Variable ramp generator

[mbartkow at ET.PUT.Poznan.PL]

Don Tillman <don at till.com> wrote:

> To do this, take a positive sawtooth going from 0v to 1v, and its
> inverse, a negative sawtooth going from 1v to 0v.  Put each through a
> current controlled gain stage, and combine them with a simple "min" or
> "and" circuit that delivers whichever of the two voltages is most
> negative.  Voltage control those gain stages like a panner circuit so
> one goes from a gain of 0.5 to some large value (like 20) while the
> other goes from the large value down to 0.5.  Inaccuracies will show
> as variations in amplitude, not variations in frequency.  

I like this. Two diodes smartly inserted into a wave mixer do the "min"
trick. There is a problem yet, how to scale the amplitudes of the rising
and the falling saw in order to ensure constant amplitude of the combined
wave. Needless to say the latter is necessary for further waveshaping.

I think the best way to achieve a constant amplitude is to scale the both
saws in such a way, that the "meeting" point is allways at the same height.
A simple calculations (for simplicity let's assume the cycle length is 1
and the meeting point is at 1) reveal, that if the rising saw is scaled by
a (a is the symmetry factor, in the range 1...+inf), the falling one should
be scaled by a/(a-1). Now, it would be great to have the symmetry voltage
controlled. Is there a simple way to realise the f(x)=x/(x-1) relation ?
I think an OTA-based one-quadrant voltage division circuit is reasonable,
but maybe there is a simpler way ?

mb


--

Maciej Bartkowiak, PhD
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