20 October 2017

LT Spice and GW3UEP PA

Anyone who intends to transmit on 630m can't avoid the GW3UEP PA design. It's a simple design which can't fail to operate. Dutch authorities limited the power of our 630m transmitter. It should not exceed the 100W, you can overshoot this with 6dB, but in order to to transmit with 5W eirp as most countries have, you should have more power available.
The 100W is just the design for us, the Dutch.

But on the other hand this restricting is more or less also for for own safety. The consequences of a short antenna is that the moment the tuned signal leaves your house it has a high voltage. Fluorescent lamps will burn spontaneously. The use of high quality insulators is a must, the feeder must be as free as possible from any environment. The best is to invest in a more efficient antenna/ground system.
Some solutions I can't publish because it wont work in other situations.

Being a pensionado, I have now more time to look at the radiohobby. One of the topics I discovered, was LT Spice. It's a simulation program for electronics.
You can download it from here Linear Technology.
In principle the company made it for themselves, so the library is limited for the stuff they produce. It's possible to load 3rd party data. There are several fora and places where you can retrive missing libraries, but the disadvantage is,  when you send an ASC file to someone who don't have the library, it will not work.
If you install it and start with these 3 short instruction videos you know enough to get started.
I'm currently in process to make a 630m transverter. In this design a small "broadband" amplifier was needed. In stead of doing it on the breadboard, I used LT Spice. In reality the design worked instantly as simulated in LT Spice.
I also played with (simple) filters and and amplifiers ect. Explored the program what happened when you change components. The program is rather popular so there is always someone who wrote solution for your question. This playground moment is very essential, to familar yourself with the program. At a certain moment I decided to try to put the GW3UEP PA in LT-Spice. Not all components are available in the  LT Spice library, but there some alternatives which will work.The totempole transistors and PA MOSFET are replaced by "alternatives".

In the simulator you can easily change the voltage AC/frequency or DC, but the GW3UEP PA is driven by a squarewave, so some study should be done to arrange a squarewave in the "Voltage".
You have to construct this as a pulse. Pulse is build in "seconds" not  in Hertz, so first you have to convert  475kHz in "seconds". The result is a ridiculous long figure. Ask your friend Google/DuckDuckGo for a "Frequency to second converter". I came to 2,1186440677966 µsec. You have to divide it by 2, because you a squarewave has a "low" and a "high" state. It looks complex but it is very versatile. You are able to construct almost any waveform.

Look at this instruction video which makes designing a voltage pulse clear.

The "Voltage Component" is filled in as follows:
1 Choose Pulse, a parameterbox will appear.
2 Vinital start with 0
3 Von  max voltage iI used 10 (V)
Trise and Tfall can be used to compose triangle wave form. I kept them on 0
4 Tdelay(s),  here you enter the time the "state" will be "delayed" at the start, "OFF" state, in our case 1µ0593320338983
5 Ton(s) here you enter the time there will be "ON" state, 1µ0593320338983
6 Tperiod(s) here you enter the total time the sequence will take, after completion it wil repeat itself.
The LT Spice ASC file for the GW3UEP  100W PA you can download here.
If you have LT Spice installed and "click" on it, it will be automatically opened in LT Spice.

If you "run" it, and measure the output, you will notice that the output is far below the promissed 100W. It's one of the issues I encountered at first when I build the GW3UEP design. I solved that with inserting a Ferrite bar (from a broadcast radio) in the filter coil (L2 16µH).
Now try to change the value in the simulator into 35µ!. And presto 100W!

 Any idea how much current is flowing thru C4? Now you know why you have to use those "expensive" Wima's!
What is the influence of L3 12µ?
What happens when you lower the input?
All these situations you can try to explore without blowing up the PA.
Rise the PA voltage to 48V, see whats happen with the output power!
Another MOSFET? Try the IRFP4668 (It's in the standard LT Spice lib) Stunning? Now you have 150W. Change the PA voltage to 48V,  Oeps... 570W. Measure the voltage ontop C5.... Almost 600V! C4 has now a current of 35A.
Yep theoretically offcourse. 😉  I noticed that the program doesn't know the maximum current an IRF520 can take. In simulation mode it didn't blow....

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