Folk,
Thanks to some good help here in the past, I have a nice clean oscillator (for my new fox transmitters), but I'm having trouble with the LTSpice simulation at higher frequencies. I'm assuming that the Intersil Spice model for the CA3046 is bad, and I know JT has a better one.
Anyhow, I have two questions about the attached LTSpice schematic.
1) When I set it to 30MHz, LTSpice says the oscillator will start nicely (even up to 150MHz in fact). Now I know that the (calculated) Colpitts capacitance ratio is bad, as is the base capacitor, but with the physical circuit built in a tight layout in SMD and a range of more sensible capacitances substituted, I can't get it to oscillate above about 12MHz. Why does it not want to run in real life, when Spice says it should? Can someone provide me a better CA3046 model please, or say what else might be going on?2) I'd like some insight into the correct formulae to calculate the Colpitts capacitive divider ratio, and the minimum safe base coupling capacitor. The CA3046 data sheet says that Cbe and Ccb are both in the range of 0.6pF, and the Ft is around 300MHz. So at 150MHz I only have
3dB of gain to play with, so that Cbe means I need at least a couple of pF to drive the base, and the divider ratio has to give reasonable drive. If anyone could suggest better "rule of thumb" for calculating these things from Ft and Cin, I'd appreciate it.Clifford Heath
-- Cut Here for OscProblem.asc -- Version 4 SHEET 1 2160 1200 WIRE 304 -336 192 -336 WIRE 752 -336 304 -336 WIRE 192 -288 192 -336 WIRE 192 -176 192 -208 WIRE 192 -176 -64 -176 WIRE 432 -176 192 -176 WIRE 528 -176 432 -176 WIRE 192 -128 192 -176 WIRE 752 -96 752 -336 WIRE 304 -48 304 -336 WIRE 432 -48 432 -176 WIRE -48 0 -64 0 WIRE 32 0 -48 0 WIRE 96 0 32 0 WIRE 192 0 192 -48 WIRE 192 0 160 0 WIRE 240 0 192 0 WIRE 32 64 32 0 WIRE 528 64 528 -176 WIRE -48 144 -48 0 WIRE 432 160 432 32 WIRE 32 208 32 128 WIRE 304 208 304 48 WIRE 304 208 32 208 WIRE 368 208 304 208 WIRE 192 256 192 0 WIRE 304 256 304 208 WIRE 32 272 32 208 WIRE -48 384 -48 224 WIRE 32 384 32 336 WIRE 32 384 -48 384 WIRE 192 384 192 336 WIRE 192 384 32 384 WIRE 304 384 304 336 WIRE 304 384 192 384 WIRE 432 384 432 256 WIRE 432 384 304 384 WIRE 528 384 528 128 WIRE 528 384 432 384 WIRE 752 384 752 -16 WIRE 752 384 528 384 WIRE 752 400 752 384 FLAG 752 400 0 FLAG -64 0 Vosc IOPIN -64 0 Out FLAG -64 -176 Vbias IOPIN -64 -176 Out SYMBOL voltage 752 -112 R0 WINDOW 123 24 126 Left 2 WINDOW 39 24 111 Left 2 SYMATTR InstName V1 SYMATTR Value 3.2v SYMBOL cap 16 64 R0 SYMATTR InstName C1 SYMATTR Value {C1} SYMBOL cap 16 272 R0 SYMATTR InstName C2 SYMATTR Value {C2} SYMBOL ind -64 128 R0 SYMATTR InstName L3 SYMATTR Value {L1} SYMBOL npn 240 -48 R0 SYMATTR InstName Q1 SYMATTR Value CA3046 SYMBOL res 176 -144 R0 SYMATTR InstName R4 SYMATTR Value 100k SYMBOL npn 368 160 R0 SYMATTR InstName Q2 SYMATTR Value CA3046 SYMBOL res 176 -304 R0 SYMATTR InstName R2 SYMATTR Value 47k SYMBOL cap 512 64 R0 SYMATTR InstName C4 SYMATTR Value {100/(F0*10k)} SYMBOL res 288 240 R0 SYMATTR InstName R6 SYMATTR Value 2.2k SYMBOL cap 160 -16 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value {C3} SYMBOL res 176 240 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL res 416 -64 R0 SYMATTR InstName R8 SYMATTR Value 10k TEXT 864 -424 Left 2 !.tran 0 {10u + (2000/F0)} 0 {1/(F0*50)} TEXT -96 -384 Left 2 !.param F0=30Meg TEXT -96 440 Left 2 !; Calculate main tank components from F0, ZT and stray capacitances:\n.param CSTRAY1=1.7p CSTRAY2=0.8pF\n.param L1={ZT/(2*PI*F0)} DIV=17 C2={1/(2*PI*F0*(ZT/DIV)) - CSTRAY1} CT={(1/(2*PI*F0*ZT)) - CSTRAY2} C1={1/((1/CT) - (1/C2))}\n.MEASURE L1_ PARAM L1\n.MEASURE C1_ PARAM C1\n.MEASURE C2_ PARAM C2 TEXT 872 232 Left 2 !.param ZCouple1=5k C3={max(1/(2*PI*F0*ZCouple1),
1pF)}\n.MEASURE C3_ PARAM C3 TEXT -96 -344 Left 2 !.param ZT=350 TEXT 872 144 Left 2 !.MEASURE C4_ PARAM {100/(F0*10k)}RIGHTS RESERVED\n*\n*CA3046 PSpice MODEL\n*REV: 2-24-97\n** ----- BJT MODEL -----\n*\n.model CA3046 NPN\n+ (IS = 10.0E-15 XTI=3.000E+00 EG=1.110E+00 VAF=1.00E+02\n+ VAR=1.000E+02 BF=145.7E+00 ISE=114.286E-15 NE=1.480E+00\n+ IKF=46.700E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.005E-15\n+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=991.71E-15\n+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.02E-12\n+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=277.01E-12\n+ ITF=1.750E-00 XTF=309.38E+00 VTF=16.37E+00 PTF=0.000E+00\n+ RE=0.0E+00 RB=0.00E+00 TEXT -96 -424 Left 2 ;Change these parameters to vary the operating frequency and tank impedance TEXT 696 560 Left 2 ;This model was initially designed and built (works fine) at 3.58 MHz.\nWhen run at any frequency up to 150MHz, it still simulates fine.\nHowever, the physical circuit only reaches half amplitude at 12MHz,\nand doesn't start at all much above that, even when larger C3 and\nsmaller C1/C2 ratio is used (to overcome lower gain, input capacitance;\n1pf for C3 cannot ever be enough with Cbe=0.5pF Ft=300MHz).\nI'd really like to know why. Is my CA3046 model wrong?