Pulse VCO Help

[Jeremy Brookes]

> I am trying to create a simple VCO which outputs very narrow pulses
> (microsecond range) at wide range of frequencies (0 to 10 khz at
> least).  It is not intended as an audio source so extreme stability is
> not an issue (it's for an experimental envelope generator).  I have come
> up with a circuit that works but I'm hoping there's a simpler way to do
> it.
>
> I couldn't get a 555 to adjust over a large enough frequency range so I
> turned to the 4046.  (Did I miss something with the 555?)  This has
> acceptable range but outputs a square wave (I don't believe there's a
> way to change this, true?).  I then had to figure out how to convert a
> square wave into a narrow pulse wave.  I ended up connecting a capacitor
> in series with the VCO output to get a sloped waveform and then ran the
> output of that through an omp-amp buffer and then into a comparator
> which is used to generate an adjustable pulse wave output from the
> sloped wave input.  A trim pot is used to provide a variable voltage for
> the pulse width adjustment.

Depending on how narrow you want the pulse to be, there may be some way of
feeding the signal from the 4046 into the XOR in the chip along with a
version of the signal after going through a cap to delay the rising edge.
This should produce a narrow pulse on the rising edge. Might be worth
breadboarding up.

jezz.