[sdiy] SAW core VCO flyback time

[Neil Johnson]

The problem with the original Michaels (sp?) saw core VCO (really a
CCO with a voltage-to-current expo converter front end) is that the
reset time is outside of the control loop, by which I mean that while
you can derive an expression relating the input current to output
frequency that includes a term for the reset time, you don't know what
that time is, so the output frequency will always be an approximation.
Franco compensation addresses this, but it is not perfect (the output
level varies with frequency).

As Roman says:

> The key is charging current always flowing via integrator cap, all the time,
> uninterrupted. 2nd key is that discharge is done by injecting precisely
> controlled amount of charge to the integrator, so reset phase is well
> defined as discharge current from VFC chip minus tuning current from expo
> converter.

The venerable Pease-designed LM331 does this so eloquently, using what
is termed "charge balancing".  If you look at the datasheet:

http://www.ti.com/lit/ds/snosbi2c/snosbi2c.pdf

you can see how it works.  Note that the critical part is the
stability of the reset pulse timing and gain - once that is set the
output frequency is very linear.

If you consider figure 16 then that _IS_ a current-controlled
oscillator (Rin does the V-to-I conversion for the integrator loop
around A1, Cf).

Granted, as you approach the high end of the frequency range the shape
slowly moves towards a triangular shape, but I would argue that by the
time you're above 20kHz you don't really care about triangle or saw or
whatnot as any harmonics will be well above hearing range.

Neil
--
http://www.njohnson.co.uk