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Class D 40/75m transmitter




 
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Author Topic: Class D 40/75m transmitter  (Read 260 times)
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ka1tdq
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« on: February 08, 2019, 11:16:05 AM »

I have duplicate heat sink that I used for the CW modulator and was thinking about what to build with it. I think I'll be able to squeeze two class D single ended transmitters on it. It'd be low power stuff, around 30 or 40 watts carrier each. It would be compact, cheap, easy to build and be a nice companion for a killer linear. You could switch which transmitter to use simply by supplying 12 volts to the selected RF deck's IXDD driver.

Attached is the layout for the heat sink. I could mount the companion matching CW modulator next to it so that the transmitter could be either AM or CW. Class D is broadband enough to cover both portions of the band.

I'm excited.

Jon




* Class D 40-80.jpg (3443.43 KB, 4032x3024 - viewed 42 times.)

* CW-modulator.jpg (3516.83 KB, 4032x3024 - viewed 45 times.)
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ka1tdq
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« Reply #1 on: February 09, 2019, 10:29:53 AM »

Here's the schematic for the 75 meter portion.

Jon


* Class D schematic.jpg (2174.13 KB, 3910x2930 - viewed 57 times.)
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ka1tdq
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« Reply #2 on: February 18, 2019, 03:22:55 PM »

I decided to just build one transmitter per heat sink and here's how the progress is going on the 75 meter version.

Jon


* 75m class D.jpg (3740.34 KB, 4032x3024 - viewed 14 times.)
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K8DI
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« Reply #3 on: February 18, 2019, 05:24:50 PM »

Hi -- couple questions for you on this circuit/design...keep in mind I am a complete novice regarding class D and RF use of FETs. (I build stuff with tubes.)

How much voltage really flies around in a circuit like this? You've got a modulated 48v power supply, so 0-96 or 0-110 (@125% positive). And a 900 volt part.

You say 30-40 watts, but that FET specs out at 300 watts dissipation. And 48 volts at 11 amps is a lot more than 40 watts. What limits the power output?

For grins, I might build something similar..I've got available a bunch of FCA47N60s, 47 amps, 600 volts, salvaged from an 85" Samsung TV that got bent in half by a forklift in shipping. So, if I get my hands on a big PSU, and modulate it with some transformer and a big audio amp..... What could go wrong? What will go wrong?

Ed Walters
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Ed, K8DI (fmr. KB8TWH)
ka1tdq
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« Reply #4 on: February 18, 2019, 08:10:00 PM »

Modulated DC does run around what you mentioned. To be honest, Iím not too sure about the rest. Others can chime in. All I know is that I ran something like this (without the shunt cap) for a few years without incident. The FET is well within its rating either way regardless.

Voltages get very high in Class E for sure though. Iím just building this to check the class D box behind my name.

Jon
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