Thomas Henry Tunable Noise Source (was 4046 VCO experiment)

[mbartkow at et.put.poznan.pl]

> Hello list,
> 
> Enough list members wanted to see the schematic for Thomas Henry's tunable 
> noise source (from Nuts & Volts, Nov. '99) that I decided to put it up on a 
> web page, so anyone else who might be interested in it could look at it at 
> their leisure. 

Dear all,

this circuit reminded me some experiments with noise modulation I've done 
two years ago. It is very true, indeed, that a kind of pitched noise is 
obtainable through frequency modulation of a periodic carrier using a noise 
source. 
The circuit of Thomas Henry employs an LM566-based VCO, but it really does not
matter. The effect is perfectly replicable by using a white noise feeding any 
kind of VCO. Even better result is achieved by using a sine waveshape of the
VCO, since there are no additional bands located around each harmonic partial
of the triangle.
According to the modulation theory, the bandwidth of the narrowband noise-like
output of the VCO is dependent rather on the modulation depth than the spectrum 
of the input noise. On the other hand, what we hear is rather reflected by
a short-term Fourier transform than a total power spectra. Concerning this,
the noise spectrum does matter. Very interesting shades of noise are obtainable 
by filtering the noise before it is fed to the VCO.

regards,

MB