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switched capacitor integrator noise simulation results (Read 7533 times)
pancho_hideboo
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Re: switched capacitor integrator noise simulation results
Reply #15 - Nov 25th, 2009, 2:18am
 
alireza wrote on Nov 24th, 2009, 10:09pm:
Therefore, it makes sense that if we are using pnoise-sources for a discrete-time switched-cap circuit we integrate the output spectrum again from 0 to fs/2.
Otherwise, it would yield a different result. Am I right?
Not correct. You have to integrate from 0 to infinity ideally.

However from practical point of view,
an integration from 0 to fs at least might be enough useful.
This range is mainlobe of output noise spectrum.

alireza wrote on Nov 24th, 2009, 10:09pm:
Is this again because of folding/aliasing issue?
I think it is due to aliasing.

alireza wrote on Nov 24th, 2009, 10:09pm:
Does this mean that for a sampled data system we only care about the pnoise-sources spectrum from 0 to fs/2??
Again, No.
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #16 - Nov 27th, 2009, 8:43pm
 
Hi,
pancho_hideboo wrote on Nov 25th, 2009, 2:18am:
Not correct. You have to integrate from 0 to infinity ideally.
However from practical point of view,
an integration from 0 to fs at least might be enough useful.
This range is mainlobe of output noise spectrum.


Yes, you were right. integration beyond fs/2 does not change the result by much.

I was also wondering, if you were able to simulate the netlist that I uploaded in the last post. If yes, was there any new results in the input-referred noise spectrum?

Thanks,
Alireza
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Re: switched capacitor integrator noise simulation results
Reply #17 - Nov 28th, 2009, 5:20am
 
alireza wrote on Nov 27th, 2009, 8:43pm:
If yes, was there any new results in the input-referred noise spectrum?
Attached figures are input and output noise spectrum comparison between type=timedomain and type=sources.
As you can see, I can get very reasonable results both for input and output noise spectrum.

Show me your definition of input noise in pnoise(type=timedomain) as OCEAN Script.

My definitios are followings:

For type=sources Code:
in1_dBV = dB20( getData("out" ?result "pnoise-pnoise") / harmonic( v("/sh" ?result "pac-pac") '(0) ) )
in2_dBV = dB20( getData("in" ?result "pnoise-pnoise") )
out_dBV = dB20( getData("out" ?result "pnoise-pnoise") ) 



For type=timedomain Code:
in_dBV = dB20( getData("out" ?result "pnoise-timedomain.pnoise") / harmonic( v("/sh" ?result "pac-pac") '(0)) ) )
out_dBV = dB20( getData("out" ?result "pnoise-timedomain.pnoise") ) 



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« Last Edit: Nov 28th, 2009, 6:22am by pancho_hideboo »  

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Re: switched capacitor integrator noise simulation results
Reply #18 - Nov 28th, 2009, 5:35am
 
These are input and output noise spectrum for type=sources.
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Re: switched capacitor integrator noise simulation results
Reply #19 - Nov 28th, 2009, 5:36am
 
Hill around fs/2 in type=sources is due to folding not aliasing.
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Re: switched capacitor integrator noise simulation results
Reply #20 - Nov 28th, 2009, 5:44am
 
Attached is archive of netlist, ocean script and etc.
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Re: switched capacitor integrator noise simulation results
Reply #21 - Nov 28th, 2009, 7:20am
 
pancho_hideboo wrote on Nov 22nd, 2009, 11:07pm:
alireza wrote on Nov 22nd, 2009, 10:04pm:
Yes, I had set the harmonics to 300 as well.
I don't know why Cadence HB-PSS/PAC can not give reasonable results.
I can make out the reason why Cadence HB-PSS/PAC can not give reasonable results.

Event-driven constructs such as @(cross) and @(timer) will not be triggered in HB Analysis.
HB Analysis also doesn't support $abstime.

You use "sh" and "nor_gate" Verilog-A modules.

"sh" include @(timer) and $abstime.
"nor_gate" include @(cross).

@(cross) in "nor_gate" is no problem actually.

But @(timer) and $abstime in "sh" are very problematic for HB Analaysis.

In Agilent RFDE(ADSsim), Sample&Hold component which is effective even in HB Analysis is provided.
http://edocs.soco.agilent.com/display/ads2009/SampleHoldSML+%28Sample+Hold%29

Also there is more sophisticated switch component in Agilent RFDE(ADSsim) more than Cadence Spectre "relay" component.
http://edocs.soco.agilent.com/display/ads2009/SwitchV+%28Voltage+Controlled+Swit...

And I can build various custom components which are effective in HB Analysis by using FDD(Frequency-Domain Defined Devices)
http://edocs.soco.agilent.com/display/ads2009/Custom+Modeling+with+Frequency-Dom...

I can use these components in Cadence Spectre Syntax netlists when I use RFDE(ADSsim) as simulator.

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« Last Edit: Nov 28th, 2009, 11:48am by pancho_hideboo »  
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #22 - Nov 28th, 2009, 8:04am
 
pancho_hideboo wrote on Nov 28th, 2009, 5:44am:
Attached is archive of netlist, ocean script and etc.


Thanks very much for your sims. Your input spectrum at different time points are (more than) slightly different. (some of the time points have more rise near fs/2) This is somewhat different that what I had. For me, they were more similar. But in any case, there is still the issue of noise folding. In your previous posts you'd mentioned there may be a solution to mitigate this problem in cadence spectre. I know you use other (more advanced) tools, but do you now have an idea of a possible solution in cadence spectre? Because this makes the integrated input-referred noise power wrong and the simulation almost useless.

Thanks,

Alireza
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Re: switched capacitor integrator noise simulation results
Reply #23 - Nov 28th, 2009, 8:10am
 
alireza wrote on Nov 28th, 2009, 8:04am:
Your input spectrum at different time points are (more than) slightly different.
(some of the time points have more rise near fs/2)
What do you mean ?

I showed spectrums at 0ns, 5ns, 10n, 20n, 25n, 30n.
There is almost no difference between them.
However If I choose time point at edge of clock signal, spectrum could be more large.

alireza wrote on Nov 28th, 2009, 8:04am:
This is somewhat different that what I had. For me, they were more similar.
You wrote your output noise spectrums are different by 4dB between tdnoise and pnoise(sources). But your input noise spectrums are same between them.
alireza wrote on Nov 22nd, 2009, 10:04pm:
I have attached the output spectrum plots both in dB  and linear scale.
There is a difference of about 4 dB at fs/2. (pnoise-td is higher that pnoise-sources)
alireza wrote on Nov 19th, 2009, 6:26pm:
the pnoise simulation has the same noise floor rise near fs/2.
(It is about 2dB and is approximately same as td-noise.)

I think your definition of input noise in tdnoise is wrong.

Again show me your definition of input noise in pnoise(type=timedomain) as OCEAN Script.

alireza wrote on Nov 28th, 2009, 8:04am:
But in any case, there is still the issue of noise folding.
Folding in pnoise(sources) is not issue at all.
Folding in pnoise(sources) is very actual.

You must not confuse folding with aliasing.

alireza wrote on Nov 28th, 2009, 8:04am:
Because this makes the integrated input-referred noise power wrong and the simulation almost useless.
Still you can't understand correctly.

The integrated input-referred noise power in pnoise(type=sources) is very correct and the simulation is very useful.

Are you very familiar with IIR digital filter which is approximation as continuous time analog filter ?
Frequency definition is (-infinity, +infinity) for continuous time analog filter.
However this range is mapped to (-fs/2, +fs/2) for approximated IIR filter.
Frequency range of tdnoise is a little similar to this.
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« Last Edit: Nov 28th, 2009, 10:51pm by pancho_hideboo »  
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Re: switched capacitor integrator noise simulation results
Reply #24 - Nov 28th, 2009, 8:43am
 
pancho_hideboo wrote on Nov 28th, 2009, 8:10am:
alireza wrote on Nov 28th, 2009, 8:04am:
Your input spectrum at different time points are (more than) slightly different.
(some of the time points have more rise near fs/2)
This is somewhat different that what I had. For me, they were more similar.
What do you mean ?
I showed spectrums at 0ns, 5ns, 10n, 20n, 25n, 30n.
There is almost no difference between them.
However If I choose time point at edge of clock signal, spectrum could be more large.


(Sorry, to clarfiy, by similar I was referring to different time points, Not sources Vs. time-domain. I meant I got similar spctra at different time points)
Please see attached. This is the waveform that I had posted earlier. In my sim, the rise was about 2 dB for almost all time points (0, 5n, 10n , 15n, 20n, 25n, 30n). But in your simulations at some of the points its about 6dB while for the others it is about 2dB.  (you have also plotted a waveform in linear scale, (in V/rHz), but the y-axis shows only dB scale, Does the yellow curve only correspond to the linear plot? I can see 7 plots in the figure legend, but can only distinguish three curves (colors=yellow,red, purple) on the plot.



pancho_hideboo wrote on Nov 28th, 2009, 8:10am:
[
alireza wrote on Nov 28th, 2009, 8:04am:
Because this makes the integrated input-referred noise power wrong and the simulation almost useless.
Still you can't understand correctly.

The integrated input-referred noise power in pnoise(type=sources) is very correct and the simulation is very useful.

Based on the theory, the integrated input-referred noise of the SC integrator should be 2kT/C. With this rise in the noise spectrum it does not lead to the expected value. However, if the noise spectrum was white (as expected, since it is thermal noise of resistors) it would yield the correct result. (i.e. if I multiply the BW (12.5MHz) by the noise floor at dc I get exactly 2kT/C, otherwise I don't.)
pancho_hideboo wrote on Nov 28th, 2009, 8:10am:
Again show me your definition of input noise in pnoise(type=timedomain) as OCEAN Script.


Sure, unfortunately, I don't have access to cadence right now, I will post it later today.

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Re: switched capacitor integrator noise simulation results
Reply #25 - Nov 28th, 2009, 8:52am
 
alireza wrote on Nov 28th, 2009, 8:43am:
But in your simulations at some of the points its about 6dB
while for the others it is about 2dB.  
you have also plotted a waveform in linear scale, (in V/rHz),
but the y-axis shows only dB scale,
Does the yellow curve only correspond to the linear plot?
I can see 7 plots in the figure legend,
but can only distinguish three curves (colors=yellow,red,purple) on the plot.
No. you are misunderstanding.

Basically I don't use Direct Plot Form in Cadence ADE where there are many bugs and definition of equations are very suspicious especially for RF applications.
http://www.designers-guide.org/Forum/YaBB.pl?num=1237656418/1#1

All noise spectrums are dBV/Hz unit. Again see OCEAN definition of these.
Only Yellow bold line is pnoise(sources). Other six lines are all tdnoise results.
See Yellow bold line in output noise spectrum. You can see only this line is from 0 to 50MHz.

We can't find out any remarkable difference between these six tdnoise input noise results.
Input noise results of tdnoise are all higher than pnoise(sources) by 4dB at fs/2.

pancho_hideboo wrote on Nov 28th, 2009, 8:10am:
Based on the theory, the integrated input-referred noise of the SC integrator should be 2kT/C.
This ignores folding effects.

Again folding is a reality.
http://www.designers-guide.org/Forum/YaBB.pl?num=1258339986/19#19

Output noise spectrum is a result of contribution from all sidebands, that is, multiple foldings.
While VGain of PAC considers only zero sideband.
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« Last Edit: Nov 28th, 2009, 10:58pm by pancho_hideboo »  
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #26 - Nov 28th, 2009, 6:56pm
 
Hi,

pancho_hideboo wrote on Nov 28th, 2009, 8:52am:
Again show me your definition of input noise in pnoise(type=timedomain) as OCEAN Script.

Here is what I have been using from Direct plot form:

dB10((pow(getData("out" ?result "pnoise_td") 2) / (mag((harmonic(v("/sh" ?result "pac") '(0)) / harmonic(_drplPacVolGnExpDen("v(\"/I100/In_sh\" ?result \"pac\")" '(0) nil) '(0))))**2)))

In this formula, gain is defined as: (output of the "output SH") / (output of "input SH"). However, if I use: (output of the "output SH") / (input of "input SH")
I get exactly what you got. "input of input SH" is where the PAC source is connected. Therefore, the difference in the input noise spectrums was because of different definitions of gain.

alireza wrote on Nov 28th, 2009, 8:43am:
Based on the theory, the integrated input-referred noise of the SC integrator should be 2kT/C.

pancho_hideboo wrote on Nov 28th, 2009, 8:52am:
This ignores folding effects.
Again folding is a reality.
http://www.designers-guide.org/Forum/YaBB.pl?num=1258339986/19#19
Output noise spectrum is a result of contribution from all sidebands, that is, multiple foldings.
While VGain of PAC considers only zero sideband.


Does it mean that in reality the input-referred noise of a SC integrator (caused by white noise sources) is "not completely white"?
(In the paper by R. Schreier, et al: "Design-oriented Estimation of Thermal Noise in Switched-Capacitor Circuits" in section III, they discuss that: "Due to folding the spectrum of the sampled noise signal Vnos(n) will be very nearly white."  But it is not clear what "very nearly white" really means.)

Another question may be: in reality on what parameters does this noise increase depend on?? For example, can it become 10dB under certain conditions? (rather than 2dB) Because in that case, a white noise assumption seems to be totally inadequate. (and/or unrealistic.)
(With 2dB increase near fs/2 the integrated input-referred noise power is about 20% higher than 2kT/C.)

Also, for the purpose of simulating discrete-time (SC) circuits would you recommend using pnoise (sources) (+ SH blocks at the input and the output of the circuit) instead of pnoise (time-domain)? In the paper: "Simulating SC Filters with SpectreRF", it mentions td noise analysis was built for this purpose, but it seems that the pnoise (sources) is more accurate? Am I right?

Thanks a lot,

Alireza






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Re: switched capacitor integrator noise simulation results
Reply #27 - Nov 28th, 2009, 7:59pm
 
alireza wrote on Nov 28th, 2009, 6:56pm:
Therefore, the difference in the input noise spectrums was because of different definitions of gain.
As I expected, you use different definition of input noise between pnoise(sources) and pnoise(timedomain).

You must not use Direct Plot Form in Cadence ADE blindly.

alireza wrote on Nov 28th, 2009, 6:56pm:
Does it mean that in reality the input-referred noise of a SC integrator (caused by white noise sources) is "not completely white"?
alireza wrote on Nov 28th, 2009, 6:56pm:
Because in that case, a white noise assumption seems to be totally inadequate. (and/or unrealistic.)
Pink noise is a result of correlation in time domain.

There is a correlation in time domain for SC integrator naturally since you use two Sample&Hold Circuits.

Note : Correlation of noises in frequency domain results in shaping of noise in time domain.

alireza wrote on Nov 28th, 2009, 6:56pm:
for the purpose of simulating discrete-time (SC) circuits
would you recommend using pnoise (sources) (+ SH blocks at the input and the output of the circuit)
instead of pnoise (time-domain)?
Unless your interest is a behavior of noise spectrum at clock edge, I don't recommend you to use pnoise(timedomain).

As you know, tdnoise takes very very much time in completion.
But even if you observe spectrum at clock edge by using tdnoise, you can never get any novel or valuable information so much.

Also see http://www.designers-guide.org/Forum/YaBB.pl?num=1220976977/3#3

alireza wrote on Nov 28th, 2009, 6:56pm:
In the paper: "Simulating SC Filters with SpectreRF", it mentions td noise analysis was built for this purpose.
This is no more than an advertisement of very specific vendor's EDA Tool.

EDA Tool Play is no more than Tool Play.
There is nothing superior to the actual measurement using actual instruments.

alireza wrote on Nov 28th, 2009, 6:56pm:
but it seems that the pnoise (sources) is more accurate?
Accuracies of them are same.
Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1242797715/1#1

"sources" and "timedomain" are no more than small signal analysis in frequency domain.
"sources" give you Time_Averaged_Noise_Power_Density(freq),
while "timedomain" give you Ensemble_Averaged_Noise_Power_Density(time,freq).
But meaning or interpretation of frequency spectrum is different between these two.
Again see my last comment in http://www.designers-guide.org/Forum/YaBB.pl?num=1258339986/23#23


The followings are general notes.

  - Use correct terminologies.
  - Warnigns are different from Errors.
  - ADS is not name of simulator.
  - There is no tool which name is Cadence.
  - Don't use Direct Plot of Cadence ADE blindly without knowing definition.
  - All gains in Direct Plot of Cadence ADE are "right", "true" and "practical" voltage gain.
  - MATLAB are different from Simulink.
  - Learn measurements using actual instruments. Not "EDA Tool Play


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alireza
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Re: switched capacitor integrator noise simulation results
Reply #28 - Nov 29th, 2009, 7:35pm
 
Thanks a lot for your help.

Alireza
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Re: switched capacitor integrator noise simulation results
Reply #29 - Dec 6th, 2009, 6:07pm
 
Hi,

I just have a couple of more questions regarding PSS/pnoise simulations. I am right now using the pnoise(type=sources) to find the input referred noise power of the SC integrator. I am using the integrator in open loop mode (without the S/H block feeding back to the input, and I don't have any stability issues, due to finite opamp gain.)
My circuit parameters are as follows: gm_opamp=1, Rout_opamp=1M, Ron_switch=600ohm, Cs=CI=1pF, R_opamp=0 (to model opamp noise) (please see the attached schematic.)

I am looking at the input-referred noise spectrum, based on your OCEAN script:  
(mag(getData("in" ?result "pnoise-pnoise"))**2)

The simulation results are fine at low frequencies (below fs). However I do have an spike occurring at fs=25MHz. (please see the attached curves.)

If I want to find the total integrated input-referred noise power, I should integrate the spectrum from 0 to inf, which of course includes the singular point.

Now, I am not sure, what to do with the singular point at 25MHz? Should this happen in reality? or is it again an artifact? (It only occurs at 25MHZ but not other harmonics!) (My PSS analysis engine is Shooting, as HB does not give reasonable results with the hidden-states free SH block I am using)

Even if I ignore this spike, the input noise spectrum does not fall off at high frequencies, so essentially if I integrate it, it will yield a noise power which is much larger than expected.

The above curve is for the noise density referred back to the input of the circuit (node:Inp), where PAC source is connected. This corresponds to the "input" of the "input SH". However, if I refer the noise back to the "output" of the "input SH" (node:In_sh), I get a different curve (second curve in the attached plot) If I integrate this curve from 0 to inf, I do not have any spikes and almost get the right input referred noise power. (10.54nV^2 for Cs=1pF). However I am not sure whether this node is the one I should refer back the noise to.

Thanks a lot,

Regards,
Alireza
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