[Pulse VCO Help]

[Harry Bissell]

Somebody already told you about the XOR gate tricks. IF you have to adjust the
pulse width and opt for the RC, make the cap at least 50pF and vary R to get
the delay. This is so that the CMOS gate input capacitance will be (@15 pF)
won't be the dominant factor controlling the time constant.
   Yes, you did miss some tricks of the 555, First, use a constant current
source to charge the timing cap, and an npn across the timing cap to
selectivly "rob" some of that charging current, as the means of varying the
VCO frequency. I have a schematic here somewhere, and hope to have a scanner
soon..... ;-) Harry


Chris MacDonald <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.

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.

I tried using a 555 as a monostable to generate a narrow pulse from the
square wave but since the trigger input overrides the threshold input I
couldn't find a way to output pulses which were narrower than the
trigger pulse.

So, what I have now works but my parts count is now up to 3 chips, a
trim pot, a capacitor and some resistors and diodes.  I can't help but
feel this is not the best solution to the problem, does anyone know of a
better way?

Thanks,

-Chris MacDonald


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