Variable ramp generator

[mbartkow at ET.PUT.Poznan.PL]

Dear all,

Taking the wavetable discussion aside, I have another problem
and would love to hear your hints or comments.
In fact, this is my idee-fixe for years:

I definitelly want to abandon the "mix several basic 
waveforms" VCO approach. I just don't like it, because
it trades too much pots for unnecessary stages of freedom.
OTOH it is boring and yet another replication of an old and
worn-out scheme. My idea for an original VCO is to generate
a flexible waveform and shape it with some variable nonlinear 
circuits.
First of all I need a variable ramp waveform, i.e. a waveform,
whereby the ratio of the rising edge to the falling one is
freely adjustable without affecting the frequency. 
I know, it is possible to independently control the charge and 
discharge times in integrator by using a pair of diodes or 
a CMOS switch. AFAIR, Juergen described such operation of KORG 
LFOs. The problem is if one applies this control scheme to VCO 
it leads to severe tracking problems.

My idea was to generate a stable saw or triangle and shape it
using a switched gain circuit. I must say I tried to design such 
a circuit, but either I am too narrow minded or blind - I just
cannot come up with anything. I hope somebody invented this before
and would not mind to share. 

mb


--

Maciej Bartkowiak, PhD
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Institute of Electronics and Telecommunication     fax: (+48 61) 8782572
Poznan University of Technology          phone: (+48 61) 8791016 int.171
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