Variable ramp again

[mbartkow at ET.PUT.Poznan.PL]

Dear list,

Few weeks ago there was a discussion on variable ramp generator I had
needed for my own project. In fact, I needed even more than adjustable
triangle/saw wave - I wanted to have it voltage controlled.
The conclusion was that it's better to built a kind of wave shaper than
to affect the charge/discharge time ratio within a VCO.

Many nice ideas came, I especially appreciated the Don Tillmann/Martin Czech
approach to appropriatelly scale two ramps going in oposite directions and
take the min of the two as the output. The problem was with the scalling, 
as it required some nonlinear relation between two scaling factors. I have
built such circuit which almost works, but it is very complicated and I thought
about an alternative approach and at last I got a simple idea. As many ideas
this one would never come out without the stimulating lecture of this very
list, and for this particular one I owe thanks to Juergen who mentioned
integrating the pulse output of a regular VCO.

The idea follows:

Take the saw wave from the VCO and compare with an adjustable threshold (or
a voltage input for VC ramp). At the output of the comparator one has a variable
width pulse train. Now, remove the DC offset (which is equal to the threshold
voltage BTW) and input it to an integrator. A variable ramp signal is observed at 
the output of the integrator with the symmetry (ramp ratio) dependent on the
voltage threshold. In order to prevent the amplitude of the signal from changing 
with frequency, the pulse train may be appropriately scaled which is very easy-
just run it through an OTA controlled by the same pitch controlling current the 
core VCO is. 

I have built this very simple circuit (note: no nonlinearities or complex depen-
dencies!) and it works great with the one obvious problem of integrator drift
resulting from some residual DC and its offset voltage. The question is now,
how to keep its output swinging around 0V ?

I have an idea again, but this does not work perfectly: take two peak detectors,
each consisting of a diode and a capacitor and measure the highest and the lowest
voltage at the inegrator output. Now, offset the integrator input by the difference
between these two.

Needless to say I would love to hear if you have some better ideas
or even some general comments/criticism. When finished, I am going
to post a complete schematics here.

yours,

mb

--

Maciej Bartkowiak
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Institute of Electronics and Telecommunication     fax: (+48 61) 8782572
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