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switched capacitor integrator noise simulation results (Read 7169 times)
alireza
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switched capacitor integrator noise simulation results
Nov 15th, 2009, 6:53pm
 
Hi,

I have been working on noise simulations of a discrete time integrator using SpectreRF. I have used PSS/PAC/td-noise tools to find the input/output noise PSD's (power spectral densities), as well as the transfer function from the input to the output of the integrator (after the S/H). My methodology has been based on the paper: "Device noise simulation of DS modulators".

However, what I get as the input noise PSD is only white noise at low frequencies, and the spectrum starts to rise as frequency gets closer to fs/2 (12.5MHz in my case.) This leads to incorrect input-referred noise power calculated by integrating the spectrum. However, if I ignore the rise of noise PSD, (assume it is white) and multiply the white PSD by the bandwidth (12.5MHz) I get the right (expected) noise power, which is approximately 2kT/C. I have attached my schematic as well as the output waveform snapshots in a single .zip file.

I would very much appreciate if someone can help me figure out why I am seeing this rise of noise floor near fs/2.

Thank you very much.

Alireza
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pancho_hideboo
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Re: switched capacitor integrator noise simulation results
Reply #1 - Nov 16th, 2009, 12:58am
 
Show me time waveform of clock(25MHz) with x-axis as timeindex.
Show me noise spectrum of all timeindex points.
Try to set timeindex other than 0 to observe noise spectrum.
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #2 - Nov 16th, 2009, 9:35am
 
Hi,

Thank you very much for your reply. I simulated and plotted the noise spectrum at 2 other points (20ns, and 40ns). "0" and "40ns" correspond to the end of phase 2 (integration phase), while 20ns corresponds to the end of phase 1 (sampling phase). The results are approximately the same. Please find the waveforms attached.

Sorry, I was not exactly sure about your first question. Did you mean the clock waveform with respect to time obtained from a transient simulation? (because if I want to plot the clk waveform when running PSS/Pnoise simulations only, Cadence doesn't plot anything.)

ps: Just to provide some more detail about my test bench, I am using ideal components for this noise simulation, my opamp is ideal (VCVS, in series with an Rout), also switches are the ideal switches from analogLib, in series with a 500ohm resistor whose noise is enabled. Basically, all the noise I am simulating is the white noise from the series Ron of switches.)

Thanks,

Alireza
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waveforms3_001.png
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pancho_hideboo
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Re: switched capacitor integrator noise simulation results
Reply #3 - Nov 17th, 2009, 5:08am
 
Show me netlist portions regarding analysis statements and signal source statements.

alireza wrote on Nov 16th, 2009, 9:35am:
Please find the waveforms attached.
Show me noise spectrum at middle point between begin and end, e.g. 5nsec, 10nsec, 30nsec, etc.
Is there any difference ?

alireza wrote on Nov 16th, 2009, 9:35am:
Sorry, I was not exactly sure about your first question. Did you mean the clock waveform with respect to time obtained from a transient simulation?
No. I don't mean Transient Analysis.
I request time domain waveform as result of PSS analysis.

alireza wrote on Nov 16th, 2009, 9:35am:
(because if I want to plot the clk waveform when running PSS/Pnoise simulations only, Cadence doesn't plot anything.)
If you set PSS properly, you can get time domain waveform as result of PSS analysis.

Again show me netlist portions regarding analysis statements and signal source statements.
Show me logfile.

Show me input_noise spectrum, output_noise spectrum and VGain as 20*log10().
Show me "output_noise spectrum" / "input_noise spectrum" as 20*log10() and compare it with VGain of PAC result.

If you compare input noise spectrums at 1Hz and 12.5MHz, they are different by 20*log10(970/660)=3.3dB.

In tdnoise of Cadence Spectre Pnoise, noise PSD is overlapped around fs/2 where noise is sampled at rate of 1/fs.
This overlapped noise is not physical.

http://www.designers-guide.org/Forum/YaBB.pl?num=1059089369/1#1
http://www.designers-guide.org/Forum/YaBB.pl?num=1207830622
http://www.designers-guide.org/Forum/YaBB.pl?num=1218622880
http://www.designers-guide.org/Forum/YaBB.pl?num=1242797715
http://www.designers-guide.org/Forum/YaBB.pl?num=1220976977

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« Last Edit: Nov 17th, 2009, 8:32am by pancho_hideboo »  
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #4 - Nov 17th, 2009, 2:35pm
 
Thanks again for your reply. I have attached the analysis and signal source parts of my netlist.

I also simulated the integrator at different time index points. The results are almost the same. (please see the attached waveforms.)
The waveform file also includes the PSS time-domain waveforms of my clocks p1 and p2.

About comparing the gain plot (in dB) obtained from PAC versus the one obtained from: output PSD / input PSD, the plots perfectly match. (As expected, as I had originally used the PAC gain to obtain the input PSD.)

About your last comment, do you think what I see in the input noise spectrum at high frequencies is an artifact caused by the way Spectre td-noise works? In other words, what I saw in my simulation results is how it should be?

Thanks a lot.

Alireza
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pancho_hideboo
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Re: switched capacitor integrator noise simulation results
Reply #5 - Nov 18th, 2009, 7:12am
 
alireza wrote on Nov 17th, 2009, 2:35pm:
I have attached the analysis and signal source parts of my netlist.
Your netlists are very fine. There is no improper setting.

alireza wrote on Nov 17th, 2009, 2:35pm:
(As expected, as I had originally used the PAC gain to obtain the input PSD.)
Output noise spectrum of tdnoise is warped around fs/2.

On the other hand, VGain as result of conventional PAC(type=sources) is time averaged gain over 1/fs period where VGain don't show any periodcity regarding frequency, so VGain of PAC is not warped at all.

If input noise spectrum is calculated by "warped output noise spectrum" / "non-warped VGain",
such input noise spectrum is also warped around fs/2.

http://www.designers-guide.org/Forum/YaBB.pl?num=1254200413/0#0
http://www.designers-guide.org/Forum/YaBB.pl?num=1221531834/0#0

alireza wrote on Nov 17th, 2009, 2:35pm:
About your last comment, do you think what I see in the input noise spectrum at high frequencies is an artifact caused by the way Spectre td-noise works?
Yes, it is an artifact.

alireza wrote on Nov 17th, 2009, 2:35pm:
In other words, what I saw in my simulation results is how it should be?
Your input noise spectrum is warped around fs/2. So you have to compensate it.
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #6 - Nov 18th, 2009, 5:56pm
 
Hi,

Thanks a lot for your comments. I just have a couple of questions.  

My main question is about your last comment: is there a way that I can compensate this effect (warped spectrum near fs/2) in Spectre, in order to obtain correct simulation results for the input-referred noise? if yes, would you please let me know how?

The other thing is that warped noise spectrum doesn't seem to happen only for td-noise analysis. In fact the same thing happens when I run a pnoise (sources) simulation, however the spectrum in that case at some points at very high frequencies (100M, 1G) looks unrealistic, with sudden jumps in the output noise floor. (Although I am not sure, if the results of pnoise (sources) simulation make sense to evaluate the noise performance of a switched capacitor circuit.)

Thank you,
Alireza
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pancho_hideboo
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Re: switched capacitor integrator noise simulation results
Reply #7 - Nov 19th, 2009, 2:47am
 
Basically I don't use Cadence Spectre mainly and intensively
because I have other more more superior simulators.
So you had better expect suggestions from Cadence Tool's devotee.

alireza wrote on Nov 18th, 2009, 5:56pm:
is there a way that I can compensate this effect (warped spectrum near fs/2) in Spectre,
in order to obtain correct simulation results for the input-referred noise?
I think it is possible although I don't use Cadence Spectre mainly.

Approximately you can compensate results of tdnoise by using results of pnoise(type=sources).
But I think strict compensation is also possible by utilizing full functionalities of Cadence Spectre intensively.

alireza wrote on Nov 18th, 2009, 5:56pm:
The other thing is that warped noise spectrum doesn't seem to happen only for td-noise analysis.
In fact the same thing happens when I run a pnoise (sources) simulation,
however the spectrum in that case at some points at very high frequencies (100M, 1G) looks unrealistic, with sudden jumps in the output noise floor.

You mean spike like following ?
http://www.designers-guide.org/Forum/YaBB.pl?num=1208334972/2#2

There's a spike in the NF curve generated from PSS/PSP.
This spike's frequency is same as fudamental frequency of PSS.

Due to numerical computation problem of Shooting Newton of Cadence Spectre's PSS,
you often see such spikes in pnoise analysis subjected to master Shooting Newton PSS.

There is another example.
See http://www.designers-guide.org/Forum/YaBB.pl?num=1208334972/1#1

Compare CDS_NFssb(Green Curve) with ADS_NFssb(Magenta Curve).
For low frequency less than 10kHz, CDS_NFssb is fairly large compared to ADS_NFssb.

If you calculate CDS_NFssb by using HB-PSS not Shooting Newton-PSS,
CDS_NFssb is consistent with ADS_NFssb completely.

alireza wrote on Nov 18th, 2009, 5:56pm:
(Although I am not sure, if the results of pnoise (sources) simulation make sense to evaluate the noise performance of a switched capacitor circuit.)
Without doubt, the results of pnoise(sources) simulation make sense to evaluate the noise performance of a switched capacitor circuit.

Why do you think pnoise(sources) simulation does not make sense ?
What procedure do you take and what instruments do you use in actual evaluation not by "EDA Tool Play ?
Consider actual situation of evaluation.

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« Last Edit: Nov 19th, 2009, 7:30am by pancho_hideboo »  
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #8 - Nov 19th, 2009, 6:26pm
 
Hi,

I have attached the input/output noise spectums that I get in my pnoise (sources) simulations with Shooting Newton PSS, with the spikes at 100MHz and 1GHz. I also tried the HB-PSS, but in that case my gain is zero! (maybe I need to change some of the simulation setup parameters to get it right?)

Other than the spikes, the pnoise simulation has the same noise floor rise near fs/2. (It is about 2dB and is approximately same as td-noise.)
So pnoise does not seem to be better than td-noise.

The reason why I was saying that I should use td-noise (but not pnoise) for my switched capacitor circuit was based on what I read in Ken Kundert's paper where he says that for discrete time circuits, this discrete time tool (td-noise) should be used.  

Since it is really important for me to solve this input-referred noise problem, I'd appreciate if you could please refer me to any source of help (paper, book, etc) in order to find a solution to this problem in Cadence spectre. Alternatively, may I ask which simulator other than Cadence spectre do you recommend for this purpose which can solve my problem?

Thanks a lot for your help.

Regards,

Alireza
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Re: switched capacitor integrator noise simulation results
Reply #9 - Nov 20th, 2009, 4:57am
 
alireza wrote on Nov 19th, 2009, 6:26pm:
I have attached the input/output noise spectums that I get in my pnoise (sources) simulations with Shooting Newton PSS, with the spikes at 100MHz and 1GHz.
If I see output noise spectrum and VGain, I think it is due to spectrum roll off characteristics of SCF hold output.
How do you calculate input noise spectrum ?
Using direct Plot Form ?
From Result Browser ?

Try to simulate with more detail frequency points such as 101points/decade including 25MHz, 50MHz, 75MHz, 100MHz, 125MHz, ...., n*Fund as additional analysis points.

alireza wrote on Nov 19th, 2009, 6:26pm:
I also tried the HB-PSS, but in that case my gain is zero!
(maybe I need to change some of the simulation setup parameters to get it right?)
Apart from slave analysis Pnoise, can master analysis HB-PSS converge for your circuit ?

alireza wrote on Nov 19th, 2009, 6:26pm:
Other than the spikes, the pnoise simulation has the same noise floor rise near fs/2.
(It is about 2dB and is approximately same as td-noise.)
So pnoise does not seem to be better than td-noise.
tdnoise is no more than pnoise where type is "timedomain".
See http://www.designers-guide.org/Forum/YaBB.pl?num=1242797715

Show me output noise spectrum for both "type=sources" and "type=timedomain" with over plot mode.

alireza wrote on Nov 19th, 2009, 6:26pm:
The reason why I was saying that I should use td-noise (but not pnoise) for my switched capacitor circuit was based on what I read in Ken Kundert's paper where he says that for discrete time circuits, this discrete time tool (td-noise) should be used.
EDA Tool play is no more than Tool Play.

There is nothing superior to the actual measurement using actual instruments.

alireza wrote on Nov 19th, 2009, 6:26pm:
Since it is really important for me to solve this input-referred noise problem, I'd appreciate if you could please refer me to any source of help (paper, book, etc) in order to find a solution to this problem in Cadence spectre.
You had better expect suggestions from Cadence Tool's devotee.

alireza wrote on Nov 19th, 2009, 6:26pm:
Alternatively, may I ask which simulator other than Cadence spectre do you recommend for this purpose which can solve my problem?
BDA's Analog FastSPICE.
http://www.berkeley-da.com/prod/oview.htm
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #10 - Nov 20th, 2009, 10:01pm
 
pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
If I see output noise spectrum and VGain, I think it is due to spectrum roll off characteristics of SCF hold output.
How do you calculate input noise spectrum ?
Using direct Plot Form ?
From Result Browser ?)
Try to simulate with more detail frequency points such as 101points/decade including 25MHz, 50MHz, 75MHz, 100MHz, 125MHz, ...., n*Fund as additional analysis points.

I use the direct plot form to plot the input noise, the output noise and the gain. I also did the simulation with more points (including those specific points.) The result was that now at all those points I got those glitches in the gain plot. (please see attached)

pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
Apart from slave analysis Pnoise, can master analysis HB-PSS converge for your circuit ?

I think so. (I say that based on the fact that the simulation completes.)

pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
Show me output noise spectrum for both "type=sources" and "type=timedomain" with over plot mode.

tdnoise and pnoise (sources) both yield the same (incorrect) spectrum for the input-referred noise. There is about 2dB rise in the noise floor near fs/2. (please see attached.)

Thanks,
Alireza
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Re: switched capacitor integrator noise simulation results
Reply #11 - Nov 21st, 2009, 12:05am
 
alireza wrote on Nov 20th, 2009, 10:01pm:
I also did the simulation with more points (including those specific points.)
The result was that now at all those points I got those glitches in the gain plot. (please see attached)
These are very natural results due to spectrum roll off characteristics of SCF hold output.

alireza wrote on Nov 20th, 2009, 10:01pm:
pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
Apart from slave analysis Pnoise, can master analysis HB-PSS converge for your circuit ?
I think so. (I say that based on the fact that the simulation completes.)
The followings are your analysis statements for Shooting-PSS.
Quote:
pss  pss  fund=25M  harms=1  errpreset=conservative  maxacfreq=15.1G
+    annotate=status

pac  pac  start=1  stop=12.5M  maxsideband=300  annotate=status

pnoise  (  sh  0  )  pnoise  start=1  stop=12.5M  maxsideband=300
+       iprobe=V4  refsideband=0  noisetype=timedomain  numberofpoints=4
+       noisetimepoints=[5n,  10n,  25n,  30n]  annotate=status

You set "hams=1" in Shooting-PSS Analysis statement.
This is OK for Shooting-PSS since you set "maxacfreq=15.1G" for Shooting-PSS.

But this is not OK for HB-PSS.
Do you set "harms=300" in HB-PSS Analysis statement ?

http://www.designers-guide.org/Forum/YaBB.pl?num=1257901096/1#1
http://www.designers-guide.org/Forum/YaBB.pl?num=1237890768

alireza wrote on Nov 20th, 2009, 10:01pm:
pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
Show me output noise spectrum for both "type=sources" and "type=timedomain" with over plot mode.
tdnoise and pnoise (sources) both yield the same (incorrect) spectrum for the input-referred noise. There is about 2dB rise in the noise floor near fs/2. (please see attached.)
Show me output noise spectrum(as dB unit)
for both "type=sources" and "type=timedomain" with over plot mode.
Not input noise spectrum.

Your noise rise near fs/2 in pnoise(type=sources) is folding not alias.
Folding in pnoise(type=sources) is physical.
On the other hand, alias in pnoise(type=timedomain) is non-physical.

Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1059089369/3#3
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alireza
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Re: switched capacitor integrator noise simulation results
Reply #12 - Nov 22nd, 2009, 10:04pm
 
Hi,

pancho_hideboo wrote on Nov 21st, 2009, 12:05am:
These are very natural results due to spectrum roll off characteristics of SCF hold output.

Sure. Given that these (downward) spikes are natural in the S/H gain plot (due to zeros at clk harmonics), the (upward) spikes in the input-referred noise should occur too.
But those spikes lead to huge input-referred noise if integrated over frequency. true? OR, do we not care about them? (Do we integrate the spectrum from 0 to inf or to fs/2?)
The other related question that I had is that do we run the pnoise-sources simulation over a wider freq. range? (than fs/2), because this is what is done in the paper: "simulating switched-cap Filters with specterRF", contrary to pnoise-td which is run until fs/2.


pancho_hideboo wrote on Nov 21st, 2009, 12:05am:
You set "hams=1" in Shooting-PSS Analysis statement.
This is OK for Shooting-PSS since you set "maxacfreq=15.1G" for Shooting-PSS.

But this is not OK for HB-PSS.
Do you set "harms=300" in HB-PSS Analysis statement ?

http://www.designers-guide.org/Forum/YaBB.pl?num=1257901096/1#1
http://www.designers-guide.org/Forum/YaBB.pl?num=1237890768


Yes, I had set the harmonics to 300 as well.


pancho_hideboo wrote on Nov 20th, 2009, 4:57am:
Show me output noise spectrum(as dB unit) for both "type=sources" and "type=timedomain" with over plot mode.
Not input noise spectrum.

Your noise rise near fs/2 in pnoise(type=sources) is folding not alias.
Folding in pnoise(type=sources) is physical.
On the other hand, alias in pnoise(type=timedomain) is non-physical.

Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1059089369/3#3


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) (pnoise-sources is in dBm, therefore the plot shows 34dB difference.) The PSS analysis was set as Shooting-PSS.. (But as I showed in the last post, when referred back to the input they both show a rise in the noise floor.)

Thanks,
Alireza
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Re: switched capacitor integrator noise simulation results
Reply #13 - Nov 22nd, 2009, 11:07pm
 
alireza wrote on Nov 22nd, 2009, 10:04pm:
the (upward) spikes in the input-referred noise should occur too.
But those spikes lead to huge input-referred noise if integrated over frequency. true?
I don't think so, as far as your analysis frequency points are enough detailed around these singular points or these singular points are excluded from analysis frequency points.

Note: n*Fund are singular points, but fs/2 is not singular point. fs/2 is nyquist point.

alireza wrote on Nov 22nd, 2009, 10:04pm:
OR, do we not care about them?
(Do we integrate the spectrum from 0 to inf or to fs/2?)
The other related question that I had is that do we run the pnoise-sources simulation over a wider freq. range? (than fs/2)
Yes.

0 to inf   for "Pnoise(sources)".
0 to fs/2 for "Pnoise(timedomain)".
Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1059089369/6#6

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.

Does iteration of HB trial reach to maximum limit without resulting in small ConvNorm ?
Confirm your simulation logfile.

If you have other simulators such as BDA's Analog FastSPICE, Nexxim, Aplac, ADSsim, GoldenGate, eldoRF, HSPICE-RF, SmartSpice-RF, etc, try to use them.

Other simulator also have both Shooting and HB solvers.
About HB solver, other simulator is far robust than Cadence Spectre.

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 thatthan pnoise-sources)
It is very reasonable.

alireza wrote on Nov 22nd, 2009, 10:04pm:
(pnoise-sources is in dBm, therefore the plot shows 34dB difference.)
Why don't you plot them with same units ?

Upload PSF directory data for both "Shooting-PSS/PAC(sources)/Pnoise(timedomain)" and "Shooting-PSS/PAC(sources)/Pnoise(sources)".

Or upload full netlist with model files and Verilog-A modules so that I can run simulation.

alireza wrote on Nov 22nd, 2009, 10:04pm:
The PSS analysis was set as Shooting-PSS.. (But as I showed in the last post, when referred back to the input they both show a rise in the noise floor.)
I will check with PSF data you upload.

Again, n*Fund are singular points, but fs/2 is not singular point. fs/2 is nyquist point.

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Re: switched capacitor integrator noise simulation results
Reply #14 - Nov 24th, 2009, 10:09pm
 
Hi,

pancho_hideboo wrote on Nov 22nd, 2009, 11:07pm:
alireza wrote on Nov 22nd, 2009, 10:04pm:
(Do we integrate the spectrum from 0 to inf or to fs/2?)
The other related question that I had is that do we run the pnoise-sources simulation over a wider freq. range? (than fs/2)
Yes.

0 to inf   for "Pnoise(sources)".
0 to fs/2 for "Pnoise(timedomain)".
Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1059089369/6#6


pnoise-td simulation and pnoise-sources simulation give almost identical PSDs (with slight difference near fs/2) from 0 to fs/2. And we know that in pnoise-td sim, we should integrate the spectrum from 0 to fs/2. 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?

pancho_hideboo wrote on Nov 22nd, 2009, 11:07pm:
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 thatthan pnoise-sources)
It is very reasonable.


Is this again because of folding/aliasing issue?


pancho_hideboo wrote on Nov 22nd, 2009, 11:07pm:
Upload PSF directory data for both "Shooting-PSS/PAC(sources)/Pnoise(timedomain)" and "Shooting-PSS/PAC(sources)/Pnoise(sources)".

Or upload full netlist with model files and Verilog-A modules so that I can run simulation.

I have attached the full netlist of my test bench. The S/H verilog-A code used in the test bench which I had downloaded from this website is also attached. The clock generator uses ideal blocks from ahdlLib, which I assume are available to you.

pancho_hideboo wrote on Nov 22nd, 2009, 11:07pm:
alireza wrote on Nov 22nd, 2009, 10:04pm:
The PSS analysis was set as Shooting-PSS.. (But as I showed in the last post, when referred back to the input they both show a rise in the noise floor.)
I will check with PSF data you upload.

Again, n*Fund are singular points, but fs/2 is not singular point. fs/2 is nyquist point.


Does this mean that for a sampled data system we only care about the pnoise-sources spectrum from 0 to fs/2??

Thanks a lot,

Alireza
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