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From: Daku on 10 May 2010 02:30
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 ??
From: Phil Allison on 10 May 2010 04:20

"Daku"
>
> 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 ??

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

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

Yawnnnnnnnnnnnn....



...... Phil


From: Michael Black on 10 May 2010 11:13
On Sun, 9 May 2010, Daku wrote:

> 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 ??
>
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

From: Tim Wescott on 10 May 2010 11:52
Daku wrote:
> 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 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.

> 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 ??

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
www.wescottdesign.com
From: George Herold on 10 May 2010 12:37
On May 10, 11:52 am, Tim Wescott <t...(a)seemywebsite.now> wrote:
> Daku wrote:
> > 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 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.
>
> > 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 ??
>
> 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.

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.


>
> 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 consultingwww.wescottdesign.com

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