SPICE and LTSPICE noise confusion

Hi folks,

I did a noise analysis with SPICE3 the other day and got confusing
results. Actually I wanted to twiddle around with the JFET pair we've
been discussing and let SPICE do the math for me. SPICE and I have
never been a good team because of mutual misunderstanding, of which
this matter is just another instance.

My circuit has a gain of about 9. After running the sim,
plot sqrt(onoise_spectrum) gives me a noise voltage density of about
15nV/rtHz, but the inoise is 1.7 uV/rtHz!

I re-checked with LT's version of SPICE, and sure enough, I get
1.65nV/rtHz which is about 1/9 of the output noise as I expected.

The SPICE netlist is below. E1 (Gain 1) was just added to reference
the circuit's output to GND. To exclude any circuit discrepancy
between LT's SPICE and the SPICE3 on my linux box I first ran the sim
on LT, then exported the netlist and sicked the Linux version on it.

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* Z:\home\bl\SWCAD\Noise1.asc
J1 N002 N004 N005 2N5912
J2 N003 0 N005 2N5912
R1 N005 N006 2000
R2 N001 N002 1000
R3 N001 N003 1000
V1 0 N006 12
V2 N001 0 9
V3 N004 0 0 AC 1e-3 0
E1 1 0 N002 N003 1
..noise V(1) v3 dec 10 1 1e4
..model 2n5912 njf (vto=-2 beta=6e-3 lambda=70e-6 cgs=3e-12 cgd=1e-12)
..end

----


Below is the .ASC file in case you want to see for yourself in
LTSPICE.

----

Version 4
*** 1 1244 680
WIRE -16 -160 -448 -160
WIRE 208 -160 -16 -160
WIRE -448 -64 -448 -160
WIRE -16 -64 -16 -160
WIRE 208 -64 208 -160
WIRE 640 -48 560 -48
WIRE 688 -48 640 -48
WIRE 560 64 560 -48
WIRE -16 80 -16 16
WIRE 512 80 -16 80
WIRE 208 128 208 16
WIRE 512 128 208 128
WIRE -448 144 -448 16
WIRE -448 144 -512 144
WIRE -16 144 -16 80
WIRE 208 144 208 128
WIRE -512 192 -512 144
WIRE -64 208 -144 208
WIRE 368 208 256 208
WIRE -144 288 -144 208
WIRE -448 336 -448 144
WIRE -16 336 -16 240
WIRE 96 336 -16 336
WIRE 208 336 208 240
WIRE 208 336 96 336
WIRE 368 352 368 208
WIRE 560 352 560 144
WIRE -144 432 -144 368
WIRE 96 432 96 336
WIRE -448 624 -448 416
WIRE 96 624 96 512
WIRE 96 624 -448 624
FLAG 368 352 0
FLAG -512 192 0
FLAG -144 432 0
FLAG 560 352 0
FLAG 640 -48 1
SYMBOL njf -64 144 R0
SYMATTR InstName J1
SYMATTR Value 2N5912
SYMBOL njf 256 144 M0
SYMATTR InstName J2
SYMATTR Value 2N5912
SYMBOL res 80 416 R0
SYMATTR InstName R1
SYMATTR Value 2000
SYMBOL res -32 -80 R0
SYMATTR InstName R2
SYMATTR Value 1000
SYMBOL res 192 -80 R0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL voltage -448 320 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 132 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL voltage -448 -80 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 9
SYMBOL voltage -144 272 R0
WINDOW 123 24 132 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1e-3 0
SYMATTR InstName V3
SYMATTR Value 0
SYMBOL e 560 48 R0
SYMATTR InstName E1
SYMATTR Value 1
TEXT 336 528 Left 0 !.noise V(1) v3 dec 10 1 1e4
TEXT 336 560 Left 0 !;ac dec 10 1 1e4
TEXT 336 480 Left 0 !.model 2n5912 njf (vto=-2 beta=6e-3 lambda=70e-6
cgs=3e-12 cgd=1e-12)

----

Thanks for any insight!

robert
.

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