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The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Simulators >> RF Simulators >> Pnoise simulation https://designers-guide.org/forum/YaBB.pl?num=1211952319 Message started by YYou on May 27th, 2008, 10:25pm |
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Title: Pnoise simulation Post by YYou on May 27th, 2008, 10:25pm Hello, I have simulated the noise performance of my direct conversion front-end recently with Pnoise. - the mixer is single balanced structure with LO buffers and DividerBy2 circuit - to minimize the noise contribution from the dividerBy2 and LO buffers, I use the inductive load in the LO buffer design. Also the LO buffer is ac coupled to the mixer core, that is, there is a HPF further filtering out the low frequency signal/noise. But after running the PSS/Pnoise, I found that most of the low frequency noise components are from the transistors in LO buffer and DividerBy2. Then I tested the noise performance of the LO buffers, which showed that the low freq. noise is very small (filtered out by LC tank and HPF). In my understanding, the noise from the LO side will leak to the output when both switching transistors are on. So as long as the LO signal has large swing, the noise contribution at the output from the LO side should be small. Is that right ? Can anybody give me some ideas what could go wrong with my circuit ? Simulation or design ? Thanks, Yyou |
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Title: Re: Pnoise simulation Post by pancho_hideboo on May 28th, 2008, 6:51pm YYou wrote on May 27th, 2008, 10:25pm:
You have to see noise at IF and LO frequencies under large signal drive. Here you must not unload LO buffer. You should load actual input impedance mixer seeing Lo input. Or you should do total simulation including LO buffer, DividerBy2 and Mixer. YYou wrote on May 27th, 2008, 10:25pm:
You don't seem to understand mixer operation. You only mention about direct feedthrough from LO to IF. Both LO buffer and DividerBy2 operate under very large signal drive, so low frequency noise of the transistor will be upconverted to around local frequency. This is LO floor noise. LO signal have noise at both IF and LO frequencies. Then you see contribution of these noise at IF frequency of Mixer. Noise around IF frequency of Local port is observed at IF frequency of Mixer. This is direct feedthrough. Noise around LO frequency of Local port is observed at IF frequency of Mixer. This is mixing effect. |
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Title: Re: Pnoise simulation Post by YYou on May 30th, 2008, 5:53pm Thank you for your reply. You are right that the low frequency noise of the transistors of LO buffers and DividerBy2 will be upconverted to around local frequency. I was considering that this noise at the LO port is stationary, so with the noise shaping by the LC tank & HPF, the noise power is only peaked at LO frequency. Referring to the paper 'noise in RF-cmos mixer: A simple physical model' by Darabi and Abidi, an interferer(noise) at a frequency of fin at LO port will produce harmonics at the mixer output at fin, 2f(LO)+/-fin, 4f(LO)+/-fin,..., so the big noise at LO frequency will not be observed at IF(DC for direct conversation receiver). Now I realize that this noise maybe cyclostationary, so the above statement is not applicable to this situation. But I still cannot get the details. I appreciate if someone can share some knowledge about it or give some references explaining this effect. I also tried to verified this by simulation. Here is the setup: -Case 1: check the noise at IF using PSS/Pnoise a. Input RF port: PAC=1 (say, RF freq band at 2G) b. LO(divider input)= 2*2G=4G c. PSS (fund=4G, Beat=2G) d. Pnoise(start=1K to 3M, max.sideband=20, output=IF_output, input=rf_input, reference band=-1) e. after simulation, I checked noise summary and saw large portion of noise(1K to 100K) is from 1/f noise from LO buffer/DividerBy2. -Case 2: check the noise at LO buffer output(using the same circuits as case 1) a. same as case1 b. same as case1 c. same as case1 d. Pnoise(start=1K to 3.5G, max. sideband=20, output=LO_buffer_output, input=rf_input, reference band=-1) e. I checked noise summary again after simulation, I expect to see the biggest noise contributor at 2G is from the 1/f noise from the transistors in LO buffer/dividerBy2, but the result is the opposite, most of it is white. Is my setup right ? What is the noise spectrum I should see at the LO buffer output ? The last question is how to solve this issue: - Now I am using single balanced mixer, if using double balanced mixer, the noise can be largely rejected since it is like common-mode signal. Right ? - If I stick to single balanced mixer, is there any way to lower the noise contribution from LO part ? Thanks again for the help! Yyou. |
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Title: Re: Pnoise simulation Post by pancho_hideboo on May 31st, 2008, 1:51am YYou wrote on May 30th, 2008, 5:53pm:
I can't understand why noise spectrum is peaked at LO only when noise is stationary. I think noise spectrum is also peaked at LO even when noise is cyclostationary. YYou wrote on May 30th, 2008, 5:53pm:
Noise at mixer output is cyclostationary. Also noise from LO buffer is cyclostationary. But whether cyclostationary or not is not issue in your case, I think. YYou wrote on May 30th, 2008, 5:53pm:
Wrong. fund=beat=2GHz. But you should not use term "beat" in PSS setting. Expression of "beat" is not proper. Fundamental frequency of PSS is not beat frequency. I can't understand why Cadence has kept to use this expression of "beat" in PSS's setting UI. There is no "beat" in analysis statement. But I don't mind "beat" since I don't use Cadence SpectreRF for RF circuits design. YYou wrote on May 30th, 2008, 5:53pm:
Of course, improper. Your simulation points in pnoise are not proper, so 1/f noise is lost in case-2. If you observe noise of IF frequency, you have to set like following. aho pss fund=2G harms=8 maxacfreq=40G errpreset=moderate annotate=status outputtype=freq boke (Any_Out_Node 0) pnoise start=1K stop=3M maxsideband=20 + sweeptype=absolute annotate=status If you observe noise of LO frequency, you have to set like following. aho pss fund=2G harms=8 maxacfreq=40G errpreset=moderate annotate=status outputtype=freq boke (Any_Out_Node 0) pnoise start=1K stop=3M maxsideband=20 + sweeptype=relative relharmnum=1 annotate=status You don't have to set iprobe=rf_input and reference band=-1 when you don't see NFssb and Gain. "reference band=-1" is not proper especially for case-2. YYou wrote on May 30th, 2008, 5:53pm:
Right. But not largely. And the noise is not like common-mode signal. YYou wrote on May 30th, 2008, 5:53pm:
It might be improved if you change LO and RF ports. |
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