Technical Topics > The RF Workbench
Hellschreiber beacon - with some oomph!
Charlie_Dont_Surf:
--- Quote from: Stretchyman on February 12, 2022, 1638 UTC ---The ripples on your O/P waveform are due to the lengthy earth lead of your 'scope probe.
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Yes, certainly good practice but also I was going to say that attention has to be paid to stray inductance in the circuit in general. It's hard to say where the problems lies since he's provided 'scope images of what appears to be different locations in the circuit and I'm not sure which is which.
redhat:
--- Quote from: Charlie_Dont_Surf on February 17, 2022, 0253 UTC ---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.
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This is the primary reason I'm sticking with CMCD. The design process is as simple as tuning the tank to resonance and going about your day. In theory you could precisely calculate the tank values taking into account the Cds and so forth, but I found it much easier to spitball it.
+-RH
Charlie_Dont_Surf:
--- Quote from: redhat on February 17, 2022, 0727 UTC ---The design process is as simple as tuning the tank to resonance and going about your day. In theory you could precisely calculate the tank values taking into account the Cds and so forth, but I found it much easier to spitball it.
--- End quote ---
OK, I'll take your word for it.
For what it is worth, I haven't built any yet but I have done some simulations with a few different power transistors in CMCD configurations for 43 meters and the optimized results have ended up with very different tank circuits; the same network (parallel RLC between the drains) but with very different component values depending upon the transistor.
Now, to be clear, my design process in the simulator is to do a bunch of sweeps of the RLC values to pick values that basically work then I put the optimizer to work to peak up the output power and efficiency, minimize power dissipation, etc. From there I usually end up tweaking it for one reason or another. The reason I mention this is because the optimizer can spend minutes trying to squeeze out every last milliwatt and that can move the design to a very different place. (I'm not watching everything that goes on at this stage - I'm usually asleep or doing something else while it does the drudgery for me.)
The end effect of this is that if you don't care about the difference between 120 and 125 Watts (picking numbers out of the air) then, yeah, it's probably fine to just swag at it. For better or worse and potentially overdoing it, the optimizer sweats the small details for me and that may be why I end up with very different tank circuits for different transistors at the same frequency.
What I do find interesting about CMCD is it doesn't have the high voltage peak on the drain at resonance like Class E; it's a current-operated mode (duh) and that seemingly permits the use of a lot of dirt cheap power MOSFETs with 100 V or lower BVdss that are plentiful with power inverters, switching supplies and motor control everywhere now.
Radiotech:
Finaly, i made some progress. I get approx 2W at 12v and 8W at 24v.
The oscillator puts 17Vpp into the IRF510, and the bias is adjusted to 2,1Volts. The DC bias at 2,1v is a sweetspot, output quickly drops of both before and after. The drive voltage is 17VPP, Efficiency is about 27-28%
One design i found online had a capacitor from the drain to ground. I just merged the two together, i have no idea what i am doing. My guess is that its operating in class C mode, due to the low efficiency.
Chanel 1(yellow) is voltage into 50ohm dummy load, Chanel 2(purple) is Gate voltage on IRF510.
Charlie_Dont_Surf:
For the original circuit, with an IRF510 and 7V gate drive, you could modify the existing circuit in the paper to get a bit more out of it. Since you said you got nothing before, maybe you can get something now.
Change the 139 pF to 124 pF (the exact value matters quite a lot here - a few pF either way makes a large difference.)
Change the 652 pF to 740 pF.
The RFC must be 1 uH or more, I would use 2 uH.
I continued with the 4.81 uH for the above simply because modifying it is more of a pain than changing a capacitor. If we pick better inductor and capacitor combination, the other two capacitors may have to change to suit.
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