HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: redhat on November 18, 2019, 1121 UTC
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This is something I threw together in about 5 hours. To be honest, I'm kinda surprised it works as well as it does. It uses a class D audio amplifier IC as the modulator, and a conventional current mode class D output stage. The output transformer serves double duty as the tank resonator. It is currently on 1720 KHz as thats the oscillator I had handy. PA fets are IRF530's, drivers are TC4452's, and the modulator is a TI TPA3122. Phase splitter is a 74LS86. More circuit details to follow soon.
(https://i.imgur.com/ozVABrz.jpg)
It is pretty ugly, but this was meant as a 'proof of concept' more than anything else.
+-RH
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Looks interesting! 1720 is a good frequency choice as 1710 is blocked by several TIS stations here in the east.
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It's...... beautiful!
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Nice! 8) But it would take me 5 hours to wind the toroids.
I wonder if you could mod one of these:
https://www.ebay.com/itm/Low-Voltage-ZVS-Induction-Heating-Power-Supply-Module-Heater-Coil-Tool-5V-12V/293206723034?epid=3023800338&hash=item44447b89da%3Ag%3A7C8AAOSw%7EcJdaN8U&LH_BIN=1
~
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It is indeed a beautiful display of homebrewingness. Let us know when you have it on the air.
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I don't have any (real) intent of that as this time. I built it mainly to have something to test my C-QUAM exciters with, and use as an RF drive source for repairing AM transmitter modules. Still, I do have a firebreathing box in the garage on 1750 KHz...
+-RH
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Medium wave is so easy to get on with minimal parts these days yet somehow I still over complicate my designs because I end up working around bin parts I happen to have on hand instead of purchasing what I need. Still happy with my 25 watt AM TX though that is class E utilizing a single IRF640 and the commonly found 3 chip PLL frequency generator circuit. That FET requires quite a bit more drive oomph though so I ended up using a totem pole bipolar circuit ahead of it, also didn't have proper fet driver chips on hand.
Then I went and got weird and modulated it with series class A instead of D and of course the heat sink for the modulator is the size of the rest of the transmitter including the PA that runs cool on a tiny slab lol.
Oh well, it gets the job done.
Your design is nice and clean, definitely a pro level build. Too bad I didn't see this a year ago when I started work on building my TX (http://darkliferadio.proboards.com/thread/1018/class-series-modulated-transmitter-build).
PS: I was wondering what I was going to use my set of 4 T130-2 cores for that didn't work out the way I intended for an 11m EFHW antenna, now you're making me want to fool around with another transmitter build or maybe linear PA for something to use them in :P
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This is a great thread, hooked me in right away.
Ok yes Kage, good stuff on your TX. The IRF640 is quite a forgiving Fet! I drive it with the old LuLu fave, 8 volts into the 74HC240 with a good 40 watts out @ 24 volts to the PA. Source for the 74HC240 is the usual DDS module, AD9850.
Ok yes, my modulator uses Class A also, which heats a small room LOL... but it sounds great! The audio driver is the LM 3886..its the biz. Working on the PWM version right now which will be on the air soon. Have worked it on MW and SW with great results.
Redhat..that is a cool layout man..love it. Neat. :) Cheers.
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I built it mainly to have something to test my C-QUAM exciters with...
I was going to ask about C-QUAM...
This is great and it looks really, really nice.
I'm anxiously awaiting the details and if sufficient details, may try to make one. I want (need?) an AM xmtr down on MW and this would do the trick.
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This is a great thread, hooked me in right away.
Ok yes Kage, good stuff on your TX. The IRF640 is quite a forgiving Fet! I drive it with the old LuLu fave, 8 volts into the 74HC240 with a good 40 watts out @ 24 volts to the PA. Source for the 74HC240 is the usual DDS module, AD9850.
Ok yes, my modulator uses Class A also, which heats a small room LOL... but it sounds great! The audio driver is the LM 3886..its the biz. Working on the PWM version right now which will be on the air soon. Have worked it on MW and SW with great results.
Redhat..that is a cool layout man..love it. Neat. :) Cheers.
I used the LM3875 for a similar purpose. Was going to drive the whole transmitter off of the output at half rail voltage like the smaller shortwave transmitters people build do but found the chip became unstable doing so and couldn't see the load properly so it would kick into protection. Ended up using it as a driver chip instead into some beefy current transistors to hang the transmitter voltage on for modulation and that worked well. My main issue was that rail to rail voltage was somewhere up at 40+ volts (15-20vdc at carrier minus losses in transistor diode junctions) which is simply too high for most OP Amps as drivers, but the Texas Instruments LM38xx series can handle those voltages fine and act like a high powered OP Amp to buffer audio into the final modulator transistors.
I wonder how many of us who have used them for this purpose happened to get the free sample parts from TI? ;)
Definitely overkill using such a chip for a modulator considering they were designed for high end HIFI amplification, but I just happened to have a handful laying around.
Anyway, didn't mean to derail the thread.
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I have an old Harris Stereo exciter on the shelf downstairs, I honestly don't think it was modded to C-QuAM, the sticker and docs for that mod are in the manual in a plastic bag, and I've never even powered it up, due to it's loaded with tantalum capacitors. It's something I would love to make into a standalone TX, and it can go up to around 160m band with flip of some dip switches. May have to change a few parts, as this one was on 580kHz originally.
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Work continues on this little guy. I was having some problems with the audio IC, I couldn't get it to work above 15V supply, the audio would cut out on peaks. I used some junkbox cores for the filter inductors, and the IC doesnt like them. I built another modulator card with different inductors, and after a few wiring snafu's were corrected its working happily up to 31V input. The datasheet says it should work up to 36V, but TI has been know to lie before. I hooked this test modulator up to the remainder of the transmitter and all seems to be well. The finger scorch factor implies something around 20W carrier balls-out...that's all she wrote. Still, not bad for a proto build. Proper scematics coming later in the week if all goes well.
Re the Harris exciter... Most of the AM stereo exciters aside from BE's were setup for a single frequency and it was a job to move them somewhere else. That said, I've never seen the docs for the STX-1.
+-RH