Ok I have to learn up on that stuff too. I just got a Signals and Systems textbook too, there's a bunch of basics in their I should do ASAP.

Yeah after measuring the beta of some BJT's I was using, no wonder the sim was so different, and now with all that measured and some higher Beta BJT's, it's looking good.

I worked out the equations for the ABCD parameters of the mic-CE amp. And for the diff.pair+tail current BJT, as a whole, or maybe just the A,C of that.

And today I made up a common collector Colpitt's oscillator, and worked out it's ABCD parameters in the s-domain, of the BJT part, and then the feedback tank. Without choosing C's. But I never worked out the total yet, huge equations already just for that. But I also did the ABCD-param's of the whole C.P. osc, if the loop was open. If I can pull off closing the loop, with those eqn's, I'll be happy.

IDK if I can get all that to really work right yet. Or working out the inverse Laplace transforms, but I can do the 2ndO.D.E's for the LC part, but I never tried with a BJT added in before. I was reducing the equations down to a few core constants and RE and IE and beta. Hoping to see what makes for a good or bad circuit. I just chose an A-param of the amp, RE, the BJT, Rbb, and it all seemed good enough, for a lot of C1/C2.

For instance, A of the CC amp is A = 1 + beta*Vt/((beta+1 )*Va) +Vt/(RE*IE)

Mine is A=1.02450V/V on paper, and very close if I make the same AC model in LTSpcie, and pretty close to the BJT, and the real circuit.

But anyways the CP osc. circuit works no problem on the breadboard, with a wide range of C1 and C2 combo's. In LTspice, it works with C1/C2 or C2/C1, so I'm not sure yet why that is from the eqn's. Once I settle on a better frequency, I'll trying to decrease the gain by splitting the emitter resistor, and make a nicer sine wave with less current spikes from C1 and less vbe I guess, flattening out the osc. when it's over powered.

With it applied to the mixer circuit and it's LC tank, I get some extra noise frequencies that make it fuzzy and look worse. Higher and lower freq's. IDK how much 60Hz get into all this as well, maybe that's my fault.

I haven't measured my LC parts on the LCR meter yet, nothing has been at the calculated freq's for either tank circuit alone, on the breadboard. Soon I'll solder up some of it.

Here's osc' output, w/ just a 2.2k load. It's a bit fuzzy, and tilted to the right. I barely played with the cap values on the BB. Here's basically the circuit in LTspice, but on the BB it more like 2MHz, not 1.4MHz. Once I measure the parts, then I'd worry about trying to calculate the difference. When I make it on a copper clad board, I hope it's a lot closer to basic calculation.

And then the yellow osc. connected to the mixer and other LC tank and 2 other BJT stages, the red trace.

See all the higher frequency ringing.

Ok the osc will work fine at about the correct freq, it was the other tank. So now matching it better to the 1st, and just passing the osc signal through a 15p bypass cap, the mixer/tank output looks a lot better.