Electronics > RF, Microwave, Ham Radio

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

(1/4) > >>

On a breadboard, I made the circuit below, with a ~30cm wire attached.

For now just using a signal gen, I used 50mVpp RF, and could apply 1.2Vpp AF, to get a fully modulated signal out.

I had about 30mVpp AM output, about 600mVpp with an added CE amp, and the radio can pick it up ~15ft away, in the kitchen where I want it. The real circuit works best at 1.3MHz, but the radio is better set at 1.15MHz.

I used BC547B's with a measured gain ~205. In LTSpice, using BC547B w/300beta, or 2N2222's with only 200Beta, and similar Is, the circuit can't even take 400mVpp AF, without looking bad, and it doesn't look well mixed to start with.

But the real circuit looks great. So why is LTSpice so different at this, is it the real BJT's are so different from the model's, and how much difference does the breadboard make ?

When the radio tunes onto a local station, most the background noise goes away. But with my signal, the 1kHz tone volume was ok, but the noise was there at about the same level, if not a bit higher. So is the AM TX still not high enough, or is a bunch of noise get mixed into my TX output aswell ?

Using a electret mic, and a bad 2 stage amp, I could hear my computer speakers, from the radio in the kitchen, but not clearly. So now to remake that part and add some bandpass filter to the input aswell I guess. And try something besides a 1N4001.

the red is CC stage and the yellow is the emitter of the diff pair

First thing I would try is increasing the length of the aerial. 30cm is way too short. Try a couple of metres or more. The noise on the radio is almost certainly due to the low signal strenth.

I'm not sure what that 1N4001 is doing in the circuit. What happens if you replace it with a link?

I can't explain the LTSpice naff output, but what matters is the actual real world results & the modulation looks excellent.

Yes, the diode may even be harmfull there. If it is fast, it may form a peak hold circuit. L1 and C6 form a resonant circuit, what is its resonant frequency. Do you really need it. So, connect C5 to C7, and remove all parts between them. You don't need both C5 and C7.
You may get a little more power, if you use a smaller R9, but the antenna has more effect, I think.
If you are worried about harmonics and other unwanted frequencies, you can use lowpass filter, not resonant circuit like here, L1 and C6. But I don't think this is your biggest or first problem.

Ok I never tried a longer wire or higher Vcc either. I tried a Germanium 1N60, and a 1N4218, and I wasn't impressed, it look distorted, but not with the 1N4001.

I read the diode was there to only provide 1/2 wave pulses to the tank circuit, and that the inductor will create the negative 1/2 wave. I'll try without too.

Yeah the tank circuit acts as a bandpass filter, if nothing else. I had already made a Colpits osc. for the RF signal, I need to use it again, and not the sig.gen. My circuit had peak magnitude at ~1.3MHz. Later when I make a receiver, and get that working, I'd go to a higher more obscure frequency.

Yes I need C5 and C7, or else something would be shorted by that inductor

Using a single CE stage for the mic, (not the tail BJT) 15mVpp in, gives full saturation w/ 29mVpp o.c. from the tank. And if I whistle, I can easily over modulate the signal. So it is sensitive enough.

When I played a 1kHz tone on my PC, and TXed it too the radio, I can make that out. But dialog, I can't really make out at all. I think the CE stage I added on the output, makes the output look a lot more jittery.


Now I don't plan to use the mic anyways, I'm going to try with opamps, to buffer and mix the L/R and centre channels, from the stereo input, or output. I could run a wire and speaker to the kitchen too, but that's not as fun.

So at 1.15MHz, that's 260m wavelength. I have tons of wire, I'll run it along a wall.


--- Quote ---I can't explain the LTSpice naff output, but what matters is the actual real world results & the modulation looks excellent.

--- End quote ---

With respect, the modulation is awful.  You are over modulating quite a bit. to the point where the sine wave is crossing over (inverting the phase) of the carrier.

You can see it in the nearly flat tops between peaks.  See the little bump?  That's from the "minimum" value of the modulating signal. 

Back off on the modulation and you'll see an improved tone, at least.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod