4046 VCO

A

Andrew Holme

Guest
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 datasheet says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
 
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 datasheet says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
Not really,.
Click to expand...
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"
 
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 datasheet 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"
Click to expand...
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.

--
Regards,

John Popelish
 
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 datasheet 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
Click to expand...
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
 
On 22 Mar, 13:28, Andrew Holme <ajho...@hotmail.com> 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 datasheet says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
Click to expand...
I'm having to post from Google Groups because I'm away from home, so I
don't know if you'll see this or not; but thanks to all for the
replies so far. I was wondering about a diode and pot in series
between pins 4 and 12 i.e. to shorten the VCO Out low period by a
variable amount; but I really want independent control of centre
frequency and duty cycle, which that is not.
 
On 3ÔÂ22ČŐ, ĎÂÎç9Ęą28ˇÖ, Andrew Holme <ajho...@hotmail.com> 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 datasheet says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
Click to expand...


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On 3ÔÂ22ČŐ, ĎÂÎç9Ęą28ˇÖ, Andrew Holme <ajho...@hotmail.com> 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 datasheet says 50% duty; but I want to make it variable;
and I'd rather not use an auxilliary monostable.

TIA
Click to expand...


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manufacturers... but lack the time to contact suppliers, negotiate
contracts, arrange shipping or monitor product quality? Don't worry -
Let seriouswholesale deal with all that for you.

*Check out the huge range of Gadgets, MP3 / MP4 Players, Car DVD /
Audio, and Computer Accessories now by visiting the online wholesale
catalog at seriouswholesale. com You'll have peace of mind thanks to
the seriouswholesale Quality Control, 12-month Warranty on all
products, and easy secure payment by credit card through Paypal.

Selling on eBay or your own online store? Send products direct from
our warehouse to your customers using our unique drop-shipping
service. You can profit by selling hundreds of different products,
without holding any of your own inventory! Any questions you have will
be answered by the seriouswholesale English-speaking customer support
team... Their aim is to make your China electronics importing business
easier to run than ever before.

Welcome to http://www.seriouswholesale.com.

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