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 ??
Unless some other aspect than frequency is being controlled by the control voltage (amplitude or phase or waveshape), your description is contradictory. According to a google search, a "ring oscillator VCO" 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.
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.
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.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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.
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 ??
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
www.wescottdesign.com
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.
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.
--- Please bottom post, or inline post when it's necessary for clarity. Thank you. :-)
As others have noted, your original post seems to indicate that you don't have a really good grasp on what's required to change the frequency of an oscillator as a function of an applied voltage.
For an LC tank, where:
1 f = -------------, 2pi sqrt LC
then either L or C must be varied as a function of voltage in order to make f change.
In the old days it was done with a saturable reactor, where the DC voltage on the control winding and the attendant current through it changed the reluctance of the core and, thererfore, the inductance described by the secondary.
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.
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.
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.
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Actually, the change in delay is quite pronounced, as shown by:
news:t6piu55sqenrovms7k5fuhrl8m7666thng@4ax.com
from TI's 1989 GMOS logic data book.
Also, for an HC00, tpd at 25C is 15ns max for Vcc = 6V, 18ns for 4.5V,
and 90ns for 2V.
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