[sdiy] VCA signal analysis

Neil Johnson neil.johnson71 at gmail.com
Mon Dec 9 12:44:39 CET 2013

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

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:


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:

Or something similar.

But definitively a notch filter.  And not that hard to make:

Or, as you  say, just get an audio analyser and be done with it:

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 :)


On 9 December 2013 11:15, Rutger Vlek <rutgervlek at gmail.com> wrote:
> Hi all,
> About two years ago I came up with moderately new discrete OTA vca design in Spice and now I finally have it prototyped and it works (yay! hope it is of sufficient quality to overcome dependency on the more and more obsolete OTA ICs), but I'd like to do some more analysis on the circuit beyond stating that "it puts signal through in a VCA-ish manner and doesn't sound too bad". I'd like to measure distortion, noise, CV bleed, etc. What is currently the easiest way to measure such aspects? I've read about hardware analyzers (Audio Precision and the likes), but was hoping for something a bit more gentle on the wallet. Can something like that be done with PCs nowadays? And do I need a special I/O card for that? Or would a high-quality DC-capable sound card (like my MOTU 828 MK3) work for that too?
> Best,
> Rutger
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