Electronics > RF, Microwave, Ham Radio

Why is this AM TX circuit so different than the sim ?

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MathWizard:
Well what I built was not quite powerful enough, not for how I was adjusting it anyways. So this time I chose 20mWdc in the output resistor. Then worked back and tried to match the power between output stages better. I should do it more for the rest too...but I'll have to do it all again for considering time constants. It's hard enough getting the DC to match the AC with out caps. And in the end, I still just went with closest common values to my calc's, for the most part.

And now it's working better than ever, and without even needing some bypass and feedback caps like yesterday, today the signal output is way way cleaner.

 The audio is not bad, a bit muffled right where the noise is the least when tuning.

I added a 100k pot in 3 places, to tune the RF, AF and overall mixer output levels, into the overpowered amp stage. It needs more fine tuning, but until I make in on a copper clad board, it's great.

Never mind that the mix levels in the schematic, but those are all the parts/values


If any ones tries it, my BJT tester said my BC547B had a beta of 431. I measured 330, with my DMM, at ~10mA =Ic
Then used Is=1.96fA, and Va=112V of a BC547B, from LTspice, and found that if I used n=0.848850, it matches my VBE and IB pretty well. But in LTSPice, a BC547C seems to match the current of my real BC547B's

I need to make a real BJT tester jig, DC and AC, I never tried to measure their AC properties closely anyways.
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MathWizard:
Yeah that was the wrong version I attached yesterday, here's the working schematic, and my layout. I built almost all of it on a proto-board, then realized I forget 2 resistors and a trimmer, so I had to squeeze them in. IDK how bad my layout is. I tried to keep the audio on 1 side, the osc. and the other side, the output above that, and the mixer in the middle.

Now to let my proto-board dry and see if it works. Maybe add a few 100n caps, I forgot them. And before I try making another better mini-TX, I'll try designing a receiver.

MathWizard:
Well I did make the tanks wrong by accident like that. I know they aren't like that at AC. But the added tank right on the emitter side of the osc, was working for ages on the breadboard with all sorts of cap combo's. It also works pretty well in LTSpice.

But on the proto-board, it didn't want to work. So after a day wasted, I removed that. And still, compared to the breadboard, I was having a lot harder time getting a stable, usable oscillation, with many cap values I tried. In the end I'm using a smaller emitter resistor.

I made a spreadsheet calculator, for doing all the phasor math. And did find the ABCD params, or the amp and the tank. And with common values, my gain of the tank w/ 682R output load, is still 9 and under 1deg phase.

And the amp's gain is now closer to 98%. But still, I'm having a hard time finding sweet spot without getting pops and wobbles on the radio station.

I haven't tried a model of BJT w/ capacitance. I'm guessing the Miller effect of a CC amp does not magnify the Cbe. And I read it's just a few pF and the external Ce to gnd should just be way bigger.

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