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LTSpice grading coefficient question

Discussion in 'CAD' started by s.d., Oct 2, 2003.

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  1. s.d.

    s.d. Guest

    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 30µ
    SYMBOL ind2 2048 2064 R0
    SYMATTR InstName L1
    SYMATTR Value 3µ
    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 250µ
    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.
     
  2. s.d.,
    Opps, that limit was there for historical reasons. It will be removed
    in the next available update.

    Thanks.

    --Mike
     
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