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40 meter 8 FET transmitter




 
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Author Topic: 40 meter 8 FET transmitter  (Read 6291 times)
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ka1tdq
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« on: July 09, 2016, 10:36:54 AM »

I'm still waiting on a few major parts to come in, but this will be the overall look of the rig.  It's very similar to my last rig.  

One heat sink will mount all of the IXDD's and the other will mount the FETs.  I'm going to lop off the top of the IXDD heatsink so that all the cores will fit nicely and have a short run to the FET bus'.  

I found 470pf Russian doorknob capacitors with solderable leads.  They're smaller and have a 2kv rating.  I'm thinking they should be perfect for the shunt capacitors.  I'll use the two 540 Transzorb zener diodes on each drain bus, and that should give me a little extra capacitance on the drain to equal around 500 pf.

I'll use one IXDD to drive each individual FET.  In the center of each IXDD bus, I'll mount an input IXDD (10 IXDD's total... 8 for drive and 2 for input wave shaping).  That way all leads will be short and neat.  The input IXDD will be connected to the output of the DDS VFO.  I'll use a sine wave with variable DC offset to drive each input IXDD.  That will give me an output square wave with variable duty cycle.  

It's a start!

Jon


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K1JJ
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« Reply #1 on: July 09, 2016, 12:56:27 PM »

Hi Jon,


Good to see you're keeping the homebrew art alive!

Suggestion:  It looks like you taped the cores together using black electrical tape?   I did the same thing on my 24 FET KW E-rig.  Unfortunately, they produced loud acoustical talk-back and sang when modulated. During a dead carrier, they would sometimes feed back into a 1 KHz note.  

The cores get warm and this is sometimes enough to make the tape slip.  This produced the tiny core gaps that made the acoustical talk-back, I suppose.

I was told by an experienced E-rig  builder that GLUE (epoxy?) is a better way to hold them together. More permanent without slip.  Hold them tight together in a clamp until the glue dries.

Later -

T
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ka1tdq
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« Reply #2 on: July 09, 2016, 11:33:37 PM »

I used Gorilla Glue to hold them together.  I don't always build class E transmitters.  But when I do, I use Gorilla Glue to hold my cores together.   Smiley

I also lopped off the top of one of the heat sinks to allow the core assembly to pass over it.

I didn't think about it until now, but the heat sink is just long enough to hold 8 FETs with a little gap in between. 

Jon


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K1JJ
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« Reply #3 on: July 10, 2016, 12:51:28 PM »

Good- you should be OK now.

I ran my cores with electrical tape for a few years and could never understand why I had the acoustical feedback.  To solve it I wrapped a towel around the cores and it did the trick. However, the towel held in the heat and eventually caused some other problems.  The cores and surrounding components need good ventilation, especially when we build 1 KW rigs.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

A Night in Tunisia:  https://www.youtube.com/watch?v=baMsQeQpUvw
KD6VXI
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« Reply #4 on: July 10, 2016, 02:00:11 PM »

I will add that super glue works as well.

I had the same 'talk back'  problem in my legal limit hf mobile amp.   I noticed the cores where stacked beads that had enough slop on the brass tubes they could move around.   

Google said super glue would work,  and it does!

--Shane
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ka1tdq
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« Reply #5 on: July 17, 2016, 01:58:57 AM »

I got a few more parts in.  I still need to place the order for all the semiconductors, but I won't be in a position to mount all that for a little while anyway.  Mounting all this big stuff is a piece of cake compared to all the fine, intricate work of the FETs and drivers.  There's a lot going on in such a small area.  I have a plan to make it all come out nicely, but I'll do that when the mood hits me. 

You can see my shunt capacitors on the photo on the heatsinks.  Russkie 470pf's. 

The terminal connector block is a little overkill.  It's a Chinese 100 amp thing.  It looked really good on Amazon, but I didn't realize it was so big. 

Jon


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ka1tdq
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« Reply #6 on: July 18, 2016, 12:35:50 AM »

I did a little more work this evening, and I'm really starting to like the way it's turning out.  It almost looks like some secret military weapon, or something.  If I get it working, I won't use this as my mobile rig.  Some well meaning law enforcement officer might mistake this for something other than a radio. 

I realized that I have to watch drain current on both sides, so I ordered another meter/shunt combo.  It's on the slow boat from China so it'll hold me back two weeks. 

Jon


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ka1tdq
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« Reply #7 on: July 31, 2016, 03:21:30 AM »

Ok, I finished the heat sink assembly.  I wanted to test it before I put it into the RF deck, so I hooked a 13.8 vdc power supply up to the IXDD bus and checked the gate of each FET.  I adjusted the DC offset pot to get roughly a 40% duty cycle. 

It seems to work fine, but it seems to draw a lot of current.  I have a radio shack 19 amp power supply (with no amp meter), and the fan on the power supply kicks on when I add the second bus of IXDD's.  Could they draw over 19 amps of current? 

It's past midnight and I don't feel like doing any more testing... going to bed.

Jon


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ka1tdq
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« Reply #8 on: July 31, 2016, 12:11:13 PM »

I went with my Astron power supply instead because it has an amp meter included.  I also lowered the voltage to 12.75 vdc.  The Astron has a pot for a slight adjustment.  And, it's drawing 6 amps total.  3 on one bus, and 3 on the other (measured both).  

I did add the grounded bus bar in between the gate and the drain busses.  Sort of like a grounded grid design... no neutralization required!  

I'm going to add three good sized electrolytics from the IXDD supply bus to ground.  After that I'm going to install it into the RF deck and start hard wiring it.  

I'm still waiting on a 30 amp, 12 volt regulated supply to come in for the IXDD bus.  For testing, I'll use 12.75 volts on the drain from my Astron power supply.  

Jon


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KD6VXI
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« Reply #9 on: July 31, 2016, 01:15:31 PM »

Jon,

What's your target carrier and mod voltage?

I,  too,  found lower vtage testing to be great lol.

--Shane
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ka1tdq
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« Reply #10 on: July 31, 2016, 01:42:30 PM »

Once I have 12 volts drain tested, then I'll move to 40 volts. I have a large transformer for that. 

If all goes well, then I'll worry about the modulator. 

Jon
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WD5JKO
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« Reply #11 on: July 31, 2016, 03:07:17 PM »

Jon,

   Your tenacity is amazing!

Looking at the waveforms, I wonder if that is the drains or the gates?

Always try to use the probes at 10X since that will have less capacitive loading..

That 19 amps on the drivers sure seems like a lot to me, but working 40m with all that capacitive loading makes getting the gate swing that fast take a lot of power.

I am pulling for you Jon!

Jim
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ka1tdq
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« Reply #12 on: July 31, 2016, 05:22:10 PM »

Done.  Ready for testing!

https://www.youtube.com/watch?v=ESidBrr2cxk


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ka1tdq
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« Reply #13 on: July 31, 2016, 10:27:59 PM »

It was fit to be tied on 40 meters.  I couldn't get it to work.

It seems to work fairly well though on 75 meters.  Once I started to get a class E waveform, I reconfigured everything for 75 meters.  I made a larger coil and put a larger doorknob capacitor in parallel with the loading cap.  In the picture is the waveform I'm getting for the gate and drain of side "b".  

For numbers, I'm using 12.63vdc drain and getting 24 watts output.  I can get more, but this was the cleanest waveform I got.  I haven't adjusted too much the duty cycle going into the gates, so I could probably squeeze some more from there.  Efficiency is at 80% right now going into a dummy load.

I didn't plan on this transmitter for 75 meters, but that's where it's taking me.

Jon


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VE3ELQ
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« Reply #14 on: August 01, 2016, 08:22:37 AM »

It was fit to be tied on 40 meters.  I couldn't get it to work.

I didn't plan on this transmitter for 75 meters, but that's where it's taking me.

Jon

Jon experimenting is great fun and the best way to learn. Like you a while back I tried many circuit configurations and lots of different FETs.  I found for 40 meters the best FETs I tried are the C2M0280120D.  They switch about 10 times faster than the 11N90 and are much easier to drive. Your driver current will drop to about 1/3 and they will sing at 7.4 mhz.

Good building, 73s  Nigel
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ka1tdq
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« Reply #15 on: August 01, 2016, 09:09:19 AM »

Yeah, this wasn't happy at all at 40 meters.  You and others have been advising me against the 11N90's at 40 meters, but live and learn.  They do seem to behave on 75 though.  I've put a lot of money into the heat sink assembly, so I think I'm just going to try to get this working well on 75. 

Now that I have the rig "tuned in" at a low drain voltage, I'm going to step it up a little bit.  I've ordered a 24 volt regulated supply that will serve as an intermediary voltage for testing.  I'll also wait until my 12 volt regulated supply comes in for the drains rather than using the 12.63 volt that I'm using now. 

In the meantime, I'll focus on the drive and getting that right.

Jon
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WD5JKO
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« Reply #16 on: August 01, 2016, 10:11:35 AM »


We could also consider an alternative driver:

http://www.ixyscolorado.com/index.php/ixys-rf/mosfets-drivers-diodes-power-modules/drivers-sic-diodes/item/ixrfd630

The rise and fall times into a 1000pf load is under 4 ns......
The rise and fall times into a 4000pf load is under 7 ns......

That is screaming guys!

Here is a pasted email from an IXYS application engineer:

"I would like to suggest that you look at our IXRFD630 driver on our driver page that is well suited to high frequency operation and 13.56MHz. The 630 replaced the 420. The IXD_614 from the IXYS IC division and all of the other drivers there are primarily limited by power dissipation Pgate=Vcc*Qgate*Freq. Their driver die are small in comparison to ours. The ones in the small SOIC packages are the ones really limited to 2MHz as they use epoxy attachment for the die to substrate, the ones in the TO-220 package I believe use solder die attachment with larger surface area and so you may be able to operate them at higher frequencies. But our driver is in a bigger package (more surface area) with a bigger die using solder attachment for the die so we can dissipate a lot more power than all of the others."

Jim
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ka1tdq
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« Reply #17 on: August 04, 2016, 08:21:44 PM »

My 24 volt regulated power supply just arrived and I'm getting 140 watts output.  Total drain current draw is 7.25 amps, so that puts efficiency at 80% into a dummy load.  The picture attached shows the gate and drain waveforms for side "a".

Now that I've tested it at a medium voltage and it works, I'll finish building my 40 volt unregulated power supply. 

Jon


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« Reply #18 on: August 04, 2016, 10:23:49 PM »

If you are still contemplating pwm,  remember you need a higher than carrier voltage power supply,  by like 60 pct plus (I use double on mine).

If your going heising,  no worries.

OR,  you could go class H modulator,  and have TWO 40 volt supplies!   Complications.....   Err,  I mean.   Contemplations!

--Shane
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ka1tdq
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« Reply #19 on: August 05, 2016, 02:32:43 AM »

I know it completely makes sense to go PWM, just like it makes sense to go class E over tubes.  I just like the simplicity of Heising.  I'm going to build the whole modulator (power supply, modulator and audio amp) on a 19" rack in my closet.  That alone will take me a while to do.  I also need to put up a 75 meter dipole and build the sequencer box. 

One thing I've found with class E is that meters tend to grow on your operating position.  Check out my collection...

Jon


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ka1tdq
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« Reply #20 on: August 07, 2016, 09:07:29 PM »

I built my power supply and I'm getting 400 watts carrier at 47.9 vdc on the drains.  Attached is scope picture of the 'a' side gate and drain waveforms.  Side 'b' draws a little more current, but it's within reason.

Total drain current is 10.08 amps.  That puts efficiency at 83%. 

When I build the Heising modulator, the choke will have a little voltage drop and bring the resting drain voltage down a little bit. 

Jon


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ka1tdq
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« Reply #21 on: August 11, 2016, 01:04:06 AM »

The modulator.  

The shelf has the 48 volt power supply and the regulated 11 vdc power supply for the IXDD bus.  The large transformer sitting there is 5 volts at 15 amps.  I used it in my last linear, but now it'll be part of the power supply for the negative peak limiter.  

The Heising components will sit on bottom since they're so heavy.  The audio amp will be a rack mount unit.  

Jon


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« Reply #22 on: August 12, 2016, 10:09:55 PM »


We could also consider an alternative driver:

http://www.ixyscolorado.com/index.php/ixys-rf/mosfets-drivers-diodes-power-modules/drivers-sic-diodes/item/ixrfd630

The rise and fall times into a 1000pf load is under 4 ns......
The rise and fall times into a 4000pf load is under 7 ns......

That is screaming guys!

Here is a pasted email from an IXYS application engineer:

"I would like to suggest that you look at our IXRFD630 driver on our driver page that is well suited to high frequency operation and 13.56MHz. The 630 replaced the 420. The IXD_614 from the IXYS IC division and all of the other drivers there are primarily limited by power dissipation Pgate=Vcc*Qgate*Freq. Their driver die are small in comparison to ours. The ones in the small SOIC packages are the ones really limited to 2MHz as they use epoxy attachment for the die to substrate, the ones in the TO-220 package I believe use solder die attachment with larger surface area and so you may be able to operate them at higher frequencies. But our driver is in a bigger package (more surface area) with a bigger die using solder attachment for the die so we can dissipate a lot more power than all of the others."

Jim
Wd5JKO

That is a good driver, but the cost is around $22 _EACH_, which is kind of expensive.  The IXDD chips are only about $5.00 each...
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« Reply #23 on: August 12, 2016, 10:11:23 PM »

I built my power supply and I'm getting 400 watts carrier at 47.9 vdc on the drains.  Attached is scope picture of the 'a' side gate and drain waveforms.  Side 'b' draws a little more current, but it's within reason.

Total drain current is 10.08 amps.  That puts efficiency at 83%. 

When I build the Heising modulator, the choke will have a little voltage drop and bring the resting drain voltage down a little bit. 

Jon

Which driver IC did you end up using?  The IXDD614 or the IXDD414?  The gate waveform looks quite good.
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ka1tdq
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« Reply #24 on: August 12, 2016, 10:59:54 PM »

I used the IXDD614's because DigiKey sells those and not the 414's. 

I'm getting things wired more permanently now than when I did the testing before, so I'll tweak the drain waveform to look a little better.  As it turns out, the easy part was building the RF deck.  There's MUCH more work getting everything else done that's needed for the transmitter, but I'm getting there.

Jon
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