# Phase noise

• posted

Could someone please explain this ridiculous thing called "phase noise" to me? I understand it perfectly well when considering a phasor vector model, but that, in itself, is a non-intuitive description. Phasor vectors are not intuitively understood (at least by me).

My troubles are rooted at this: If I consider that the phase of a perfect sine is simply a position on a time scale (i.e. a phase shift of a signal is the displacement, or delay, of the signal along the time axis) and noise is a jitter induced by random perturbations, then phase noise must be a horizontal jitter of a signal. But does that make since? Not to me. I can understand amplitude noise because the amplitude can have noise "riding" on it as time passes. But with phase noise, it sounds as though the noise is forcing a horizontal random motion. If I draw this on paper I end up with more than a single amplitude value per unit of time.

I don't think that I explained that well. I can draw (and then post) a picture of my problems if necessary. Long story short, I am trying to increase the signal-to-noise ratio in my brain!

Thanks, Blair

• posted

in article 3fd7deed_2@127.0.0.1, Blair at snipped-for-privacy@hotmail.com wrote on

12/10/03 7:04 PM:

Any modulation sitting on top of a pure sinusoidal signal will broaden its spectrum. That off carrier frequency content IS the noise.

Bill

• posted

Using the common model of a rotating vector, and its instantaneous position, the Phasor, (as described in, "Reply Part One")....

(And always remember that you can create your own model to help you visualise the problem; using bananas if you so wish; you don't have to use phasors!)

For a sine wave, the vector is rotating round at a constant angular velocity, measured in Radians Per Second.

Now, imagine that there is some malicious agency that makes our rotating vector sometimes slow down ever so slightly.

Sometimes it makes it speed up ever so slightly.

The speeding up and slowing down only happens in very small tads.

We are never too far from where we are supposed to be, and averaged out we are always where we are expected to be.

At any one time, we are only ever in one position, and can only have one amplitude at that time. (Which I think answers your difficulty)

If we're slowing down, we'll speed up later.

If we're going too fast, we'll slow down later.

The malicious agency causing our speed variations is noise.

Because of our change in speed, we're never at the right position at any one time.

Position in angular terms is, "Phase".

Hence we get, "Phase Noise".

Voila! Zat is Cointreau!

• posted

A sine wave motion repeats itself every cycle.

We want to represent and understand behaviour that is the same in every cycle, irrespective of whether it happens now, or in 10 minutes' time.

Behaviour against time, the traditional, "horizontal axis" is not helpful in visualising this "once every cycle" behaviour.

Sine wave motion is a projection from a rotating vector.

That behaviour seems to fit in with our need to visualise behaviour "once every cycle".

We choose the (admittedly strange) convention of the starting position pointing to the right and then rotating backwards, when a Westerner's inclination might be to follow the clock from 12 o'clock!

Having got our rotating model, we still have a problem with visualising it, and that is that it is rotating!

So what we do is to consider the instantaneous position, i.e., wherever "we" happen to be at any one time (even though "we" are rotating) and plot the phase of other things relative to us.

Thus we end up with the "phasor vectors"

There is nothing "magic" about phasors!

There is nothing "right" about phasors!

There is nothing "electrical" about phasors!

They are nothing more than the model we have chosen to represent "once every cycle". If you want to contrive a different model to help your understanding, then you are free to do so!

If there is some characteristic of the Frozen Banana that you find to be analogous to understanding "once every cycle", then go ahead and use it.

e.g..... Q. "What is yellow, and always points to the North?"

A. "A Magnetic Banana".

• posted

Ah-ha! I think that helps clear it up.Thanks. But now I have follow-up questions.

So, the phasor vector never goes in reverse? That makes since. Going in reverse is like going reverse in time, is it not? And then, if it only goes faster or slower than the nominal rate of rotation, what happens at dc?

Then, lastly, if amplitude noise and phase noise are both modulated frequencies, how are the two discernable from each other? Or, are they not and we just know that the both exist - because they must!

Thanks, Blair

• posted

I do not know the answer to that, anybody else help?

However, both amplitude and frequency modulation result in sidebands, noise sidebands, so my guess is that you cannot disambiguate the two.

• posted

Generally the model does not apply to DC, and only applies at one specific frequency of interest.

I say, generally, because when you come on to Fourier Analysis, you'll express DC effects using a similar model.

• posted

That cant be right surely? If you are speeding up and slowing down repeatedly you must often be in the right place at the right time - a bit like the accuracy of a stopped clock (perfect time twice a day).

73

Richard G3CWI

• posted

Help make Usenet a better place: English is read downwards, please don't top post. Trim messages to quote only relevent text.

• posted

Why not?

In one case (FM) the total power of the signal never changes. In the other case (AM) at any instant the average frequency of the energy is always the same.

This can be done with mathematics (e.g., with a DSP) or using electrical circuits.

The difference can, at the simplest level, be observed in an electrical signal displayed on an oscilliscope.

It isn't quite correct to say that both are "modulated frequencies". There is a modulated _carrier_, and in one case it is the frequency of the carrier that is changed while in the other it is the amplitude. Either one will generate characteristic sidebands that have a specific relationship to the carrier frequency and the modulating signal. The relationship is very different, and hence the two sets of "noise" sidebands have *very* different characteristics, and can easily be analyzed to determine which is which.

An amplitude modulated signal will have a carrier, which never changes in frequency or power, and there will be _two_ sidebands equally spaced in frequency, one above and one below, the carrier by the exact frequency of a modulating signal (i.e., if there is one modulating signal, there will be one set of sidebands). The amplitude of the sidebands increases in direct proportion to the modulation index, and peaks at 100% modulation, while the carrier power does not change.

A frequency or phase modulated signal has _multiple_ sidebands above and below the carrier at _intervals_ equal to the frequency of a modulating signal. The power distribution between the sidebands and the carrier always adds to a constant power and only the distribution changes as the modulation index is increased. For example, at a modulation index of approximately 2.4, there is *zero* power at the carrier frequency! At that point, all of the power is in the sidebands.

In the simplest form the difference can readily be observed for an electrical signal with nothing more than an oscilliscope.

If a signal is set to display one complete cycle across the screen, and is triggered so that the cycle *always* starts at the exact same point (the same physical point on the screen for the same electrical point of the cycle display) either type of noise can be observed.

To see AM modulation, select the point on the horizontal scale where the vertical signal displacement is at its peak. The variations in the vertical displacement, when the signal crosses the selected horizontal line, from one cycle to the next... is AM!

To see FM modulation, select the point on the vertical scale where the horizontal signal displacement is at its peak (which is to say, look at the zero crossover point on the right side of the screen). The variations in horizontal displacement, when the signal crosses the selected vertical line, from one cycle to the next... is FM!

Variations from cycle to cycle on the | / vertical scale here are AM. ,*^*,

• posted

"Airy R. Bean" wrote on Thursday (11/12/2003) :

Why is this idiot deliberately cross posting?

• posted

to get you to bite ?

• posted

What would be wrong with cross posting that?????

• posted

Good lord. Are you serious? Who the hell are you to make such a post? Which is more useful a post such as yours "this idiot...cross posting", or a thorough response such as Bean's to a topical question? The newsgroup is for EE discussions. That's what we got. Keep your absurd posts to yourself.

On second thought. This was probably a flame. Wasn't it?

• posted

If you take the trouble to check, you will find the thread is being cross posted to uk.radio.amateur as well.

• posted

"Floyd Davidson" wrote on Friday (12/12/2003) :

You obviously have not had much experience of Mr Bean's antics in the newsgroups.

• posted

• posted

The article was reasonable, and was cross posted to two groups where it appears to be on topic.

You obviously don't have a clue.

• posted

So what? My comment was regarding the caliber of post; his compared to yours. If there was a contest, and the topic was "which post is better suited for an electrical engineering forum," I'm afraid he would have you beat.

• posted

I prefer top posting because I don't have to scroll down through previously read material to see the response.

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