Technical Topics > The RF Workbench
U-LULU (Updated LULU) for 48 meters, 20 Watt Class-E AM Transmitter
Albert H:
I've done a little more to the U-LULU. I tried a few other FETs with varying degrees of success. I've included my own design of power supply sequencing (based on a bit of simple CMOS logic). With some work, I got efficiency up to around 92% with tuning that's open enough to allow it to go anywhere from 6.25 - 6.45 MHz without much change in power output. The carrier cleanliness is also now up to commercial standards, since I decided to extend the output LPF by an extra section. Just for the sake of amusement (and with a nod towards possible automatic mod depth adjustment) I introduced envelope feedback to the modulator. The audio distortion that I previously had on the unmodified version has completely disappeared (it was previously in the order of 9% at peak mod). Measurements now show the distortion to be well below 0.2%, and I've been able to add some audio response shaping to make it sound a bit "nicer" (according to my wife!).
I also built a couple of these things for MW (1.2 - 1.4 MHz) with really good results. They run virtually cold, provide really good harmonic cleanliness, and have really good sounding modulation.
We've made a few of the early 6.3 MHz versions and they've been snapped up by our Eastern European friends (as predicted). Perhaps the English commercial constructor earlier in this thread has upset potential Eastern European clients, because we've had a lot of expressions of interest from over there - for both versions (SW and MW). The enhanced version - with automatic modulation level control (a three-band audio processor), a built-in MP3 player and a "Universal Input" SMPSU to power it up looks like it should be particularly popular, and I may well make the details available on here for everyone to use. We have also shipped a couple to South America, and received good reports.
It was quite fun to hear one of my hand-built early prototypes - being run in southern Spain - up here at home in the Benelux at New Year! That one produces ~26 W carrier / ~102 W PEP, and uses one of those "ProgRock" synthesiser modules.
We don't intend these to ever be a commercial product, but we're in touch with many clandestine SW operators, and we'll be sure to supply a few of them to interested parties.
In closing, I'd like to thank CDS for his efforts in improving the basic LULU. The original was a pretty clever idea despite its flaws, and the CDS improvements made it into a great little transmitter. There's a whole lot more that can be done to these little beasts, and - when I find the time - I'll be doing some additional design work.
Charlie_Dont_Surf:
Albert - Sounds very interesting and thank you for nudging this project along.
I'm very interested to see how you implemented the envelope feedback. I never thought to do something like that because - as far as I could tell - much of the distortion came from the Class-D amplifier. I assumed this because I was operating it at essentially max load on peaks, plus a DC load on top of it (which it was never really built for). The THD dramatically increases from ~10 W to ~100 W (max) output and I just assumed that short of implementing some sort of pre-distortion (which I really was not interested in delving into), that there was probably not much I could do about it, other than use a much bigger Class-D amplifier. Apparently I was wrong.
As for other the use of other transistors, there are a limited supply of TO-220-packaged transistors that could be used and of course it must be said that the TO-220 package is going the way of the dinosaur over the coming years. As far as I know the Nexperia PHP18NQ11T that I used in the U-LULU has not been put on NRND ("not recommended for new designs") or EOL ("end of life") status yet, the former being the precursor to the latter. The Infineon IPP530 is really good in this application too. The TO-220 version is now in EOL but they have a TO-252 version that will likely be available for quite some time. The Toshiba TK11S10N1L should work too, but it only comes in a TO-252.
--- Quote from: Charlie_Dont_Surf on July 11, 2023, 2251 UTC ---I have already made the schematic and created the PCB layout for a follow-up that is similar to what you describe:
1) frequency source of either a) a selectable crystal oscillator (with a real crystal driver chip that is designed to work at HF) or b) an external oscillator input.*
2) selectable audio preamp circuit for those that want to use laptop computer, mobile phone or MP3 player as the audio source
3) RF filtering on the input of the preamp to keep the amount of RF injection hopefully low enough
3) continue to use a Class-E output, tuned for a lower Q than the U-LULU that should be able to deliver roughly equal power output from 6200-6300 KHz,
4) probably with an output somewhere between 30 to 40 Watts, better than U-LULU, largely due to being able to run the gate drive voltage much higher than 6 Volts.
5) PCB footprint currently equal to the Sure audio module I recommend for U-LULU so that the two PCBs can be stacked one on top of the other.
--- End quote ---
Well, I did eventually get the bare PCBs back from fab but I have had to devote a lot of time to activities other than radio in the past 6 months. As a result nothing has been done to test it out.
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