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Measurements >> Phase Noise and Jitter Measurements >> Positive values of phase noise in dBc/Hz??
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Message started by Tommy on Dec 3^rd, 2004, 5:49pm

Title: Positive values of phase noise in dBc/Hz??
Post by Tommy on Dec 3^rd, 2004, 5:49pm

Hi,
I carried out a PNoise simulation on a ring VCO I have. The plot I got was typical to the phase noise plot of a VCO, but It gives positive values of phase noise in dBc/Hz for offset of 1Hz to about 500Hz relative to the carrier. The corner frequency of the CMOS process is around 100k.
1. Is positive values meaningful & can it be expected
if the design is'nt very good?
2. If yes, is these positive values of phase noise going to
greatly affect the measured phase jitter?

Regards
Tom

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Frank Wiedmann on Dec 3^rd, 2004, 11:09pm

See http://www.designers-guide.com/Forum/?board=rfsim;action=display;num=1050465395;start=1#1.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by HighwayStar on Apr 26^th, 2005, 12:54pm

I also get similar response when i use the Matlab script given in "Predicting the Phase noise and Jitter ...." tutorial. ???

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 2^nd, 2005, 10:15am

Hi!
I am experiencing too the presence of positive phase noise values in Cadence.

I went through the link Frank suggested, the tutorial and Egan's book, that Eugene suggests. My problem is this. S_phi is everywhere referred to as power spectral density, so the units should be V²/Hz. However the equation L=S_phi/2 doesn’t make sense in terms of units. If L is 1/Hz than S_phi should be too. Also, this is exactly what Egan suggests. His deviation at page 305 (noted as greek sigma) is actually a ratio of the amplitudes of the noise and the signal. So...he has already taken into account the Power Ratio in the S_phi. Egan's _phi has units of 1/Hz. Why is S_phithen referred to as spectral density?

Apart from this, if Cadence is plotting L as given by equation 26 of the tutorial, I fail to understand how it comes up with positive values in phase noise. It seems like it is not subtracting (in dBs) the power of the carrier. Do u think this is the case? Could you help me solve this?

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 2^nd, 2005, 11:48am

Nick,
Check out page 31 of http://www.designers-guide.org/Theory/cyclo-preso.pdf.

The term "Power Spectral Density" implies the spectral density of a power-like quantity, in the case of S_phi, the units are radians²/Hz, which is power-like because it is in terms of radians² rather than radians. In this case, the power-like quanity is not a true power.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 3^rd, 2005, 2:14am

This has been quite enlightning.
So, Cadence cannot figure out the corner frequency. Is there a way to estimate with Cadence the corner frequency so I know up to which point I can trust the simulation? And a last thing. If what Cadence plots is Sv, and not L, then if I want to get ''phase noise'' normalized to the power of the fundamental, I still have to subtract (meaning in dB) the power of the carrier from what I see in Pnoise , isn't it?

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 3^rd, 2005, 8:35am

Nick,
Just so we are clear, the inability to compute the corner frequency is a weakness shared by all simulators that use small-signal assumtions in their noise analysis. Thus it is not just Cadence's simulator SpectreRF that has this characteristic, it is shared by all commercial RF simulators.

You can estimate the corner frequency by using the fact that the total area under the noise curve must equal oscillator output power (noise in phase does not add power to the signal, it simply redistributes the power in frequency). So if P_out is the output power and you can model the noise with a/(1+f_c²/f²), then you solve for a by matching the model to the noise at frequencies above the corner, and then you solve for f_c using P_out = a/(1+f_c²/f²).

Artist does compute L, the normalized noise. This is what you get when you ask for "phase noise" when using the basic PNoise analysis (when you have not gone to the trouble of specifically asking Spectre to compute the PM and AM components to the noise).

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 6^th, 2005, 7:19am

Ken,
I haven't resolved yet the corner frequency issue. I have more questions, but I will give you numbers this time to be more exact. My PSS gives me a differential voltage peak to peak amplitude of 250 mV. For a relative Harmonic 1, maximum sidebands 20 and a Beat Frequ equal to my oscillation frequ, I get the following results from pnoise analysis:

Phase noise (dBc/Hz): -18 dBc/Hz@1 KHz and -74 dBc/Hz@100 KHz

Output noise(A/sqrt(Hz)):-45 A/sqrt(Hz)@1 KHz and -102 A/sqrt(Hz)@100 KHz

My questions are these:
1) What does exactly output noise represent? Is it PM+AM noise? If that is the case, then shouldn't it have a 3 dB difference from the phase noise graph? I am getting an offset, but not a 3dB one.

2)My phase noise graph is exacly the same as the output noise one, but with an offset of about 28 dBs in this case. Is there a specific relation between these two graphs, like a scaling factor related to their units, or a normalization factor maybe related to my carrier? Can I somehow do the maths do get one graph from the other given the numbers I have from my PSS?

Finally, also my output noise gets positive dB values, and as you can see from the numbers above, it seems to be bigger than phase noise. I am guessing again a scaling factor or a normalization. Could you please help?

Thanks!

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 6^th, 2005, 9:45am

Nick,
1. Given the units of dBc/Hz, I would say that you used Artist to compute these number and did not use the new AM/PM noise capabilities. When you ask Artist to compute the "phase noise", what it really gives you is the normalized noise, or S_v/V_out.

The numbers you give for output noise are nonsensical for two reasons. First, the numbers look like they are in dB, but you do not indicate they are in dB. Let's assume they are. The second problem is that the noise is measured in A/rt(Hz) while the output is described as being in terms of volts. This prevents the computing of the normalized noise.

2. The phase noise and the output noise should differ by a constant factor because the reported phase noise (which is actually L, the normalized noise) equals the output noise power divided by the output power.

3. At low frequencies SpectreRF will report the normalized noise L as being greater than 1 (positive in dB). That only occurs when the frequency you choose is below the corner frequency. As such, it is not real.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 7^th, 2005, 3:17am

Ken,

thanks for the tips.
My output noise is indeed in dB. However, I have no choice over its units. l looked up the examples of Cadence and output noise is measured in V/sqrt(Hz).
Why am I getting A/sqrt(Hz)?

Thanks,
Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Eugene on Jun 7^th, 2005, 10:16am

I have noticed the same problem with units. I select output voltage, click on a node, then see A/sqrt(Hz) dispayed. The numbers seem to make sense if I simply replace "A" with "V". Personally, I suspect it's a display bug but it would be nice to hear from Cadence on this one.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 7^th, 2005, 10:55am

Thanks Eugene!
I verified that phase noise by pnoise is just ouput noise divided by the power of the carrier.

Back to the original problem now. I get positive dB values in my phase noise analysis (the simple phase noise performed by pnoise). I understand the problem with corner frequency, but do you think there is any chance of getting more realistic values by oversampling in my PSS analysis? Also, the way I have been doing things so far, PM noise analysis gives me far bigger values than phase noise (from pnoise). Any ideas?

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 7^th, 2005, 2:36pm

Nick,
Just to be clear, when you say phase noise, you mean L as computed with the standard PNoise analysis. When you say PM noise, you mean S_ph as computed using the new AM/PM capabilities of PNoise. Correct?

Remember that L = S_v/P_out. Also remember that it is S_v that has the corner, not S_ph. Thus for low frequencies, S_ph >> L. Above the corner, S_ph and L should be within a factor of two of each other. If they are not, that is an indication something is going wrong.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 8^th, 2005, 7:00am

Nick,
I misspoke in my last post. What I said was true when working with measured results, however S_v as produced by the simulator does not have the corner. So L = 1/2 S_ph (eq. 27 from http://www.designers-guide.org/Analysis/PLLnoise+jitter.pdf) for all frequencies where the phase noise dominates.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 8^th, 2005, 9:12am

Hi again,

Ken thanks. You were crystal clear.

I was expecting too something like a constant 3dB difference between my pnoise-Phase Noise and my PM noise. Let me explain a bit.

I do pnoise analysis from freq offset 100Hz up to 100MHz. Then I ask cadence to plot PM and AM noise. What I get as a plot (by default) is AM and PM from frequ offset of 10MHz and above. The plot is nothing but a straight horizontal line. Despite the graph being wrong, AM and PM are equal for the plotted range of frequencies.

If I manually ask it to show me the waveforms for the whole range of freq I have simulated (i.e. for freq 100 Hz till 100 MHz) I get the 3dB difference as expected, but only at the very beginning of the waveforms. Meaning, if I simulate from 1Hz to 100 MHz, I will see that 3dB difference only at 1Hz. If I simulate from 10 KHz to 100 MHz, I will see that difference at 10 KHz.

What happens afterwards is this. My PM noise drops with a big slope, forming a notch and rises up again to a value of about 10 dBs lower to the initial value it has. It then maintains that value for the rest of the frequencies. What Cadence actually shows me when I plot the graph is the part right after the notch, when AM and PM have settled to that value. Both PM and AM keep that value for the rest of the frequencies simulated, resulting in that horizontal line on my plot. To this I should add that the y-value of this horizontal line varies depending on the frequ from which I start the pnoise simulation.

To sum up: the only correct part of the graph is at its beginning, where I get the 3 dB difference. Whatever is wrong, I am guessing that it is the reason why cadence by default chooses not to plot the peculiar part of the graph. I don’t get any error messages. However, I think it is somehow related to the fact that I start my oscillator with a damped vsin. What do you think of all this?

Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 8^th, 2005, 12:33pm

Are you expecting a 3dB difference between L and S_ph (the PM noise), or between the PM and AM noise components?

These results seem completely wrong for a free-running oscillator. I would expect to see the L and S_ph to drop two decades for every decade of offset frequency at low offset frequencies (say less than 1MHZ). I would expect to see the AM noise to be relatively flat and much smaller than the PM noise except at high offset frequencies (say greater than 1MHz). So at low frequencies there would not be a 3dB difference between the two, it would be much greater.

I cannot explain why Artist starts the AM/PM noise plot at 10MHz, it sounds like a bug.

I also cannot explain why there would be notches in the PM noise at low frequencies unless there are resonant circuits in the bias circuits.

Finally, I cannot explain why the PM noise does not follow the traditional -2:1 slope.

All of this sounds like something is going terribly wrong. Have you checked the steady-state behavior as computed by SpectreRF? Is the oscillator really oscillating at the expected amplitude and frequency? If everything else look good, then perhaps you have encountered a bug.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 9^th, 2005, 4:49am

Ken,

To be more clear, Cadence by default was only showing me the AM / PM waveforms only after 10MHz. So, for such big frequency offsets I was expecting to see that AM and PM have the same value and that they are 3dB less than the USB noise. My USB noise is exactly the same as phase noise of pnoise.

Instead, what I get is AM=PM=USB-3dB at the beginning of my sim (whatever frequency it is I begin the sim from), then the notch, and finally again AM=PM = constant value for the rest of the frequ.

My PSS is working. The frequency I get is the one I designed for. Also my transient response shows oscillation.

Also, I went through the examples of Cadence Documentation. Specifically I loaded the schematic oscDiff from the rfExamples of Cadence. I run AM and PM and the waveforms I got look like mine. So, I guess it is not just my design. You could check that out too. If this is a bug, what do I do to get the PM working?

Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 9^th, 2005, 6:48am

At the moment I do not have access to the Cadence software, so I cannot take a look at oscDiff. Your description of the results seems quite inconsistent with my expectation of what the results should be, and I can think of no obvious user errors that could be causing them, so I would contact Cadence if I were you and ask them for help.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 9^th, 2005, 9:57am

Ken,

Thank you for all the help. It is much appreciated.
I will seek further advice from Cadence itself and I will get back to you, just for the completeness of the matter.

Many thanks again,
Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Andrew Beckett on Jun 9^th, 2005, 2:37pm

Nick,

I just looked into this - and ran through the oscDiff example.
I don't see the

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Andrew Beckett on Jun 9^th, 2005, 2:50pm

Whoops - hit the wrong button. Sorry. I'll start again. I was messing around trying to get an image included, and made a mistake...

I just tried this (oscDiff) using both the latest subversion of IC5141 (5.10.41.500.1.18) together with spectre from MMSIM60 USR1 - and then I also tried using the Base CD version of IC5141 - with spectre from the same version.

I tried with and without highorder=yes in case that made any difference (it didn't to the phase noise results).

I got consistent results as the picture shows:

The blue line shows the traditional phase noise plot - and the yellow line (which is almost identical except at higher frequencies) as you can see. The am noise (the red line) is a fair way down compared with the pm/phase noise - exactly as you'd expect. Note, I'm plotting the am and pm noise in dBc for comparison.

So, what version were you using - the above looks reasonable to me, and not like what you described.

Regards,

Andrew.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jun 9^th, 2005, 4:02pm

Andrew,
Those results also look reasonable to me. The rise in AM noise at low frequencies is probably a simulation artifact due to limited ability to resolve very small amounts of AM noise in the face of large amounts of PM noise.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Andrew Beckett on Jun 9^th, 2005, 9:38pm

BTW, the subversion of IC5141 came out wrongly as I forgot to hit the button to disable smileys. It should have been 5.10.41.500.1.18.

Regards,

Andrew.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 10^th, 2005, 2:36am

Hi,

thanks for going through this.

The version I am currently using is this:
CDS: icfb.exe version 5.0.0 04/30/2004
sub-version 5.0.33_USR2.34.8
Do you think that the problem I am facing is related to the fact I am using an earlier version?

Thank you very much,
Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Andrew Beckett on Jun 12^th, 2005, 12:26pm

Nick,

I'll try the same example tomorrow using IC5033 USR2 and let you know what I get.

Regards,

Andrew.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Andrew Beckett on Jun 13^th, 2005, 1:24am

Nick,

The direct plot appears to be quite broken in IC5033 USR2. I see the problem you're having. In fact if you pan left, the axis does continue, but the results are clearly wrong...

I found this PCR:

PCR: 699011
Title: AM/PM noise has incorrect X-axis

which from the discussion in the PCR looks as if it could well be the problem. This was fixed in IC5033 USR3.

Having just tried IC5033 USR3, the problem is gone there and the results I get look the same as those I posted earlier in this thread.

So in summary, this appears to be a version related problem.

Best Regards,

Andrew.

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Nick on Jun 14^th, 2005, 1:51am

Thank you all for your help.
I have much appreciated it. :)

Best Regards,

Nick

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 6^th, 2005, 2:18am

Hello all experts,

In the phase noise/AM/PM noise shown by Mr. Beckett, how do you explain the PM noise larger than 0dBc/Hz ?

with Best regards,
Daniel Lai

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 6^th, 2005, 8:49am

Read this thread from the beginning.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 6^th, 2005, 6:35pm

Hi Ken,
Thanks for your tip.

For PM noise, the unit should be radian^2/Hz, so my problem is why PM noise expressed in the figure is dBc/Hz? (Does the PM noise transfer to PSD of voltage as phase noise or just a unit mistake?) Besides, I was confused if the PM noise could be shown in the same figure with AM noise(dBc/Hz) or phase noise(dBc/Hz).

On the other hand, if the PM noise is radian^2/Hz, how can we explain the PM noise over 0dB-radian/Hz at the near offset?

Thank you very much for your interest.

Daniel Lai

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 6^th, 2005, 9:41pm

There are several noise metrics that are often confused. They are:
S_phi - The power spectral density of the phase noise with units of rad²/Hz
S_v - The power spectral density of the voltage noise with units of v²/Hz
L - The power spectral density of the voltage noise normalized to the power in the carrier with units of dBc/Hz

As f -> 0, S_v and L are bounded but S_phi goes to infinity.

Three things add to this confusion:
1. When you request the "phase noise" from Artist, it actually gives you L, which is technically is not phase noise.
2. When computed with SpectreRF, S_v and L are unbounded as f -> 0 as a direct result that the noise analysis is a small signal analysis, as discussed starting at (9) in www.designers-guide.org/Analysis/rf-sim.pdf.
3. S_phi and L are often confused because they can have the same value (S_phi = L when using the double sided transform and S_phi = 2L when using the single sided transform, as can be seen from (27) of www.designers-guide.org/Analysis/PLLnoise+jitter.pdf).

Finally, it is possible to separate S_v (L) into its AM and PM component pieces. These would have units of v²/Hz (dBc/Hz) and could be plotted on the same graph.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 11^th, 2005, 8:26pm

Hello Ken,
Thanks very much for your answer, which is quite enlightening me.

Indeed, not only the S_v and S_phi noise are confusing, but also AM and PM noise are.

In the limiter cycle, we have:

Quote:

Delta y(t) = (1 + a(t))y(t + phi(t)/2*pi*fc) - y(t)

Can I determine a(t) means all the AM noise, and (phi(t)/2*pi*fc) means all the PM noise? And are these two noise observable at SpectreRF simulation result as AM and PM noise?
Besides, due to S_v includes AM and PM noise, and S_phi is only for PM noise. Does that mean a(t) belongs only to S_v noise, and (phi(t)/2*pi*fc) belongs to S_v and S_phi noise?

I am very appreciated for your answers, those help me a lot.
with Best regards,
Daniel lai

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 12^th, 2005, 8:32am

Daniel,
I believe you've got it.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 14^th, 2005, 7:37pm

dear Ken,

Because S_V including AM and PM noise, the PM noise should equals to phi(t)/2*pi*fc in:

Quote:

Delta y(t) = (1 + a(t))y(t + phi(t)/2*pi*fc) - y(t)

At the question of my last post, the PM noise (phi(t)/2*pi*fc) contribued not only in S_V but S_phi. However, does PM noise contribute the same noise in S_V and S_phi? If yes, we may use small signal approximation to calculate all the PM noise to voltage noise to generate S_V. If no, maybe we need to know which parts in PM noise contributed in S_V.

Thanks you for answering my questions with great patience.
Best regards,
Daniel

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 17^th, 2005, 8:43am

Remember that the phase and voltage are related in a nonlinear way (because y is a nonlinear function). So S_phi and the PM component of S_V are not the same. However, in oscillators at high offset frequencies the phase noise is small and so through a small-signal assumption there is a linear relationship between them. The relationship depends on which form of the transform you use:
For single-sided transform (assuming large f_m):
S_phi = L
For double-sided transform (assuming large f_m):
S_phi = 2L
where in this case the AM component of S_V is assumed negligible and L is S_V normalilzed to the total power in the fundamental component.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 18^th, 2005, 7:38pm

Hi Ken,
Thanks very much for your response, that really helps me a lot.

Assumed that thermal noise generates AM and PM noise in a VCO system as a(t) and phi(t):

Quote:

V(t) = (1 + a(t)) * cos(2*pi*fc*t + phi(t))
=(1 + a(t)) * [cos(2*pi*fc*t) * cos(phi(t)) - sin(2*pi*fc*t) * sin(phi(t))]
If sin(phi(t)) is very small (much smaller than 1),
then V(t) ~ (1 + a(t)) * [phi(t) * sin(2*pi*fc*t)]

To be more clear, phi(t) should be at one specified large offset frequency fm, which is phi(t) * cos (2*pi*f_mt). Besides, all PM noise phi(t) transfer to voltage (power) domain and value is:

Quote:

|V(t)| = |1+a(t)| * |phi(t)|

However, if all PM noise use this way transfering to power domain noise S_V, S_phi and the PM component of S_V should be the same. Actually, if I assume there are three kinds of noise in nature: AM+PM noise, pure PM noise, and pure AM noise. AM+PM noise (for example: thermal noise) should contribute same PM noise in S_phi and S_V. In S_V, PM noise transfer to power domain using above formula. On the other hand, if we have a pure PM noise, it couldn't generate power and transfered its PM noise to power domain.

My opinion is all PM noise (not only AM+PM noise but PM noise) contribute to S_phi, and only PM components of AM+PM noise contributes to S_V, pure PM noise couldn't. So S_phi and the PM component of S_V are not the same. Is this correct?

Sincerely yours,
Daniel

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 18^th, 2005, 9:38pm

I'm afraid I do not understand your question. Nor do I understand why or how you have broken noise into three components, AM, PM, and AM+PM.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 18^th, 2005, 10:54pm

dear Ken,
Sorry to confuse you.

Noise in nature are from several reasons: (Information from noise book)
Thermal noise: Due to random thermal fluctuation
Shot noise: Due to the discrete nature of charge flow
Flicker noise: Due to random capture and release of charge
Because the noise are generated from different reasons, they also have different properties, such as AM+PM noise or pure PM noise only.

In MOSFET, the flicker noise is generated from the capturing and releasing charge at discrete interface between SiO₂ and Si. My opinion is the charge captured and released behavior should be periodic without generating new noise power. (The period of this behavior depends on the charging and releasing frequency.) So, I would regard flicker noise as a pure PM noise, and this PM noise would not transfer to power domain noise, which makes S_V.

My question is S_phi are not the same with the PM components of S_V, but we don't distinguish the noise properties to which PM noise part would transfer to S_V. I assumed that noise should be seperated into several group as AM+PM noise, and pure PM noise. (and even more the pure AM noise) If the noise is AM+PM noise, such as thermal noise, could transfer all its PM part to S_V; on the contrary, pure PM noise, such as flicker noise, could not transfer any PM noise to S_V.

with best regards,
Daniel

Title: Re: Positive values of phase noise in dBc/Hz??
Post by Ken Kundert on Jul 19^th, 2005, 12:09am

Whether noise is a PM or AM noise is not a property of the noise source, it is a property of the circuit. Thermal noise produces PM noise when present in an oscillator, and additive noise (what you call AM+PM noise) when produced by a resistor in a linear circuit. And if thermal noise were present on the control line of a variable gain amplifier, it would produce AM noise at the output of the VGA.

The M in both AM and PM stands for modulation, which implies that there must be a carrier signal present that is being modulated by the noise. Without the carrier, there can be no AM or PM noise.

-Ken

Title: Re: Positive values of phase noise in dBc/Hz??
Post by chlai on Jul 19^th, 2005, 12:58am

dear Ken,

Thank you very much for the correction and fast reply.

You are right, the noise property would be specified as AM or PM when it applied on a circuit. So, I should modify my last question to the situation in a VCO (or an oscillator).

Some kinds of noise exist and have different properties in VCO. Thermal noise and flicker noise exist together with different effects to the phase noise. As Ken's last post (or in cyclo-paper.pdf), thermal noise generates PM noise and suppress AM noise in VCO. However, this PM noise should transfer to power noise at S_V. (observable with a conventional Spectrum Analyzer.) Is that right?
Question in last post is: PM parts of S_V is different (and less than) to S_phi, the total PM noise. Can we determine which part of PM noise would contribute to S_V?

Sorry to ask so many basic questions, but these questions are really difficult to me, and I couldn't find them in all the papers. (rf-sim.pdf, cyclo-paper.pdf, PLLnoise+jitter.pdf) Your paper and post are quite useful to me to go ahead to understand the VCO noise.

Sincerely yours,
Daniel