Re: 4046 VCO

John Popelish wrote:
Jamie wrote:
Andrew Holme wrote:
Is there an easy way (e.g. diode and resistor) to change the duty
cycle of the CD4046 VCO? I would try on breadboard; but I'm away from
home. The data*** says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
Not really,.
The best you can hope for is to simply lock with the signal coming
in to generate an out put ref.
You could how ever, use that output ref to drive a simple PWM via
a dual op-amp that is composed of a triangle wave form, driving a
voltage comparator input while your ref is driving the other input
with some tailoring of components to scale it of course. This can
yield a 0..100% duty cycle..
You could get tempted to use the OSC from the PLL as the PWM ref
how ever, this can cause a problem in the PWM filters unless the
PLL shift is very little of course.

Not knowing your full intend of the circuit makes it very hard to
visualize your ideas.

http://webpages.charter.net/jamie_5";


I don't see the original question, so I will answer the O.P. here. Yes, there is an easy and predictable way to change the output duty cycle (if you are willing to swap out parts).

Since each half cycle charges the timing capacitor with current supplied to its opposite end, while grounding the other one, you can replace the timing capacitor with two capacitors of different values, each connected between one of the timing pins and ground. Now, this is changeable, rather than variable, but if you use variable capacitors or a butterfly tuning capacitor (center movable grounded plate that interacts with two isolated sets of plates, one increasing capacitance and one decreasing capacitance as you turn the shaft, you would have a variable duty cycle. It could also be done with a pair of varactor diodes in the proper circuit, but I wouldn't call it simple.

That would imply that you could also modulate the VCO command voltage at the VCO output frequency -- with an analog multiplexer you could (maybe) select the + and - times independently.

By the time you're done you may have even managed to use less than 10x the board space of a 555.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

Quantcast