# Voltage controlled oscillator question

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Could someone please clarify the following ?
Consider a LC-tank and a ring oscillator based voltage controlled
oscillator. In a LC-tank, the control voltage influences the
oscillation frequency - hence "voltage controlled oscillator". In a
ring oscillator based  voltage controlled oscillator, the oscillation
frequency is independent of a control voltage.  The control voltage
basically switches the inverters on and off. So how does one control
the oscillation frequency ??

Re: Voltage controlled oscillator question

"Daku"

** Ever heard of Google,  you PITA time waster  ??

http://en.wikipedia.org/wiki/Ring_oscillator

Yawnnnnnnnnnnnn....

.....  Phil

Re: Voltage controlled oscillator question

Unless some other aspect than frequency is being controlled by the
control voltage (amplitude or phase or waveshape), your description is
is a string of inverting stages connected in a circle, whose frequency
of oscillation is changed by a control voltage that modifies
resistance or capacitance so as to change the delay. This could also
be called a "But...!" gate.

http://tinyurl.com/2564vxw

--
John

Re: Voltage controlled oscillator question

Lots of things can change frequency with voltage, some schemes work
better than others.

If you are really seeing a voltage variable ring oscillator (your
description almost sounds like it's merely a ring counter with a means
of turning it on and off, but that's not clear), there are various
things that might work.

For instance, for really cheap applications, or hobbyists playing around,
changing the voltage to the IC would change the frequency, though within a
limited range and it may impact on some other aspects of the oscillator.
But if you need a cheap and dirty VCO for some limited application, it
might work.

Another scheme is that by injecting a DC voltage into the right point, you
control the switching point of that stage, and that changes the frequency
of the ring counter.

Otherwise, you're back to changing the capacitor or resistor of the
ring counter to change frequency.

Note that one popular VCO is to use a 555 timer, and vary the voltage
on pin 5, the threshold level pin.  That changes when the timer switches,
and thus allows for a limited variation of frequency by control voltage.
Similar schemes have been cooked up, though with such things one is
usually limited by how much you can vary the frequency, and it may
affect some other part of the oscillator.  But again, if you only
need a small variation, it works and is simpler circuit wise than
a more complicated VCO.

Michael

Re: Voltage controlled oscillator question

In general, LC tanks don't have control voltage ports, so their
resonance frequency can't be controlled by a voltage.  In VCO usage,
there's a voltage-controlled part -- nearly always a varactor diode,
although there are other ways to control the frequency of an LC
oscillator, if you're feeling perverse.

Like the LC oscillator, a ring oscillator is not an inherently voltage
controlled device.  Like the LC oscillator, if one wants to base a VCO
on a ring oscillator, one must modify one's oscillator to respond to a
control voltage.

There must be a real question buried in here, but you've worked so hard
to abstract your question from what you're doing that you've lost the
sense of it.  Try again, starting with what you want to _do_ -- even if
it's to understand why professor X and university Y has published an
article about phase locked loops using ring oscillators.

With a sensible question, perhaps we can make sensible answers.

With a more detailed but still mangled question, perhaps we can help you
make a sensible question.

--
Tim Wescott
Control system and signal processing consulting
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Re: Voltage controlled oscillator question

Hey, does CMOS have a time delay that depends on the supply voltage?
(It seems like it might.. or you could add some R's between stages.)
Then you could control the frequency somewhat by changing the supply
voltage.

George H.

Re: Voltage controlled oscillator question

[on the subject of VCO circuitry]

The output impedance of CMOS does depend on the power supply
voltage, of course, as well as on temperature and unit-to-unit
variations.  This quality is better developed, IMHO, in OTA
(operational transconductance amplifier) circuits.
CMOS impedance can vary by a factor of six (3 to 18V range of
4000 series gates), but the old CA3080 was good for a factor
of 1000.

Re: Voltage controlled oscillator question

On Mon, 10 May 2010 09:37:56 -0700 (PDT), George Herold

---
From TI's 1989 High Speed CMOS Logic Data Book:

Re: Voltage controlled oscillator question

Thanks for the feedback. My question, basically is:
How do I control the oscillation frequency of a ring oscillator based
voltage
controlled oscillator? I have created SPICE code for both varactor-LC
tank
and ring oscillator VCOs, and while I am testing the ring oscillator
based
code, I am not sure what parameters I need to tweak to see the
oscillation
frequency change.

Yes, as I said before, I do have a working SPICE model of a varactor
-LC VCO.

Exactly. Consider the simplest ring oscillator consisting of 3 CMOS
inverters, wirh a PMOS and NMOS at the top and bottom of each, so
that each of the PMOS acts a pull-up and the NMOS as the pull-down.
Each of the pull-up PMOS have their gates tied, and each of the NMOS
have their gates tied as well. By applying an appropriate gate
voltage, I
can pull down each of the inverters and start the ring oscillator to
oscillate.
Now if the voltage applied to each of the NMOS is below the NMOS
threshold, there is no oscillation. However, once the start the
oscillator,
how do I vary the frequency ??

Re: Voltage controlled oscillator question

If you don't have any clue how to vary the frequency of your ring
oscillator, why do you want to use one?

Have you searched the literature?  Have you Googled?

As far as I know a ring oscillator is a really crappy oscillator that
semiconductor companies build for bragging rights about how fast their
new processes go.  To what practical use are you planning on putting
your ring oscillator, why won't something like an RC oscillator work,
and why would you want to control the speed of a ring oscillator if all
they're good for is going fast?

If you have no clue, if you can't find the answer from colleagues or
fellow students, and if you really do need to make the thing work -- how
about "bleeder" FETS at each stage, to lightly pull the node up or down?
You'll kill gain, which will make the thing not want to oscillate, but
you'll also slow it down.

--
Tim Wescott
Control system and signal processing consulting
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Re: Voltage controlled oscillator question

Dear Sir,
I do not intend to use a ring oscillator based VCO.
However, as you have said below, literature search
shows a lot of people using the ring oscillator based
voltage controlled oscillator in phase locked loop
designs, which is sounds strange to me, since in a
phase locked loop, the VCO oscillation frequency
must be very sensitive to the input voltage level.
Maybe the ring oscillator is designed to oscillate
at the central frequency, but frequency variation is
difficult and tricky.

Re: Voltage controlled oscillator question

---
Please bottom post, or inline post when it's necessary for clarity.
Thank you. :-)

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Re: Voltage controlled oscillator question

Dear Sir,
I am fully aware of the LC-tank circuit, and that a varactor needs to
be
added in the tank to get the voltage control. However, a simple
literature
search on voltage controlled oscillators indicates that a large number
of
designs are based on the CMOS ring oscillator. As one of other posters
noted before (and I fully agree with him) that it is difficult to
dynamically
control the oscillation frequency of the CMOS ring oscillator - this
frequency
is based on the characteristics of the MOSFETs in the inverters of
the
ring oscillator. Given that, how are these ring oscillator based
designs
being labelled  "voltage controlled" since the voltage control merely
switches the oscillations on and off, NOT control the frequency as in
a
LC tank.

Re: Voltage controlled oscillator question

Show us a sample circuit, and we'll explain it.  Until we actually
see what you are talking about, then "how" can only be conjecture.
Show us a sample circuit, and then that circuit can be explained.

Michael

Re: Voltage controlled oscillator question

No, it doesn't

also feedback helps things.

Re: Voltage controlled oscillator question

number

---
After doing a cursory search for "ring counter VCO", such seems notv
to be the case.

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Re: Voltage controlled oscillator question

On Wed, 12 May 2010 19:09:42 -0500, John Fields

It can be done with ring oscillators, too.  Gate delay isn't a strong function
of Vcc, though so it's not done often.  DLLs generally use a mux to select the
number of gates in the ring.

Re: Voltage controlled oscillator question

On Wed, 12 May 2010 22:07:32 -0500, " snipped-for-privacy@att.bizzzzzzzzzzzz"

---
Actually, the change in delay is quite pronounced, as shown by:

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Re: Voltage controlled oscillator question

On Thu, 13 May 2010 08:50:37 -0500, John Fields

+/- 10% isn't very good for the capture ratio.  The normal (lot-to-lot)
variation is likely much larger, making it more or less useless for a VCO.

Re: Voltage controlled oscillator question

John Fields

The ratio is 6:1.

....  Phil