Technical Topics > The RF Workbench
Hellschreiber beacon - with some oomph!
Charlie_Dont_Surf:
--- Quote from: Stretchyman on February 22, 2022, 0946 UTC ---I started by simulating in LTSpice whilst working for a 3G base station design company in 2005. I had the best engineers in the world to help, scientists too!
Initially started with 160m as the low frequencies mean you can use virtually any FET and wow couldn't believe how simple LTSpice was to use (still free!) and then when construction of the circuit was complete how close the simulation was to the real world.
Of course such a simple program has its shortfalls but you can see waveforms change shape when adjusting circuit values and hence get a 'feel' for what's going on.
--- End quote ---
Of course, the above is you agreeing with me without realizing it, as is usual. ::)
And you know, earlier in this thread, while poo-pooing simulation, you just had to go drag out a simulation (at the wrong frequency, BTW) to "demonstrate" a point. (https://www.hfunderground.com/board/index.php/topic,92791.msg297415.html#msg297415) You know, because simulation doesn't matter, right? ::)
--- Quote from: Stretchyman on February 22, 2022, 0946 UTC ---Like RH I've done all the simulations I need to do and actually know that the 'idealised' values don't matter at all. The circuit values are published and the circuit works well with off the shelf (not junk box!) modern components.
--- End quote ---
Disagree that they don't matter. That you can't figure out that the idealized values do matter - especially based upon what you wrote in the paragraph quoted above - tells us everything we need to know.
Think about what you are saying before posting. If the idealized values had no bearing then the circuit would be dominated by, or entirely composed of, parasitics. Does that seem right to you? You would be tuning your LC networks by "feeling your way in the dark", stumbling around, with no predictability. I don't know about you but mine aren't that bad.
I'm done with you on this topic. There's no logic coming from you at all, with contradictions up the wazoo. As is typical, you are trying to pretend to be relevant and knowledgeable and you just end up making yourself look foolish to anyone paying attention.
Charlie_Dont_Surf:
--- Quote from: Radiotech on February 26, 2022, 1058 UTC ---I got rid of the Arduino Micro and got a Attiny45 to do the same job, programmed it with the Tiny AVR Programmer(using Arduino language).
--- End quote ---
I assume that this is for the outgoing message, correct?
Radiotech:
--- Quote from: Charlie_Dont_Surf on February 27, 2022, 0034 UTC ---
--- Quote from: Radiotech on February 26, 2022, 1058 UTC ---I got rid of the Arduino Micro and got a Attiny45 to do the same job, programmed it with the Tiny AVR Programmer(using Arduino language).
--- End quote ---
I assume that this is for the outgoing message, correct?
--- End quote ---
Yes, the Attiny45 carries the code for keying the transmitter and sending the Hellschreiber message. The Attiny works well, despite beeing in close proximity with the 10W transmitter.
Radiotech:
I gave the LULU rf a try, the square wave output from the 74HC240 worked very well, but the output was behaving the same way as before. Output slowly rising for 1-2s and after that it looked like a mess on the oscilloscope. This was the same problem i had with my E class experiments last time.
Perhaps its my 50v ceramic capacitors that doesnt hold upp?
Charlie_Dont_Surf:
Hi. I have a few things to say.
I assume that the first o-scope image is the gate drive. That's actually pretty good. The remaining ringing is possibly from your o-scope ground but also the 74HC240 package (DIP or SMD) was not designed to be a low inductance package and not designed to operate well at ~7 MHz. (Short version of the story: the limitation is chip internal layout and the wirebonding inside the DIP package. They were not designed for 7 MHz and not designed to be a massive-parallel FET driver.) The frequency indication in the upper right is 6 MHz. I thought you were trying to use 7 MHz? This frequency indication should be very accurate if the trigger is working well.
The drift with time is undoubtedly due to self-heating thermal drift. While you have a nice heatsink on what I assume is the final transistor, there will always be some thermal drift regardless. Do you have heatsink grease or a thermal pad between the back of the transistor and the heatsink?
You should also understand that the HC240 is being asked to deliver (what is for it) a lot of current at high frequency. The Icc/Idd will increase quite a lot just simply because of the frequency involved then to deliver the drive current to the transistor will also create a lot of heat. Most of the heat in this particular internal chip layout will come out of the bottom (because that is where the leadframe, which is what the wirebonds attach to inside, is located) but you have the bottom of the chip pointed up in the air ("dead bug"). (Yes, heat rises but the plastic-to-air thermal interface is never very good at transmitting heat.) You might be able to slightly improve things by trying to put some sort of heat sink or heat spreader on the bottom of the HC240.
There may also be other sources of thermal drift that I am not thinking of right now.
The other thing to note is that the messy o-scope image (the second one) probably has a lot to do with the trigger level you have set. Try moving it lower and experiment to find what gives the clearest result. If that is the output into a 50 Ohm load, it should be a reasonably nice sinewave, once the trigger level is adjusted correctly. Also, check the grounding of your probe. If that is the drain of the transistor, it should not look like that at all.
If you want to know more about the limitations of the LULU, you might want to see this: https://www.hfunderground.com/board/index.php/topic,89881.msg288345.html#msg288345
(I did see some thermal drift on my circuit as well. I just did not write about it.)
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