Technical Topics > The RF Workbench
Hellschreiber beacon - with some oomph!
Radiotech:
I changed the filter to a Buttersworth of the 7.th order, and it made the wave form look a bit nicer. Found the calculator for it here https://rf-tools.com/lc-filter/
At 13,8v i get approx 2W of RF out, and its farly stable. At 24v however, it starts at approx 4 watts, and during about 3 seconds it increases to 10,5W where the supply goes into current limitation and power stabalises. Is this some kind of thermal run away?
The oscillator drive voltage doesnt increase, its stable. I am driving the IRF510 with a high VPP, so the bias doesnt really do anything, it doesnt affect RF output at least.
Edit: I read that the IRF510 should never have more than 8vpp on the gate, and i have double that. This might be the problem, i will try to power down the oscillator.
Edit 2: Problem percists, even at 7vpp on the gate, power is surprisingly low and unstable at higher voltages. Too high bias and the power increases until current limitation cuts in. Too low, and it doesnt give any output. 1w at 13.8v is a bit low, it should be able to prove higher output.
Stretchyman:
The ripples on your O/P waveform are due to the lengthy earth lead of your 'scope probe.
Please use the class E amp from the same article, far better efficiency and no need for bias either.
So why use class C I wonder?
I'd forget the IRF510. I've never found any good ones, far better devices around now. Try the 520 if your happy with old devices, usually works well.
Oh the filter is overkill, no harm in that but a single inductor and a pair of caps is fine for this power level at attn is approx 100 times (20dB).
Str.
Radiotech:
I have played around some with the cuircuit and discovered that it was the bifilar transformer that made the output power non-stable. Once it was removed(I put a T80-2 with 19 turns) the circuit was very stable, no increasing/decreasing over time. Also made the filter simpler with only one inductor and two capacitors, output waveform still looks nice.
I now get the following power:
30v supply = 4,50W RF output(400mA), efficiency = 38%
25v supply = 3,25W RF output(360mA), efficiency = 36%
20v supply = 2,25W RF output(300mA), efficiency = 37%
15v supply = 1,45W RF output(270mA), efficiency = 35%
12v supply = 0,85W RF output(250mA), efficiency = 28%
After removing some components, there is more room on the board. 4,5W is nice, but it can be a bit tricky to supply when you havent got a variable powersupply at hand.
I will have a look at alternatives to IRF510, that doesnt require as high voltage, it seams to be the most common HF amp on diy projects, cant find many examples with other ones.
Radiotech:
I found a couple of usefull pages on how to build an E-class amplifier, and gave it a try. With 12v i get about 1.5W out, so its usefull, but not as high output as i expected.
http://www.wa0itp.com/class%20e%20design.html
Edit:
Also tried this circuit from https://www.researchgate.net/publication/320623200_Notes_on_designing_Class-E_RF_power_amplifiers But i got no usable power from it.
This Class-E stuff is clearly more complex than i expected.
Charlie_Dont_Surf:
--- Quote from: Radiotech on February 15, 2022, 1347 UTC ---Also tried this circuit from https://www.researchgate.net/publication/320623200_Notes_on_designing_Class-E_RF_power_amplifiers But i got no usable power from it.
--- End quote ---
My biggest problem with this paper and this circuit is that to specify a Class-E output network as he has without specifying a transistor is somewhat misleading; it fools the user into thinking that they can plug any old transistor (even a MOSFET) into it and expect it to work. Maybe this would work at 50 KHz with old power MOSFET transistors (which tend to have humongous output capacitance that swamps any other performance differences between models of transistors) but not at 6.9 MHz (as in the paper). The reality is more complicated and the component values of a Class-E network absolutely have to be tuned and optimized for each transistor at each frequency.
Also, it is humorous to me that this author does mention that he performed SPICE simulation but again without specifying what transistor was used and without providing any actual measured results to go with it. Had he substituted another transistor into his SPICE model, he would have seen exactly what I wrote above - the matching network has to be tuned and optimized for a particular model of transistor, which would make it obvious that specifying a network needs to be accompanied by naming the actual transistor used.
This paper is one of these garbage undergrad papers that people put out to pad their resume/CV; there's nothing new in there and it's lacking aspects that would make it useful to others. He references Sokol's book on Class E but then goes through a bunch of derivations that Sokol already did in his QEX article from 6 years before the book. I don't see the point.
I'm sorry that you wasted your time expecting this kid's navel-gazing project to work. That's a few hours of your life that you won't get back.
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