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flicker noise (Read 9382 times)
aaron_do
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flicker noise
Feb 14th, 2008, 10:40pm
 
Hi,

just asking for opinions. Does the flicker noise in the graph below seem to have a very high corner frequency? It is a simulation of a MOSFET in a new PDK I am using. The plot is the input referred noise in dB. VDS = 0.4V, VGS = 0.6 V, IDS = 175 uA. W/L = 10u/0.18u. It is a simple common source stage with the drain supplied by a 0.4V source. I've tried using other dimensions/biasing conditions and they all come out with this shape...



thanks for the help,
Aaron
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vivkr
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Re: flicker noise
Reply #1 - Feb 14th, 2008, 10:57pm
 
Indeed, it does seem astonishingly high even for a small transistor. Perhaps you should do a reality check and ask the fab why this is so,
or look into the model file. Maybe they messed up a few orders of magnitude in one of the terms controlling the flicker noise.

The thermal noise floor looks ok, and so the problem would really lie with the flicker noise.

Regards
Vivek
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aaron_do
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Re: flicker noise
Reply #2 - Feb 15th, 2008, 12:24am
 
Hi,

thanks for the reply. I just noticed that the performance seems to be the same with my old PDK. I'll snoop around...

thanks,
Aaron
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Re: flicker noise
Reply #3 - Feb 18th, 2008, 4:14pm
 
Flicker noise corner at 1GHz? I really doubt that.

Quick sanity check - change W/L and see if the noise corner shifts as is theoretically defined. Probably won't.

Frequently noise coefficients in models are NOT properly defined. TSMC, UMC, and other "mostly digital" foundries mess these up all the time.

Jerry
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Re: flicker noise
Reply #4 - Mar 20th, 2008, 9:35am
 
hi aaron,
can u please tell me how did u obtained that plot.
thank in advance

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Re: flicker noise
Reply #5 - Mar 21st, 2008, 12:09am
 
Hi, Aaron.

Maybe you use "noise analysis" in Spectre.
So I think you set Vgs voltage source as input noise.
But how to observe output noise ?
Output noise is restricted to probe or voltage in Spectre noise analysis.
Do you convert drain current to voltage by using CCVS(Current Controled Voltage Source) ?

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Geoffrey_Coram
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Re: flicker noise
Reply #6 - Mar 21st, 2008, 7:27am
 
If you use the "oprobe" parameter for the noise analysis, doesn't this use the current of the v-source?
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pancho_hideboo
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Re: flicker noise
Reply #7 - Mar 21st, 2008, 7:51am
 
Geoffrey_Coram wrote on Mar 21st, 2008, 7:27am:
If you use the "oprobe" parameter for the noise analysis, doesn't this use the current of the v-source?

What I'm worrying is whether there exists any coupling capacitor or inductor in circuit.

If we use "port" as both load and Vds bias voltage source purpose, DC blocking capacitor is not needed, but we have to adjust dc voltage parameter of port accroding to Ids and resistance of port.

If we use "port" as load and "vdc" as Vds bias voltage source with feed resistor or inductor to drain, DC blocking capacitor is needed, so there exists some frequency characteristics especially in low frequency.

In simple test, I don't use "port" and I use "vdc" as both Vgs and Vds bias voltage sources, here I don't use any capacitor, resistor and inductor. So there is no frequency characteristics except for DUT.
Then I observe output current with using CCVS.

I have a ideal_bias_tee model built by Verilog-A. In more detailed test, I use this ideal_bias_tee with "port" as loas and "vdc" as bias voltage source when I see transistor characteristics without worrying frequency characteristics related to bias circuit.

I don't use new analogLib component "port3t", since this is realized by capacitor and inductor.
So there exist frequency characteristics.

My ideal_bias_tee model is realized without capacitor and inductor, so there is no frequency characteristics just same as DCFeed and DCBlock components in ADS.
In attached figure, inductor and capacitor symbols are drawn in ideal_bias_tee component, but there is no inductor and capacitor.

Since my ideal_bias_tee model is valid in both Spectre and ADS, I'm comparing transistor characteristics with both Spectre and ADS under completely same condition.

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« Last Edit: Mar 21st, 2008, 10:50pm by pancho_hideboo »  

test_DUT.jpg
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aaron_do
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Re: flicker noise
Reply #8 - Mar 21st, 2008, 8:01am
 
Hi,

sorry i didn't see the last few replies. If i remember correctly, I set the output in "noise" analysis to probe and probed the voltage source (VDS). Then i plotted the input-referred noise. Is this correct? I'll go back and check with iprobe...I definately didn't use CCVS.

I recently found out why the flicker noise was so high. Its actually a worst case model from the foundry. They gave us two models, as_extracted and worst_case but the as_extracted had been disabled without telling us, so we were always simulating worst_case which was about 7 times worse than as_extracted.

Does anybody have any background info on as_extracted and worst_case models for flicker noise? Like how likely is worst_case?

The corner frequency does change correctly with W and L...

thanks,
Aaron
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