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350W 3 band Class D FET AM Transmitter




 
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Author Topic: 350W 3 band Class D FET AM Transmitter  (Read 1138 times)
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VE3ELQ
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« on: February 09, 2017, 12:02:20 PM »

After countless hours of fun experimenting over several years, my pw modulated FET AM transmitter is now completed. It is a 350W carrier 1500W PEP band switched (160/80/40) high efficiency desktop design running in class D.  The RF deck employs 6 Cree C3M0280090D SiC FETs in single ended push pull running at 70V carrier at 5.3A driven by a single IXDD614 per phase. The output transformer is a T200A-2 core with opposed windings. The 3 relay switched output filters are fixed tuned 3 pole butterworth low pass using 2 layer air core inductors and silver mica capacitors. The modulator pulse generator is based on the LTC6992-1 chip fed by 2 audio stages from a TL072 op amp with feed forward ripple cancellation. The output is a half bridge using a pair of IRFP260 FETs driven by a IR2110 with high side bootstrap feeding a 4 pole butterworth modulator filter using a 6 layer aircore 1st inductor and a ferite 2nd inductor. A DDS VFO followed by an amplifier/buffer and co-phase driver provides the RF source. The case is wood construction with aluminum front and rear panels (I hate metal work). Tests have shown very high efficiency on 160 and 80m, the RF deck barely gets warm and on 40M just slightly warm.  Likewise the modulator runs just slightly warm. Convection cooling is all thats needed, no fans.

Still to do:  Make new meter scales 0 to 10A and RF power/SWR. (looking for a PC app)
Experiment with the LTC6992-4 variant PW mod chip which limits the negative peak PW to 5% instead of zero which may (or not) work out as a negative peak limiter.
Re-design the negative peak clip LED circuit which seems prone to RF or something.

Look forward to working some of you with this transmitter.  If you hear me on please say hello.

73s  Nigel


* 20170209_113835.jpg (552.49 KB, 2048x1536 - viewed 135 times.)

* 20170209_113726.jpg (563.5 KB, 2048x1536 - viewed 94 times.)

* 20170209_112535.jpg (892.64 KB, 2048x1536 - viewed 202 times.)
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WB5IRI
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« Reply #1 on: February 09, 2017, 12:15:29 PM »

Absolutely beautiful, Nigel. Hope to work you soon!

Doug
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KQ6F
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« Reply #2 on: February 09, 2017, 01:10:51 PM »

Nice work, Nigel.

Here's a meter graphics program that might work for you.
http://http://tonnesoftware.com/meter2.html
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steve_qix
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« Reply #3 on: February 10, 2017, 12:36:24 AM »

That's fine construction - looks great.  Near layout of the modulator.  Do you have any pics of the RF deck?
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VE3ELQ
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« Reply #4 on: February 10, 2017, 08:19:50 AM »

That's fine construction - looks great.  Near layout of the modulator.  Do you have any pics of the RF deck?

Steve,
In my haste to get this done I neglected to take enough pictures. The RF deck is the same one I used for fixed tuned 40M class E experiments only I changed the transformer to the double thick 200A-2 and added 4 relays, 2 per phase, and additional drain caps for band switching.  Attached is a picture of that deck before the changes.  Hope that helps.

BTW how is the high power GaN FET project progressing??

Edit:  Added a close up which shows the band switch relays and caps, the bottom half looks the same.

73s  Nigel


* 20161019_113456.jpg (776.96 KB, 2048x1536 - viewed 119 times.)

* 20170211_142812.jpg (837.35 KB, 2048x1536 - viewed 94 times.)
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pw fallon
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« Reply #5 on: February 12, 2017, 08:50:53 PM »

Hi Nigel,

Can you post a schematic of the transmitter?

Thanks,

Joe (aka PW Fallon)
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VE3ELQ
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« Reply #6 on: February 13, 2017, 01:37:29 PM »

Hi Nigel,

Can you post a schematic of the transmitter?

Thanks,

Joe (aka PW Fallon)
WA1IWQ
Joe,  
The schematics are currently hand drawn in several notebooks with lots of scribbles and test notes, so need to be re-drawn. I will be happy to post them but it will take some time.  
I have a kind offer from forum member and fellow builder Rod KQ6F who is skilled with CAD to draw up the schematics from hand drawn PDF scans. We can certainly do this if there is any interest.
In the meantime I can post any part of it with a Word format narrative that you or others may be interested in.  It consists of VFO buffer/driver, RF deck, 3 band output filters, Power/SWR bridge and TR relay, modulator audio and pulse generator, modulator output and filter, and the basic low and high voltage power supplies.

In the past few days after some intitial modulator pulse ratio and mic gain tweaks, I have had some nice QSOs with this transmitter receiving good signal and "broadcast audio" reports.  Think I will take the rest of the day off.  Cheesy
73s  Nigel
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W1AEX
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« Reply #7 on: February 14, 2017, 02:04:17 PM »

Looks beautiful Nigel! Bet it sounds as good as it looks too! I look forward to finding you on the bands again.

Rob
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VE3ELQ
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« Reply #8 on: February 15, 2017, 02:49:43 PM »

Had a few Emails regarding the modulator design.  Attached is a PDF scan of the hand draw schematic. Perhaps KQ6F Rod will make it pretty.

Its a "Plain Jane" design, no peak limiter or audio band passing. I use a RE-27 mic and a W2IHY 8 band EQ externally which nicely tailors the audio and rolls off pretty quick above about 3200 hz. Several output FETs were tried but the old reliable IRFP260s worked best. The Gate Rs of 4.7 and 16 ohms are critical to prevent shoot through and overheating. My carrier PW is at 45% to allow for my own natural positive peak asymmetry, I see just under 400V RF peaks which is almost 1600W PEP. A "Key Up" circuit is included to key the TR relay and also provide a 25ms transmit delay with zero turn off delay from a single switch.  This timing is important to allow the TR relay to settle before RF is applied. Mine is spec'd at 15ms so the delay was set to 25ms.  Open with Adobe, click view rotate CW.   Hope this is useful.

73s  Nigel

A revised copy of schematic attached which corrects some errors.

Also a pic of the modulator output board. Amazing how much audio power this little thing can produce at about 99% efficiency.

* Modulator.pdf (491.22 KB - downloaded 72 times.)

* 20170220_102511.jpg (831.85 KB, 2048x1536 - viewed 102 times.)
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VE3ELQ
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« Reply #9 on: February 19, 2017, 07:22:40 PM »

When trying to draw a schematic from an assembly of notes in different locations the inevitable happens, mistakes occur.
I have replaced the schematic above with a revised one which I think is now correct.  It does work, honest.

73s  Nigel
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M0VRF
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« Reply #10 on: March 18, 2017, 01:38:33 PM »

Here's my version!

Same design but I'm using TC4422's as drivers, same C3M Cree Fets in O/P.

You can just alter the turns on the toroids to change the O/P impedance.

I'm using 8T and getting 300W O/P @ 60V, efficiency is around 90% Will use my PWM modulator for this one.

I've also built one for 24V and used (4T on toroids) and using a Modtranny and 100W class D amp gives a 100W carrier @ 85% efficiency.

The PCB is my first attempt, have made a few minor changes in a new version.


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* 300W amp.jpg (343.87 KB, 2159x1456 - viewed 32 times.)

* amp pcb.jpg (132.76 KB, 808x970 - viewed 27 times.)
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