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Class D 40/75m transmitter




 
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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 157 times.)

* CW-modulator.jpg (3516.83 KB, 4032x3024 - viewed 151 times.)
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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 218 times.)
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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 136 times.)
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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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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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« Reply #5 on: February 23, 2019, 04:00:32 AM »

Making more progress.

Jon


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« Reply #6 on: February 24, 2019, 08:29:23 AM »

Almost done.

I even have a small loop of wire (yellow) for DC shunt to measure drain current.

Jon


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« Reply #7 on: February 24, 2019, 11:42:24 AM »

Finished!  Now just testing.

https://www.youtube.com/watch?v=3av3sjOBua0

(I take my projects seriously)

Jon


* 75m Class D Transmitter.jpg (4331.36 KB, 4032x3024 - viewed 149 times.)
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« Reply #8 on: March 17, 2019, 10:33:06 AM »

I finally got around to testing it and the drain waveform isn't exactly what I had hoped for. Efficiency is in the dirt at 56%, but I am getting 10 watts out with 13.8vdc on the drain. Efficiency stays about the same but power rises as I go up in frequency and down as I go down.

I'm going to mess with the value of the shunt cap. I get more power output higher in frequency and I'm using 470pf. It seems to want more capacitive reactance so I'll use a lower value and see what happens.

The gate drive looks perfect. It's a nice square wave around 43% duty cycle with almost an unnoticeable change under load conditions.

It'll work under it's current design, but efficiency is nothing to brag about. If I raise drain voltage to around 32 volts, this would make a nice 50 watt carrier transmitter.

Jon


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« Reply #9 on: March 17, 2019, 12:04:33 PM »

The waveform looks a little better and efficiency rose a little with 840pf of shunt capacitance. It's not there yet though.

Jon


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« Reply #10 on: March 17, 2019, 01:23:37 PM »

It seem to me that your transformer suffers from too little inductance and too much spread inductance, not sufficient coupling. You can see that at the resonances of the higher frequencies.
May be you should try two ferrite tubes next too each other and use a U-shaped tube as primary and wind the sec through these tubes. Like the pigs-nose transformers. They have less spread inductance and, because you use twice the ammount of ferrite, more inductance.
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« Reply #11 on: March 17, 2019, 07:52:34 PM »

I'm having difficulty with the screen capture on my scope, so I'm doing the picture method.

I'm using two cores in a binocular configuration and now I'm at about half the capacitance for the 'sweet' spot. I'm getting a little better efficiency at 66% and the waveform is looking a little more class D, but not there yet. I can move the variable cap around to vary the appearance, but it's not the classic D waveform.

I did a quick-and-dirty job on the transformer. I didn't use tubes or sleeves and just wound it straight with #12 solid.

Jon


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« Reply #12 on: March 17, 2019, 08:40:54 PM »

Using tubes can make quite a difference with plain wire.
Look also if the higher frequencies are also present at the +DC side of the transformer. There it should be just DC. If not there is a decoupling problem. But I guess that it is the coupling factor of the transformer which can be increased quite a bit using a tube primary connected with copper foil or PCB (not just wire) and winding through the tubes. Connect the primary also with wide foil to the drain and to the DC decoupling as short and low inductance as possible. Due to the high drain capacitance every nano Henry counts.
Following the results with great interest
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ka1tdq
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« Reply #13 on: March 17, 2019, 09:43:15 PM »

Ah, man... I was about to wrap this one up with 'Good enough.' Burning 5 watts vs 2 watts at carrier with the extra efficiency doesn't matter too much. But absolutely, it should be right.

When I disconnected the variable capacitor to measure it's value, I almost took it as a sign from God. The value was exactly 470pf, the value of the doorknob I started with.

To do it right, I might just order a binocular core rather than taping two cores together.

By the way, I really do appreciate your input on this. This would make a really interesting transmitter to pair with a linear. Audio coming from this will be absolutely phenomenal! And it's easy to do since the transmitter is so small.

Jon
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« Reply #14 on: March 20, 2019, 01:39:20 PM »

I just made the ends for the transformer using one-sided PCB material. I will use two FB-43-1020 cores and the holes will accept 1/4" copper tubing.

Jon


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« Reply #15 on: March 20, 2019, 11:41:46 PM »

This is fun Jon
When it works fb, I like to make one as well, but I will try to modulate with a switching  powersupply instead of a standard PWM amplifier . The powersupply chips are becoming very fast and it should be possible to modulate using the PWM loop in the chip if the frequency is high compared to the highest audio frequency.
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« Reply #16 on: March 21, 2019, 12:36:06 AM »

I put the new transformer in and I'm still getting about 66% efficiency and 10.5 watts output at 13.85vdc on the drain. Here's the waveform with a 470pf doorknob capacitor.

I've been doing some Googling, but I can't determine what the waveform is supposed to look like. What waveform am I looking for for single-ended class D in this configuration? I say this configuration because I'm not exactly sure if this is voltage mode or current mode.

Jon


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« Reply #17 on: March 21, 2019, 08:25:13 AM »

I haven't had time/been home enough to experiment with this circuit, although I plan to once the day job settles down (always busy as school sports seasons kick in).

But I've been googling and doodling schematics as well.  Found this paper online someplace, has some schematics, and some waveforms, which may be of interest.

Ed

* class d.pdf (394.15 KB - downloaded 75 times.)
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« Reply #18 on: March 21, 2019, 08:48:00 AM »

That's some good reading material, thanks!

I seem to have found the maximum efficiency point in terms of the capacitor value. I think at this point the problem is impedance matching between the primary and the 50 ohm load.

The FET's impedance is (.5)(Edrain/Idrain)

so (.5)(13.85v/1.1a) = 6.2 ohms

My 1:2 transformer ratio brings that up do 24.5 ohms (I think). So I still have a 25 to 50 ohms impedance mismatch. I think if I add another turn it will bring it close to 50 ohms.


Jon
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« Reply #19 on: March 21, 2019, 12:46:59 PM »

The impedance indeed may be an issue. On the other hand, I see that the voltage is an a half sine BUT is going up again when the FET switches on. So it does switch when there is voltage across it. The voltage should be half a sine and when crossing zero the FET should switch ON The scope pic. points to a too low inductance in the drain or too high a resonance frequency. The FET should switch on when there is no current or voltage.
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ka1tdq
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« Reply #20 on: March 21, 2019, 01:03:14 PM »

Ok! So I'm looking for a half sine wave... nothing too fancy. When the gate switches on, the voltage across the FET should be dropped to zero at that point. I can vary the capacitance to make that happen.

I'll make the impedance adjustment with the extra turn first and then throw the variable cap back on to get the correct drain waveform.

Good stuff!  Thanks!

Jon
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« Reply #21 on: March 22, 2019, 01:08:12 AM »

I was able to get a cleaner waveform with more capacitance. I used a 680pf doorknob to get here and adjusting the duty cycle of the gate.

The transmitter wasn't stable with 3 turns on the secondary, so I went back to 2 turns.

I'm only able to get about 68% efficiency out of it with 13 watts carrier and 13.8vdc on the drain. At 50 watts carrier that would be burning 24 watts. The heat sink should be able to take that without too much difficulty. 

I ordered an Antek 32 volt transformer to make the drain power supply.

Jon


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« Reply #22 on: March 22, 2019, 02:13:54 PM »

Quite strange that the amp isn't stable with 3 turns sec. Switching amps normally are quite stable due to the ow drive impedance. That means that there is some feed back. Do you see a difference in the gate voltage when you change the loading ( e.g. put a cap or extra R across the load) If so, may be a weak driver. Look as well at the source connection and tho power of the driver if there is an RF voltage. That can be difficult, you need a very short and good ground at your probe  to avoid pick-up via the ground.
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