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Simulators >> RF Simulators >> Pnoise for lna+mixer
https://designers-guide.org/forum/YaBB.pl?num=1042537924

Message started by hugh on Jan 14^th, 2003, 1:52am

Title: Pnoise for lna+mixer
Post by hugh on Jan 14^th, 2003, 1:52am

Hello all friends,

I'm a new comer to spectre simulater. I was just designing a LNA and mixer. The noise figure of LNA is 1.2dB, and the input noise voltage for mixer is 2nV now. However, if I connect the LNA and mixer together, the total noise figure is 4.5dB. The gain of LNA now is about 15. With a hand caculation, the noise figure should not be that much. I ran the simulation for LNA+mixer with the same configuration as mixer for pss+pnoise analysis. My question now is if this setup is correct or not? Or how to do the analysis for LNA+mixer together?

Thank you,

Hugh

Title: Re: Pnoise for lna+mixer
Post by Eugene on Jan 14^th, 2003, 8:55am

I found that it takes some effort to make manual calculations match pnoise analysis. It is easy to forget about the assumptions you make for a manual calculation and it is also easy for a new SpectreRF user to set the analysis up incorrectly. Without seeing your pnoise set up, I only have the following comments: Are you sure the assumptions you make for hand calculations and simulations are consistent, especially regarding terminal impedances and interstage loading? Also, pss will account for noise mixing from other harmonics besides the desired ones, whereas the typical manual calculation may not. It is very very easy to end up with an errors of 3dB, which seems to be in the neighborhood of your error. You might try playing around with the number of sidebands you are using in your pnoise analysis. That might give you some idea of where any inconsistent assumptions lie.

Title: Re: Pnoise for lna+mixer
Post by hugh on Jan 14^th, 2003, 11:04pm

Thank you very much. Maybe it's helpful to give you my analysis configurations.

The setup for mixer is as below:
in pss, only LO signal (LO=1G) is considered; the number of harmonics = 0; errpreset=conservative.

in pnoise, sweep type = absolute; star=1k, stop=2G, points per decade=50; maximum sideband = 40; reference sidebands = -1.

These setup are copied from the tutorial of cadence.

When simulating the mixer, I put the output impedance of LNA at the input of the mixer.

Then when simulating the LNA+mixer, I use the same pss and pnoise setup as for the mixer.

Would you please tell me if something is error with my analysis?

Thank you.

Title: Re: Pnoise for lna+mixer
Post by Eugene on Jan 15^th, 2003, 12:58pm

Assuming the objective is to resolve the descrepancy between hand calculations and simulation, I would start by using a relative sweep only over the band of interest. I would also limit the the maximum number of sidebands to 1, and then maybe even use an array of sidebands and select only the + or - 1 side band. These changes may bring your simulation in line with your manual calculation. Then you can decide which is more appropriate. Also, as I said before, make sure your manual calculation accounts for changes in loading. For example, if everything in the chain is driven from 50 ohms and loaded with 50 ohms, the usual Friis formula applies. But if you start out with a 50 ohm system then down stream switch to high input impedances, you will have to modify the Friis formula.

Title: Re: Pnoise for lna+mixer
Post by Harry on Dec 2^nd, 2005, 10:27pm

Hi, Hugh, have you figured out why yet? I have tried cascading two LNAs and the pnoise result matches to that of hand calculation based on the Friis equation. I believe it has to do with the mixer (or frquency conversion in spectreRF).

Title: Re: Pnoise for lna+mixer
Post by RC on Dec 28^th, 2005, 9:18am

i have another more interesting issue to share with. it is base on measurement, the measured nf is much less than the simulated one. don't know why after many people and a lot of time spent on figuring out how simulation should go, so the company is thinking of switching to other software. we are group of experience rf/rfic/mmic designer, just by forced to used this software.
i am looking for the solution of this discrepancy now, so don't need to switch software.

Title: Re: Pnoise for lna+mixer
Post by Eugene on Dec 28^th, 2005, 9:32am

Among the three numbers (measured, simulated, and manually calculated), do any two agree? If so, which ones?

Title: Re: Pnoise for lna+mixer
Post by RC on Dec 28^th, 2005, 9:48am

measure agree with manual. any suggestion in debugging the simulation? such as
-looking at the gain first,
-single stage nf,
-noise summary,
-transient,
-op, dc,
-etc.
but we are not spectre common user, that's why need some advice.