Hi, Aaron.
You can't apply straight Fris equation to cascaded systems when DUT's temperature isn't 290[K].
Straight Fris equation is valid when temperature of source and DUT are all 290[K].
Of course you can modify equation to deal with temperature difference.
See
http://www.designers-guide.org/Forum/YaBB.pl?num=1190112138 Run the following netlist using Cadence Spectre.
(1) Tambient=-250[degC], T0=16.85[degC]
(2) Tambient=-250[degC], T0=-250[degC]
(1) gives NF=4.00182[dB]
(2) gives NF=13.0000[dB]
Do you think which results express DUT's thermal noise effect reasonably ?
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simulator lang=spectre
global 0
aho options temp=-250 // Analysis Temperature in [degC], (Tambient)
parameters T0_degC=temp // Reference Temperature in [degC]
parameters att=13 // Attenuation in [dB]
parameters k=10**(att/20)
parameters z0=50 // Reference Impedance [ohm]
parameters Rp=z0*(k+1)/(k-1)
parameters Rs=0.5*z0*(k*k-1)/k
R1 (in 0) resistor r=Rp
R2 (out 0) resistor r=Rp
R3 (in out) resistor r=Rs
port1 (in 0) port r=z0 num=1 type=dc
//+ noisetemp=T0_degC
port2 (out 0) port r=z0 num=2 type=dc
//+ noisetemp=T0_degC
boke options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 \
tnom=27 scalem=1.0 scale=1u gmin=1.0e-15 rforce=1 maxnotes=5 \
maxwarns=5 digits=5 cols=80 pivrel=1e-3 ckptclock=1800 \
sensfile="../psf/sens.output" rawfmt=psfascii
sp sp values=[100M] donoise=yes oprobe=port2 iprobe=port1
+ annotate=status
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