an AP, but then if you
> hunt around you can pick up a used one for less than the price of a
> new MOTU 828 Mk 3, and its inputs will handle several hundred volts
> (you can hear the relays clicking as it autoranges), and it is built
> like a tank.  And other than the control and GPIB part, it is
> all-analogue :)

...and with 70 dB dynamic range perfect for testing the fidelity of
transatlantic phone lines :-)

Cheers,
D.