On 4.4.14 11:44, Clifford Heath wrote:> > But first, I'd like a better guess (or I know, math...) at plausible > values for these parts to make it oscillate at 30MHz, before I try 50 or > 150.You are running the transistor at a very low current. It may be limiting its capability of handling the strays from the ground plane at higher frequencies. Out of curiosity, I dropped the resistances by a factor of 10, and the thing at least simulates well. Another vote to dropping the R and C from the top of the varactor chain. Also, another vote to taking the output from the emitter. -- Tauno Voipio
LTSpice simulator of my oscillator is far from reality; bad models?
Started by ●April 4, 2014
Reply by ●April 4, 20142014-04-04
Reply by ●April 4, 20142014-04-04
On Fri, 04 Apr 2014 19:28:23 +1100, Clifford Heath <no.spam@please.net> wrote:>On 04/04/14 15:04, Jim Thompson wrote: >> On Fri, 04 Apr 2014 14:03:58 +1100, Clifford Heath >> <no.spam@please.net> wrote: >> From my archives (and recently provided to Robert Macy for his noise >> experiments)... >> Some IC models (NPN) from that era... > >Thanks, that second one is the same one I copied into the ASC file >below. is the CA3046_ORG the model originally from Intersil, and the >other is yours?Yes.> >My device is actually LM3046, but I don't expect any differences that >would affect things at 30MHz, though perhaps at 150. > >Layouts are referenced in my other response.Have you figured in the pin-to-pin capacitances of your package? DIL's can run as high as 4pF.> >Clifford Heath. > >> ****************************************************************** >> .MODEL CA3046_ORG NPN IS=3.860200F BF=120 NF=1.04845 VAF=61.1026 IKF= >> + 50.000000M ISE=3.100000P NE=2.16533 BR=100.101000M NR=1.04845 ISC=0 >> + NC=1 RB=214.644 RBM=214.644 RE=721.362980M RC=9.2065 CJE=1.249000P >> + VJE=899.999940M MJE=499.999970M TF=210.000000P XTF=1.85 VTF=1.585 >> + ITF=50.000000M PTF=0 CJC=1.000000P VJC=749.999940M MJC=333.000000M >> + XCJC=499.999970M TR=10.000000N CJS=6.300000P VJS=749.999940M MJS= >> + 499.999970M XTB=1.5 EG=1.11 XTI=3 KF=0 AF=1 FC=499.999970M >> * >> *COPYRIGHT � 1997 INTERSIL CORPORATION >> *ALL RIGHTS RESERVED >> * >> *CA3046 PSpice MODEL >> *REV: 2-24-97 >> ** ----- BJT MODEL ----- >> * >> .model CA3046 NPN >> + (IS = 10.0E-15 XTI= 3.000E+00 EG = 1.110E+00 VAF= 1.00E+02 >> + VAR = 1.000E+02 BF = 145.7E+00 ISE = 114.286E-15 NE = 1.480E+00 >> + IKF = 46.700E-03 XTB = 0.000E+00 BR = .1000E+00 ISC = 10.005E-15 >> + NC = 2.000E+00 IKR = 10.00E-03 RC = 10.000E+00 CJC = 991.71E-15 >> + MJC = 0.333E-00 VJC = 0.7500E-00 FC = 5.000E-01 CJE = 1.02E-12 >> + MJE = .336E-00 VJE = 0.750E-00 TR = 10.000E-09 TF = 277.01E-12 >> + ITF = 1.750E-00 XTF = 309.38E+00 VTF= 16.37E+00 PTF = 0.000E+00 >> + RE = 0.0E+00 RB = 0.00E+00 >> * >> *COPYRIGHT � 1997 INTERSIL CORPORATION >> *ALL RIGHTS RESERVED >> * >> *CA3086 PSpice MODEL >> *REV: 2-24-97 >> ** ----- BJT MODEL ----- >> * >> .model CA3086 NPN >> + (IS = 10.0E-15 XTI= 3.000E+00 EG = 1.110E+00 VAF= 1.00E+02 >> + VAR = 1.000E+02 BF = 156.6E+00 ISE = 114.886E-15 NE = 1.470E+00 >> + IKF = 36.700E-03 XTB = 0.000E+00 BR = .1000E+00 ISC = 10.005E-15 >> + NC = 2.000E+00 IKR = 10.00E-03 RC = 10.000E+00 CJC = 991.79E-15 >> + MJC = 0.333E-00 VJC = 0.7500E-00 FC = 5.000E-01 CJE = 1.02E-12 >> + MJE = .336E- 00 VJE = 0.750E-00 TR = 10.000E-09 TF = 278.55E-12 >> + ITF = .770E-00 XTF = 91.38E+00 VTF= 18.37E+00 PTF = 0.000E+00 >> + RE = 0.0E+00 RB = 0.00E+00 >> * >> *COPYRIGHT � 1997 INTERSIL CORPORATION >> *ALL RIGHTS RESERVED >> * >> *CA3127 PSpice MODEL >> *REV: 2-13-97 >> ** ----- BJT MODEL ----- >> * >> .model CA3127 NPN >> + (IS = 3.20p XTI= 3.000 EG = 1.110 VAF= 100 >> + VAR = 100 BF = 95.2E ISE = 20.586p NE = 1.990 >> + IKF = 61.500m XTB = 0 BR =100m ISC = 10.805n >> + NC = 2.000n IKR = 10m RC = 10m CJC = 281.1f >> + MJC = 0.138 VJC = 0.75 FC = 0.5 CJE = 651.9f >> + MJE = .336 VJE = 0.750 TR = 10n TF = 122.61p >> + ITF = 1.600p XTF = 2.050K VTF= 307 PTF = 0 >> + RE = 0 RB = 0 >> * Application Note MM9701 >> ****************************************************************** >> .SUBCKT CA3046PAK 1 2 3 4 5 6 7 8 9 10 11 12 13 14 >> Q1 1 2 3 [SUB] CA3046 >> Q2 5 4 3 [SUB] CA3046 >> Q3 8 6 7 [SUB] CA3046 >> Q4 11 9 10 [SUB] CA3046 >> Q5 14 12 13 [SUB] CA3046 >> RSUB SUB 13 1 >> .ENDS CA3046PAK >> ****************************************************************** >> >>> 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)} >>> TEXT 864 -320 Left 2 !*COPYRIGHT � 1997 INTERSIL CORPORATION\n*ALL >>> 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? >> >> ...Jim Thompson >>...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●April 4, 20142014-04-04
On Fri, 04 Apr 2014 01:28:23 -0700, Clifford Heath <no.spam@please.net> wrote:>> ...snip, way too much ....Alex Bordordunov [incredibly prolific producer of effective models and able electronic designer!] just sent me this model: "Hi Robert, I have found the more neat (exact) model a model transistor CA3046. .MODEL CA3046 NPN(IS=10f BF=145.76 VAF=100 IKF=46.747m ISE=114.23f NE=1.483 BR=100.1m VAR=100 IKR=10.01m ISC=10f RC=10 CJE=1.026p MJE=333.33m CJC=991.79f MJC=333.33m TF=277.09p XTF=309.38 VTF=16.364 ITF=1.7597 TR=10n CJS=6.3P VJS=0.749 MJS=0.5 VCEO=20 ICRATING=50m MFG=RCA) To LTspice used the additional parameters to models (CJS, VJS, MJS) necessary to use the symbol NPN4. Alex." He has over 35MB of models, which he posted to the LTspice group. I am trying to put the two of you in contact. send me your email address. Meanwhile, try his model and let us know if it acts much differently. Don't be offended, ...wrap the lines properly.
Reply by ●April 4, 20142014-04-04
On Fri, 04 Apr 2014 08:38:50 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:>> ...snip.... > > Have you figured in the pin-to-pin capacitances of your package? DIL's > can run as high as 4pF. > >>4 pf ?!!! 400 ohms at 100MHz!! Words of, "Don't even THINK about trying to get impedances above free space." echo in my ears. should we also add the tiny 'common mode' transformer action inside those leads, too? After all, they are parallel conductors too, right?
Reply by ●April 4, 20142014-04-04
On 05/04/14 02:38, Jim Thompson wrote:> [snip] > Have you figured in the pin-to-pin capacitances of your package? DIL's > can run as high as 4pF.No - but it's an SSOP. Pin capacitances? No idea.
Reply by ●April 4, 20142014-04-04
On 05/04/14 00:41, Tauno Voipio wrote:> On 4.4.14 11:44, Clifford Heath wrote: >> >> But first, I'd like a better guess (or I know, math...) at plausible >> values for these parts to make it oscillate at 30MHz, before I try 50 or >> 150. > > You are running the transistor at a very low current.Ahh, good point. I'll run it a little hotter. Battery power, but a couple more mA won't hurt.> It may be limiting its capability of handling the > strays from the ground plane at higher frequencies. > > Out of curiosity, I dropped the resistances by a factor > of 10, and the thing at least simulates well. > > Another vote to dropping the R and C from the top > of the varactor chain. Also, another vote to taking > the output from the emitter. >
Reply by ●April 4, 20142014-04-04
On Sat, 05 Apr 2014 07:37:05 +1100, Clifford Heath <no.spam@please.net> wrote:>On 05/04/14 02:38, Jim Thompson wrote: >> [snip] >> Have you figured in the pin-to-pin capacitances of your package? DIL's >> can run as high as 4pF. > >No - but it's an SSOP. Pin capacitances? No idea.Some of those package are as low as 0.15 to 0.2pF between pins. Add pin-to-pin capacitance to your model as a parameter and play with it to see what matters. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●April 4, 20142014-04-04
On 05/04/14 00:25, Chris Jones wrote:> On 04/04/2014 19:44, Clifford Heath wrote: > [snip] >>> any photos? of the circuitry? Don't wish to cast aspersions, but layout >>> above 10 MHz starts to become important. up there a short connection is >>> no longer a connection, >> >> Yes, I'm aware of that. I've routed it single-sided on a copper rear >> plane, using 0805 components throughout. Within that constraint, I'll be >> very surprised if you can find a way to substantially improve it... but >> here are the snapshots of this part of the circuit (from Eagle): >> <http://cjh.polyplex.org/electronics/fox/Oscillator.brd.png> >> <http://cjh.polyplex.org/electronics/fox/Oscillator.sch.png> > [snip] > > Thanks for posting the diagrams. > > Some loss in the inductor would certainly be more realistic. I hope the > inductor is not sitting right on the PCB unless you make a big hole in > the ground plane under the inductor, at least 2x the diameter of the > inductor.I wasn't aware that much flux will escape the toroid? I'm sure I've seen this done successfully elsewhere. In any case in my prototype the toroid is floating on 2cm leads, so could flop into different places (this is the low-HF version that works fine) and at VHF I have an air coil mounted a few mm above the board. Otherwise it is going to reduce the Q like a shorted turn. As> long the hole is big enough so that the ground plane is spaced far > enough from the turns, it is no longer a problem and actually helps with > shielding the inductor. > > I note that you are taking the output via C9 from the top of the tank, > which is a very sensitive node. I hope it is going to something with > really low loss, or that could be a cause for reducing the Q and > tendency to oscillate. I would suggest taking the output from some > low-impedance node, such as a tap on the inductor, or the top of R6 or > something like that.I have a very small coupling cap to a BJT buffer/phase-splitter node with pretty high impedance. The simulator shows the tank tap is cleaner than the signal at the emitter, and I was trying to keep things pure and linear as possible to avoid later filtering (note it's a VCO). I'll keep that in mind at higher frequencies though, perhaps switch to a JFET buffer/splitter.> Also not related to the present problem, I think you could probably get > rid of C6 and R7 - the DC voltage on each side of C6 is the same, > assuming the varactors stay reverse biased.Ahh, good point. I cargo-culted that from a circuit where the tank was hot. Thanks for your thoughts. Clifford Heath.
Reply by ●April 4, 20142014-04-04
On 05/04/14 02:45, RobertMacy wrote:> On Fri, 04 Apr 2014 01:28:23 -0700, Clifford Heath <no.spam@please.net> > wrote: > >>> ...snip, way too much .... > > Alex Bordordunov [incredibly prolific producer of effective models and > able electronic designer!] just sent me this model:Fantastic, I'll try it later today.> "Hi Robert, > I have found the more neat (exact) model a model transistor CA3046. > .MODEL CA3046 NPN(IS=10f BF=145.76 VAF=100 IKF=46.747m ISE=114.23f > NE=1.483 BR=100.1m VAR=100 IKR=10.01m ISC=10f RC=10 CJE=1.026p > MJE=333.33m CJC=991.79f MJC=333.33m TF=277.09p XTF=309.38 VTF=16.364 > ITF=1.7597 TR=10n CJS=6.3P VJS=0.749 MJS=0.5 VCEO=20 ICRATING=50m MFG=RCA) > To LTspice used the additional parameters to models (CJS, VJS, MJS) > necessary to use the symbol NPN4. > > Alex." > He has over 35MB of models, which he posted to the LTspice group. > I am trying to put the two of you in contact. send me your email > address.firstname.lastnmae@gmail.com>Meanwhile, try his model and let us know if it acts much > differently.Thanks, I will do.> Don't be offended, ...wrap the lines properly.Do you mean "let the reader wrap the lines"? Yes, ok, a lot of people read on displays narrower than 80 columns now. Really? I have a 40-year habit of wrapping text, which I'm trying to break... bear with me :) Clifford Heath.
Reply by ●April 4, 20142014-04-04
On 04/04/14 22:49, Chris Jones wrote:> On 04/04/2014 14:03, Clifford Heath wrote: >> 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? > [snip] > I would suggest estimating the length of some of the more critical > physical wires on the hardware version, and putting appropriate > parasitics back into the simulation model. Particularly in series with > the emitter of transistors, it can affect gain at RF. I would suggest > modelling the emitter pin and leadframe and the wiring as 1nH per > millimetre of length, as a first guess, plus another nanohenry for the > bondwire (unless it is already in your transistor model). Whilst this > probably isn't the cause, it might start to expain something.The whole oscillator fits in 1cmx2cm - leads are really short. I'd be surprised if parasitics are a problem as low as 30MHz. Fair enough to worry at UHF, but even at 150MHz I don't think my layout is bad. I need find out though! I think I could pretty easily switch to discretes (say MMBT2222A) which have a similar-ish Ft etc, and that could improve the layout a little. Especially by getting pin 8 (collector of the phase-splitter) away from the oscillator device.> Another thing that may be worth trying, though it isn't usually > necessary until you get to GHz frequencies: The capacitance across > unwanted junctions on chips (perhaps collector to substrate in your > case) can be lossy.What sort of capacitance would I expect between two SSOP pins? and for a 10mil trace, what capacitance through ordinary 1.6mm FR4? If you model it as a pure capacitor then you can> simulate better Q or gain or noise figure than reality. Of course if you > add an infinite value resistor in series with the parasitic capacitances > then that also gives zero losses and artificially good performance. > There is some intermediate value of resistance to put in the model in > series with the parasitic capacitance, between zero and infinity Ohms, > that will result in the worst possible losses. There is some other value > of series resistance that is the best model of reality. Often the most > representative value is close to the worst case losses value, except > when you don't want things to oscillate in which case it is very different. > > It is important to put some losses in the model of your inductor also, > as another poster mentioned. Beware of metal very close to the inductor. > I sometimes like to put inductors in screened cans, but all metalwork > should ideally be spaced away from the turns of the inductor by at least > the inductor diameter, to avoid causing more losses.I planned to box the whole oscillator, but it's hard to develop that way :P.> Hopefully you have some low-ESR decoupling (e.g. 100n chip ceramic) > capacitor(s) across your supply right near the oscillator. Otherwise the > impedance of the supply wiring might stop things from working, in a way > that would not be simulated.Yes, the collector of both oscillator and AGC transistor have 0805 caps directly to a ground via. I'm using caps from a cheap Chinese SMD book... I hope they're ok.> I suggest you get one of those label machines, and affix a label to your > prototype that says "unconditionally stable amplifier". It will surely > oscillate then.:)> Are you able to verify the DC collector current and Vce of your > not-oscillating transistor? I would check that the collector current is > close to (but slightly lower than) the value that gives the peak Ft > value.I don't even know what current that is. Will check the data sheet.>(I have been warned that above the current that gives peak Ft, > models are often not very good and/or device performance is more > unpredictable, and to therefore stay somewhat below that current if good > Ft is important). Also make sure the Vce is well over a volt,Ahhh, ok, it's just over a volt - about 1.7v I think. Supply is only 3.2V. and more> if the transistor has a poorly contacted collector with lots of > distributed resistance (which could cause parts of the device to > saturate before other parts, and wouldn't usually be modelled). > > For oscillators, I like to pick a transistor with an Ft that is about 10 > times higher than the highest desired oscillation frequency. Something > like an old BFY90, in your case, or one of the many surface mount RF > transistors that you can get these days.Thought I had some BFR93A's here and wanted to splice one in to the existing layout (cut pins and hot-wire the SOT23 on top of the LM3046), but I can't find them. Maybe too fast, but still...>Actually I prefer FET LC > oscillators but none of this is really relevant to the simulation problem.Well, I'll experiment with BF862's for VHF work. Not sure how well that will go with a 3.2V supply (Vgs is a bit unpredictable). This existing design is useful even if I can't get it to run at VHF, but I really want to learn what I've done wrong anyhow. -- Clifford Heath
