The AM Transmitter on a Printed Circuit Board! - AM2PC pt. 7

In AM2PC part 7, I make a further attempt at building an AM transmitter design sent in by Awais Mushtaq. In conjunction with PCBWay, I solder it all together on a printed circuit board. While there are still some issues in this revision, there are some improvements.
Thanks again to PCBWay for providing a wonderful circuit board for this project!
Check them out: www.pcbway.com/
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[@daisywong-ke1kz]

Really glad to be a part of this great work. :D

[@awaismushtaq5719]

Greetings sir. I'm glad you pointed out number of short comings of my previous TX circuit. Audio amplifier design was good but problem was with that laptop jack output increased level of audio which disturbed biasing voltages and distorted the output. I got rid of it and used TI's LM386 WITH 10 k-ohm Potentiometer at the input to control audio levels. Also, please, use a smart phone in place of laptop as it's output impedance is better matched with audio reproduction circuits. Second problem, the oscillator, I replaced it with Colpitts crystal oscillator which outputs 200 mV enough to drive modulator. I shall still suggest to use an emitter follower for isolation purposes so modulator stage doesn't load the oscillator directly. That way the frequency won't shift. I re-calculated the modulator circuit values for 100 mW of power and wound a T-50-2 type toroid with 23 SWG enameled wire equivalent to 22 AWG; 28 turns on primary and 9 on secondary gave me what I wanted. Needed load at modulator stage is 385 ohms and with 9 secondary turns on toroid, I matched it to 50 ohm coax transmission line/cable. I also designed a Band-Pass filter for it that works as it should. If you agree, I shall send you the improved design that still is targeted for 7.2 MHz. I have built it and almost got caught by FAB (frequency allocation board) our equivalent of American FCC. Just say it, if you need it. Nice video BTW. O and the N-channel JFET is cursed. Also, if I may make a humble suggestion, try using an EMI filter on AC mains to your PSU. With RF work, it's an utmost necessity. It does excellent work of removing RF remnants from power rails. If you agree sir, make the changes and it should work. My apologies that I got your money wasted. Let me know, if I can repay. Regards!

[@rcat256]

Thanks again for some more advice! I would like to stay with the simple nature of the current transmitter design, but I would not mind taking a look at your newer design (send via email again). I've been trying to build my own band pass filters for this and other transmitter circuits, but I have trouble figuring it out. In the setup in this video, I used an iPod nano as an audio source. I have used laptops and cellphones during other testing, but most of the time, I use the iPod. As for a power supply, I usually use a 9v battery. This time around, I got a hold of a 12v battery that gives it a bit more power. My circuit was completely isolated from mains (even the portable oscilloscope was battery powered), although I did not apply any shielding this time. Interference has not been a big issue for me, but I keep it in mind. I have gotten a hold of some T50-2 toroids as I plan to use them for impedance matching later on. I still want to make a ham radio enabled version of the circuit in the future. I think for now, I will use my usual oscillator in later circuits because I cannot seem to get the JFET version to work. As I said in the video, I built it many times and I have used several different 2N4117's with no luck. Thanks again and best wishes!

[@awaismushtaq5719]

@@rcat256 I'm sure you'll get it working soon enough, sir. A simple suggestion on audio amplifier part, the previous audio stage of mine was to be fed from a mic, an electret capsule to be exact; that vital part I forgot about. Please, keep in mind no single amplifier stage can get you the gain and power you need. You'll have to use cascaded stages, two at least. Excellent idea using PP3 and other PP series batteries. They surely solve the clean power rail issue. I use a PSU because at 100 mW power, PP-3 gets depleted fast. Also, LM386 consumes quite a lot of power so PP-3 life is shortened even further. Previous circuit was designed for 50 mW and for that PP-3 did well. I used LM386 when I realised my mistake that a pre-amp for mic won't do at all with then prevalent circuit conditions and that I needed multi-stage amplifier circuit. Instead of re-inventing the wheel, I used the existing low power LM386 which served my purpose excellently. If I may make a humble suggestion, you can drive modulator with LM386 without having to use direct power input. You see that LM386 internal circuitry reduces the applied input by a half; means 9 volts shall be halved to 4.5 volts, sufficiently high to drive modulator (with a choke of 10 uH at amp output pin to block RF getting in) stage and still give you up to 200 mW of RF power. Choice is yours, sir. I'll send you the new circuit in digitised form soon. Stay safe!

[@Stefan0719]

Analyzing the circuit a bit. It might work, but some of the components don´t make much sense. The audio amplifier provides (based on it´s values) to less audio level to get a "full" modulation out of the transmitter. It would work much better, if "R6" would not go to ground, instead it would go directly to the collector of "Q1" "R4" removed, and the bias re-aligned. Instead of the voltage divider R1 / R2 you could just use the 91k and connect it from the base to collector. Also connect Q1´s Emitter directly to ground. That would form a proper AM modulator, and would create a good and loud modulation. For shortwave, "C10" can be reduced to .022 instead of .1. That would improve modulation quality a bit. The Resistor in series with the coil... has what... exact... purpose? Talking about the 22µH coil - at least the value of the coil is "okay-ish", if you want a high impedance output (Which is okay if a short antenna is used). The oscillator could use a safety resisto, bc as it is now, the fet is directly between + / - so, if sth goes wrong, it´ll go "pop". Also there´s no antenna matching, but for the said distance "up to 70 yards", it will do without matching. The transmitter, as is, is most likely going to work... but I doubt the modulation level it can achieve is anywhere near a "makes sense" useable level. How should the modulator drive the "pA" if it´s current limited by high value resistors opposed to low value resistors in the final amp? If such a methode would "supposed to be used" the modulator would require far lower resistor(s) to provide enough Audio signal to the final amp. You said you do co- work building transmitters...? I can make you a very-very- simple 2 transistor am transmitter / shortwave transmitter, that has >70yards range, and far-far-far less components than this circuit... ... With more than 20 years of RF device (build receivers, transmitters) experience I just *had* to leave a comment here. Good luck getting the current circuit to work tho (not meant sarcastic)... I may check here for the results ... eventually.

[@hobbyrob313]

I've been watching the videos you made about AM transmitters and it looks like your receiver isn't very good either! if I do/make the same thing then everything I do is better, the signal and the sound? do a Google search for grenade transmitter and look at images? pretty much the same oscillator and it works perfectly for me! (I'll keep an eye on you!) Next video I expect nice sound ;-) Healthy and Friendly Greetings from the Netherlands! RadioRob (MW, SW and WFM Piraat!)