flamingo
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Thank you, and I have learned something from your reply. The fft "noise" floor can be decrease to about -75dB while increasing the tran run time from 10us to 50us. the reason??? The following is tran and fft statements:
**input nodes:in1 and in2 vid1 s1 m0 ac 1 0 sin(0 0.3m 35meg) $$to keep output swing equal 1Vpp vid2 m0 ic sin(0 0.3m 37meg) evid1 s2 m1 m0 s1 1 evid2 m1 ic ic m0 1 $diff Signal use transformers vic ic 0 2 rs1 s1 in1 50 rs2 s2 in2 50 .options acct post nomod brief dcstep=2 CONVERGE=1 .options fft_accurate acout=0 lvltim=3 reltol=1e-4 .tran 0.2n 10u $when use longer duration the lower "noise" floor of fft .fft v(outn,outp) np=8192 window=kaiser alfa=3 $default start=0 and stop=10u in tran. .end
althogh longer tran simulation time can imporve the "noise" floor, I don't know the limit due to this, and it takes long time (Typically 50us needs about 4-5hours). Since the fft "noise" floor does not represent the actual noise of amplifier, I just need to lower the floor level enough to manifest the distortion. For example, IM3 for a given output is about -60dBc, then I just need "noise" floor -65dB or lower. As you mentioned, If want to do tran with noise analysis simultaneously, what simulator can be used? I have SpectreRF and ADS, can one of them do this?
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