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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> Negative NF for LNA in Hspice simulation and 0dB of input and output matching https://designers-guide.org/forum/YaBB.pl?num=1431362155 Message started by nguks on May 11th, 2015, 9:35am |
Title: Negative NF for LNA in Hspice simulation and 0dB of input and output matching Post by nguks on May 11th, 2015, 9:35am Currently I am trying to simulate the noise figure, S11 and S22 parameters for LNA at 2.4GHz in HSPICE. Below is the code I used: Calculate noise figure: .ac dec 50 400MEG 3g .NOISE v(vout) vin 1 Calculate S11 and S22: .net v(vout) vin rin=50 rout=50 .print s11(db) s11(m) s11(p) .print s22(db) s22(m) s22(p) However, I get negative NF after calculate from the following: **** resistor squared noise voltages (sq v/hz) element 0:r3 rs 9.1186f 1/f 0. total 9.1186f rx 5.2622k ... **** total output noise voltage = 433.9253f sq v/hz NF = 10log(9.1186f/433.9253f) = -16.77dB and get 0 value for S11 and S22: freq s11 s11 s11 db mag phase 400.00000x 0. 1.0000 180.0000 418.85142x 0. 1.0000 180.0000 ... 2.41024g 0. 1.0000 180.0000 freq s22 s22 s22 db mag phase 400.00000x 0. 1.0000 180.0000 ... 2.41024g 0. 1.0000 180.0000 Can I know did I do wrongly? Or why these values appear? Thank you. |
Title: Re: Negative NF for LNA in Hspice simulation and 0dB of input and output matching Post by Geoffrey_Coram on May 12th, 2015, 10:44am NF = 10log(9.1186f/433.9253f) Isn't NF the ratio of the total to the input resistor noise, rather than the other way around? |
Title: Re: Negative NF for LNA in Hspice simulation and 0dB of input and output matching Post by nguks on May 13th, 2015, 11:59am I think NF is 10log(SNRin/SNRout) |
Title: Re: Negative NF for LNA in Hspice simulation and 0dB of input and output matching Post by Geoffrey_Coram on May 15th, 2015, 11:46am But SNR should be the ratio of signal power to noise power, not the "total output noise voltage" If you have (S/N)_in / (S/N)_out, then you do end up with something that has N_out / N_in, which is what I was trying to say, though I agree I didn't get the formula right. |
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