LTSpice grading coefficient question

[s.d.]

Hello,

Quite unexpectedly I found out LTS complaining about M (grading
coefficient) being too large in a varactor diode model:

bby40: grading coefficient too large, limited to 0.9

That limitation seems surprising in regard of the varactor model
coefficients found in litterature. see:
http://www.eetkorea.com/ARTICLES/2002SEP/2002SEP20_BD_MSD_HBM_MAT_AN.PDF
page 4

It is not really an issue that matters much, Spice is seldom used for RF
modelling. But one would like to know if LTS always reset M to 0.9 if
too large, and the reasons of. Maybe I overlooked some configuration
switch to overide the limitation.

Best regards

s.d.

The diode model is from Zetex library

The circuit I use: vhartley.asc

Version 4
SHEET 1 2640 2644
WIRE 2400 1968 2400 2096
WIRE 2400 2176 2400 2192
WIRE 2192 1984 2192 1968
WIRE 2192 1968 2400 1968
WIRE 2064 2048 2064 2080
WIRE 2064 2160 2064 2192
WIRE 2064 2304 2064 2336
WIRE 2144 2048 2064 2048
WIRE 1936 2048 1936 2080
WIRE 1936 2176 1936 2144
WIRE 2064 2048 1936 2048
WIRE 2192 2080 2192 2112
WIRE 2192 2192 2064 2192
WIRE 2064 2192 2064 2224
WIRE 2288 2112 2192 2112
WIRE 2192 2112 2192 2192
WIRE 1744 2272 1744 2256
WIRE 1744 2192 1744 2160
WIRE 1744 2080 1744 2048
WIRE 1744 2048 1808 2048
WIRE 1424 2160 1472 2160
WIRE 1552 2160 1600 2160
WIRE 1680 2160 1744 2160
WIRE 1744 2160 1744 2144
WIRE 1536 2048 1600 2048
WIRE 1680 2048 1744 2048
WIRE 1872 2048 1936 2048
FLAG 2400 2192 GND
FLAG 2064 2336 GND
FLAG 2288 2112 out
FLAG 1936 2176 0
FLAG 1744 2272 0
FLAG 1536 2048 0
FLAG 1424 2160 0
SYMBOL njf 2144 1984 R0
SYMATTR InstName J1
SYMATTR Value 2N5484
SYMBOL voltage 2400 2080 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12.
SYMBOL ind2 2048 2208 R0
SYMATTR InstName L2
SYMATTR Value 30ľ
SYMBOL ind2 2048 2064 R0
SYMATTR InstName L1
SYMATTR Value 3ľ
SYMBOL cap 1920 2080 R0
SYMATTR InstName C1
SYMATTR Value 470pF
SYMBOL varactor 1728 2080 R0
SYMATTR InstName D1
SYMATTR Value bby40
SYMBOL varactor 1760 2256 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value bby40
SYMBOL res 1696 2144 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL voltage 1568 2160 R90
WINDOW 0 -32 56 VBottom 0
WINDOW 3 32 56 VTop 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(1 30 0 100n 100n 50u 100u)
SYMBOL res 1696 2032 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL cap 1872 2032 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 100p
TEXT 1896 1976 Left 0 !.tran 250ľ
TEXT 2216 2248 Center 0 !K1 L1 L2 1.
TEXT 1584 2376 Left 0 !*ZETEX BBY40 Spice Model Last revision
18/8/92\n*\n.MODEL BBY40 D IS=7.417E-15 N=1.058 RS=.1259 XTI=3 \n+
EG=1.11 CJO=64.39E-12 M=1.013 VJ=2.566 FC=.5 BV=45.12 \n+ IBV=.1232
TT=215E-9\n*\n* + ISR=1.731E-12 NR=2.27 IKF 9.882 (LATER SPICE
VERSIONS ONLY)\n*\n*NOTES: FOR RF OPERATION ADD PACKAGE INDUCTANCE OF
2.5E-9H AND SET\n* RS=0.47 FOR 2V, 0.39 FOR 5V, 0.32 FOR 10V OR 0.28
FOR 20V BIAS.

 

[Mike Engelhardt]

s.d.,

Quite unexpectedly I found out LTS complaining about M (grading
coefficient) being too large in a varactor diode model...

Opps, that limit was there for historical reasons. It will be removed
in the next available update.

Thanks.

--Mike