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pss+pnoise: cascaded system with 2 freq translations (Read 14073 times)
mixed_signal
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pss+pnoise: cascaded system with 2 freq translations
Sep 04th, 2014, 7:18am
 
Hi,

I have done pss+pnoise analysis and found noise gain of drain noise of M1 to Vout1 and Vout2 from sideband = +1.

Does pss+pnoise do a second frequency translation using -LO while finding the gain from M1 to Vout2? What does sideband=+1 mean at the second output
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Ken Kundert
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #1 - Sep 6th, 2014, 11:55pm
 
I cannot understand your question.

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #2 - Sep 22nd, 2014, 7:47pm
 
Hi Ken,
Sorry for the late reply.
The system has 2 mixers in cascade, 1st driven by +LO and 2nd driven by –LO. The input noise profile of a MOS device in the system is shown. I want to know how much noise at fRF (red) of M1 appears at the output at  fIF where fIF=fRF-fLO.

I tried to find it out using pss+pnoise with  noise separation by following these steps in ADE
Results>Direct plot> noise separation> source noise gain>sideband=+1>select device M1>plot

Is my procedure correct?
There are two frequency translations at X and Y. If I choose sideband=+1, will everything be taken care of?
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #3 - Sep 22nd, 2014, 8:25pm
 
Still having trouble understanding your question. Are you trying to determine how to compute the output noise of a chopper stabilized amplifier?

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #4 - Sep 22nd, 2014, 9:30pm
 
Hi Ken,

It is not a chopper stabilized amplifier but a circuit under research, some sort of mixer with differential LO.
If the scenerio is as depicted as in the figure,  are the simulation steps correct to find the noise?
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #5 - Sep 23rd, 2014, 4:45am
 
Okay, well you have drawn a chopper stabilized amplifier. So, it seems that this circuit can be thought of as a chopper stabilized amplifier where the input signal is at the first upper sideband of the clock. This is the same as a down-conversion mixer and would be simulated in the same way.

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #6 - Sep 23rd, 2014, 8:10am
 
Hi Ken,
Thank you very much for the reply. One more question.

Please check the attached single gate nonlinearity based mixer. The LO frequency is 2.4 GHz.

I have simulated noise transfer function of thermal & flicker noise of M1 from RF to IF using pss+pnoise and noise separation. (sideband=+1). Why are they different? I believe both thermal and flicker are modeled as current source and hence transfer function should be same. The transistor model is BSIM 4.5 and noise model is holistic. Is it because contribution from noise source modulation is different for them?

I tried modeling the noise using a current source (red) and use PXF analysis. But the gain is around 150 in contrast with 1.2K from pnoise simulation. Why is pxf resulting in such low value? Is this method of modeling noise  faulty?
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #7 - Sep 23rd, 2014, 8:12am
 
Simulations are here
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #8 - Sep 23rd, 2014, 8:33pm
 
There are two important differences between the thermal (channel) noise and flicker noise produced by a MOSFET. Of course, flicker noise as a 1/f characteristic whereas the channel noise is white. The second is that the thermal noise is modeled as coming from a voltage source in series with the gate whereas the channel noise is modeled as coming from a current source across the channel. Thus, the flicker noise is being modulated by a possibly time varying gm.

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #9 - Sep 23rd, 2014, 8:57pm
 
Hi Ken,
Thanks again for the reply. I have still some doubts:
1. Even if channel/thermal noise is modeled as a current source, it is expressed as 4kTgm and hence modulated gm will also affect it in the same way as flicker.

2. The 1/f should not be an issue because I plotted noise transfer function and not the noise. The transfer function should be independent of input noise profile.
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #10 - Sep 23rd, 2014, 10:31pm
 
The gm term in the channel noise is considered to be part of the noise source and so is not included in the transfer function. The gm term in the flicker noise is consider part of the circuit not the noise source, and so is included in the transfer function.

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #11 - Sep 24th, 2014, 8:19pm
 
Hi Ken,

But flicker is modeled as current source in my kit that uses BSIM 4.5. I have attached the equations from BSIM 4.5 documentation. Also spectre plots transfer function in voltage/ampere units which indicates current noise model fro flicker.
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #12 - Sep 25th, 2014, 8:30pm
 
I believe SpectreRF treats the gm term in the channel noise as part of the noise source and so is not included in the transfer function. The gm term in the flicker noise is treated as part of the circuit not the noise source, and so is included in the transfer function. That suggests that the two transfer functions differ by a factor of gm. Is that what you are seeing?

-Ken
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #13 - Sep 26th, 2014, 6:16pm
 
Hi Ken,

Thanks again for your reply.
The gm is only 1.15mS. In contrast the ratio is 1.13 which is 1000 times more. I have thoroughly checked both the units (V/A).

I am more concerned with the failure of PXF analysis in determining the noise gain (150 in contrast with 1000). I have tried doing both pnoise and pxf  before to find noise transfer function from resistor thermal noise to different circuit nodes and they always agree.

Is modeling noise by external current source not correct?
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Re: pss+pnoise: cascaded system with 2 freq translations
Reply #14 - Sep 27th, 2014, 4:14am
 
Hmmm. Okay, I am starting to see some of the subtlety. The simplest model for flicker noise is a stationary noise voltage in the gate, so I think of it that way, but the simulator does not actually put a voltage source in the gate because that would require extra nodes, which would make the simulation more expensive. However, the simulator can still compute the transfer function from the gate if it wanted to by dividing through by gm, but it seems like SpectreRF is not doing that. If it were, then the units should be V/V rather than V/A.

Having said that, the units given in your screen capture are very confusing to me. What is the meaning of :sqrt(Hz). A transfer function is not a noise and so should the units should not include :sqrt(Hz). I also do not know how to interpret the colon.

Now, as to your question as to why the transfer function for channel noise and flicker noise are different, I cannot say. You might ask Cadence about that.

Also, if the gm term is not being included in the flicker noise transfer function, then it does seem as if the PXF and PNoise transfer functions should be exactly the same. I cannot explain why they would be different. Perhaps there is something in the documentation that describes it. After all, there must be some reason why they provide PNoise separations as opposed to just advising you to use PXF.

-Ken
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