Pulse VCO Help

[Gene Zumchak]

John,

    Sorry, this doesn't do the job, and it is simpler than you describe.  If
you use an XOR gate, it will differenentiate BOTH edges of the applied
rectangular wave.  If it is exactly square (not easy with a 555) you will get a
pulses at twice the frequency, otherwise you get twice the pulses, but with
irregular spacing.

    To differentiate with a CMOS XOR gate you run the signal directly to one
input and run it through a resistor to the other input.  The input impedance of
the CMOS gate is very high.  It has some gate capacitance, however, of a few
pf.  The resistor and input C give you an RC delay. Vary the R to vary the
pulse width.  This is good for very narrow pulse widths.  If you need wider
ones you'll probably have to add your own C.

    If you want to differentiate just one edge, then use an AND, NAND, OR or
NOR, CMOS of course.  What you use depends upon which edge you want to
differenentiate and whether you want your output to be high going or low
going.  The choice is left as an exercise to the student.

Gene Z.



John Speth wrote:

> On Thursday, April 15, 1999 11:55 AM, Chris MacDonald
> [SMTP:macdonald at evenfall.com] wrote:
> > 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.
>
> You can use an XOR gate with some sort of delay line with whatever simple
> VCO you need to get a slim pulse from a square wave.  Simply run the square
> wave into one input of the XOR.  Also run the square wave into a delay line
> (made of 2 XOR gates in series).  Now run the output of the delay line into
> the other input of the XOR gate.  At the output you'll see a slim pulse
> with on-time equal to the delay line's delay time.  The downside is that
> you can't vary the pulse width.
>
> Maybe you can experiment with a simple RC lowpass configuration and use a
> CMOS XOR gate which will allow you to vary the pulse width.
>
> John Speth
> Object Engineering, Inc
> mailto:johns at oei.com