I was reading the other day how you can use two transmitters coupled together on the same frequency, one out of phase with the other and one of them phase modulated to magically create AM w/o the need of a huge high level modulator. So if I read this correctly you need no real modulator no matter what the power output of the transmitter, all you need is two signals of the same power being one out of phase and combined with the other and phase modulated (kind of like FM modulation) to create really good AM. Has anyone tried building a hobby transmitter or even ham transmitter using this method? I was thinking maybe use two class E finals and trying this modulating method. Since the modulation would easily get to 100% and from reports I have read of actual AM stations trying this the audio sounds better than good compared to other modulation methods for AM. Anyone have any info on this? Maybe take two mosfet PA stages and just rig one to be out of phase with the other and phase modulate it? I guess the phase relationship has to be just right and the need of a good dual scope would be needed.
i think radio disney AM does this in my home town. they use a digital type TX. it sounds like ass. a phone call sounds better.
Probably their audio processing. Radio Disney is notorious for cranking the bass and compressing the shit out of their audio. It would sound like shit through the studio probably before it even got to the transmitter
Phase modulated AM (Ampliphase) should technically sound as good as full 100% modulated AM using standard methods. I'm sure if you played enough with PM-AM giving it a basic sinewave from 20hz to 20khz it could produce a perfect RF output on a scope. The science is there and should provide exactly as it would on paper.
The pirate Ross Revenge did this and has a great webpage on their ship and transmitter used with lots of pictures and information which is what originally got me interested... www.rossrevenge.co.uk/tx/txroom.htm
The cool thing about ampliphase is no need of a high powered modulator. You can modulate very low level much like with FM radio broadcasting.
I just thought it was neat to read into. Always love learning new stuff about radio technology.
Next time I build an IRF510 transmitter I may just experiment with this. The more I think about it the more it sounds highly practical. Just build like the grenade only using two IRF510s. One being completely out of phase with the other right from the gate input (but balanced enough to reach 50% of the total RF output power). Mix both IRF510s outputs using a basic toroidal. Then run the oscillator so that it has two outputs, one direct to the first IRF, and the other going through a phase modulated buffer into the other IRF. If all is tuned well it should allow 100% modulation at a low level and because of no need to series modulate a single IRF mosfet output you'd get the full VCC voltage creating RF rather than half like in many peoples circuits. It could be incredibly efficient on energy usage. So if all goes as planned in my head running it off of a car battery you could get around 13.8volts^2*52ohms output=3.7 watts PEP. If you wired it to have a flyback output network effect you could get up to around 5 watts carrier off of car battery juice. Of course the limits are the IRF mosfet type. Using 24 volts or more could yield much more RF output all at a perfect 100% modulated and hifi and no need for a modulator. Just a random idea.
There's a "magic" point for the no-mod condition of an "Ampliphase" rig - the carriers are set to be 135° apart, and then they modulate in opposing directions to get phase addition and cancellation at the output bridge. My pseudo-Ampliphase circuit uses a CMOS PLL to generate a carrier at 64 times the output frequency. I then use a ring counter to divide by eight, taking signals from the first and fifth outputs of the divider.... These are inherently 135° apart.... These signals then provide references for a further pair of PLL circuits, which have very "light" control loops, allowing their frequency modulation. The PLL outputs are further divided by 8 and the resulting carriers feed Class D amplifiers. The Class Ds meet in an output bridge - inductors to the aerial and capacitors to ground. The "aerial" feed goes through a 7-pole filter to keep things clean, and I have gorgeous AM fully modulated by just 1.55V of audio! I've built several of these rigs, at varying powers from 20W carrier / 80W peak up to 2.5kW carrier / 10 kW peak. Several of them are in use as local stations in African cities.
Incidentally, "Ampliphase" was an RCA creation (based on the Cheirix patents from the '30s), and proved that "RCA" stands for "Requires Constant Attention"! I've worked on their 5kW, 10kW and 50kW versions (on ships, rocking around the North Sea!) and they were horrible to maintain.
The principle is brilliant, and with an ideal site, with an ideal, matched load (good antenna and good earth) they can work well. They're very power efficient (when compared to the 25% - at best - that you can get from Heising modulation, and (at best) 55 - 60% you get from screen-grid modulation), with my all Solid-State, FET-output rigs (93% efficient with two Class D PAs), but the load has to be perfect. Remember - the modulation actually occurs at the output bridge, where the two PAs combine. If the match is just fractionally off, the modulation really suffers!
My latest design uses pseudo-sinewave carriers digitally derived from a frequency synthesiser, and modulates by means of a pair of high-speed comparators driven with antiphase carriers and anti-phase audio. The two resulting squarewave carriers, effectively phase modulated, are amplified at very high efficiency by Class D amplifiers, and the output bridge is somewhat simplified, making the output match much less critical. There is envelope feedback - demodulating the final audio to apply to the incoming audio as negative feedback - which makes the audio sound perfect, and we're getting >96% efficiency at 250 Watts carrier / 1 kW peak! This looks like we might - finally - have got it exactly right! It's only taken a quarter of a century to get to this point......
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