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Design >> Analog Design >> gm-c filter OIP3 simulation result
https://designers-guide.org/forum/YaBB.pl?num=1373337545

Message started by abcyin_DG on Jul 8^th, 2013, 7:39pm

Title: gm-c filter OIP3 simulation result
Post by abcyin_DG on Jul 8^th, 2013, 7:39pm

Hi all,

the attached is the oip3 simulation result of a Gm-C filter, it's confusing that the IM3 is away from 3× slope line, and not smooth at all, could anyone explain it to me the reason?

Thanks in advance

Title: Re: gm-c filter OIP3 simulation result
Post by aaron_do on Jul 8^th, 2013, 9:19pm

Hi,

I suggest you run the simulation from a lower power level where you should get the correct slope. Higher order odd nonlinearity also results in 2*f2- f1 IM3 components but with different slope. For example, a 5th order nonlinearity will still produce IM3 but with a slope of 5dB/dB. So when the amplifier is heavily non-linear, high-order nonlinearity can cause the slope to go haywire.

That's my initial guess. It could be due to something else though.

regards,
Aaron

Title: Re: gm-c filter OIP3 simulation result
Post by abcyin_DG on Jul 8^th, 2013, 10:27pm

Thanks for the reply,
As the IM3 could be buried in the noise when the simulation started from a lower level, it still couldn't give the correct slope at this power level.

And I tried to involve less intermodulation products during the simulation, but still results the similar result, thus to say, the higher order intermodulation products don't give much influence as we expected.

And do you think it's because of the gm-C structure, because other circuits will not give such kind of results, for example VGA.

Title: Re: gm-c filter OIP3 simulation result
Post by Frank Wiedmann on Jul 9^th, 2013, 12:27am

You might want to take a look at http://www.designers-guide.org/Forum/YaBB.pl?num=1207474039.

Title: Re: gm-c filter OIP3 simulation result
Post by aaron_do on Jul 9^th, 2013, 12:34am

Hi,

I guess we can't know unless you post the schematic. There is some discussion on this forum about IM3 results showing the wrong slope even at low power when using BSIM models, but this is for transistors biased in the deep triode region so it might not apply to your circuit. If you have PSP models, you could try using them.

What do you mean by buried in the noise? There isn't any noise in the simulation. If you are talking about the simulator accuracy, you can try increasing the accuracy setting to conservative for example.

If you say that the slope is wrong even at low power levels then it shouldn't be due to high-order harmonics.

regards,
Aaron

Title: Re: gm-c filter OIP3 simulation result
Post by abcyin_DG on Jul 9^th, 2013, 12:46am

Hi,

The reason I think IM3 would be buried in the noise is as follows:
For example, when I was simulating a circuit, I would expect that the IM3 curve would be flat in the low input power range, and then with 3× slope in some power range, then saturate finally. I think the flat IM3 range is due to the circuit-self noise, even I set the accuracy mode to conservative, as always the filter itself contributes lots of noise.

So how do you think?

BR

Title: Re: gm-c filter OIP3 simulation result
Post by aaron_do on Jul 9^th, 2013, 2:20am

Hi BR,

that would be true in measurement, but in simulation, noise has to be simulated seperately. You only see circuit noise when you run noise simulations like "noise", "pnoise", or "transient noise". I'm not too sure about the whole matter, but I believe the "noise floor" you see in simulation is due to numerical inaccuracy related to the limited number of bits used in calculations. I don't remember the numbers, but I believe that "noise floor" is a lot lower than the -50 dBm I see in your plot.

cheers,
Aaron

Title: Re: gm-c filter OIP3 simulation result
Post by abcyin_DG on Jul 9^th, 2013, 2:33am

Thus, Could you tell me how to increase the accuracy besides setting to conservative?

BR

Title: Re: gm-c filter OIP3 simulation result
Post by aaron_do on Jul 9^th, 2013, 5:55pm

Here is what I found through a quick search online...

http://www.designers-guide.org/books/dg-spice/chA.pdf

Why don't you post the plot with the input starting from a much lower value...btw, seems that the numerical noise floor may be due to the point where your accuracy setting tells the simulator that the tolerance is acceptable enough and stops the simulation.

Aaron

Title: Re: gm-c filter OIP3 simulation result
Post by abcyin_DG on Jul 9^th, 2013, 9:46pm

aaron_do wrote on Jul 9^th, 2013, 5:55pm:

Thanks, Aaron, I set the accuracy to much tighter, as the document tells, I do see the difference, the output from a lower input power decreases as the accuracy increases, so I'm sure now it's all because of the accuracy, but not the noise.

Best wishes,
Li