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24 FET 75 Meter Transmitter




 
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ka1tdq
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« on: January 17, 2017, 11:47:04 PM »

I've started gathering parts for a 75 meter 24 FET transmitter.  

Before I get too involved, I want to throw this design variation out there.  I do something similar with my current 75 meter 8 FET rig.  I use the output of a single IXDD614 to drive one phase of the other IXDD614's.  The picture shows a block diagram of what I'm talking about.  It seems to work out well driving 4 of them, I'm just wondering about the possibility of driving 6.  

This way, there's only two feedlines going into the rig to drive the two phases.  I'm currently using twisted pair wire for this purpose with both ends terminated with about 1k resistors.  

Jon


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ka1tdq
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« Reply #1 on: January 18, 2017, 04:19:31 AM »

...to further illustrate my point, I would have to change the layout a little bit.  Instead of the elongated core layout, I would have a 4-on-4 bundle.  Each group of 6 MOSFETs for each phase would be next to each other.  My drawing is way over simplified, but I'm just trying to show the concept.  

I think this would greatly reduce the lead length supplying each IXDD buss.  Now, you just have one IXDD chip supplying the input to all the other IXDD chips for that phase on one copper strip.  That single IXDD chip for each phase would then have the twisted pair feeding it from the DDS VFO.  

It's basically just folding the transformer in half on itself (also, the best I've got at 2 o'clock in the morning).  Smiley

Jon


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steve_qix
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« Reply #2 on: January 18, 2017, 09:41:58 AM »

You probably don't want to do any sort of input bus.  I have had varying problems with input busses with respect to parasitics, and finally standardized on this: 

A piece of thin coax going to each IXDD driver, with a 300 ohm resistor to ground at the end of each thin coax (that number comes from 50 ohms * the number of drivers connected in a single phase) and a 200 ohm resistor in SERIES with the input pin of each IXDD.  All of the thin coax lines are the same length and are all paralleled together, with a single line then coming from the VFO (which has an IXDD in the output of each phase).  The thin coax lines should be able to be less than 12 inches in length.

You do not need to have another IXDD to drive the IXDD FET drivers if you're only supplying drive to 6 IXDD devices.

Sounds like a good project!  Of course you'll need a good PWM modulator to go with it.  I'm guessing the RF amplifier DC load presented to the modulator will be in the range of 1.7 to 2 ohms.
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ka1tdq
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« Reply #3 on: January 18, 2017, 11:40:38 AM »

I used input busses on my 75 meter, 8 FET rig.  That probably explains why it wouldn't behave on 40 meters.  

The 200 ohm resistor in series with the IXDD is interesting.  It's like a grid swamping resistor.

Thanks for the tips.  I'll do that, and I think I'm still going to wrap the cores around to make a 4x4 bundle.  The heat sink I ordered is about a 12" square, so it should work out nicely.

I have a plan to do open frame construction on this transmitter as well.  I have some ideas about welding up a frame.   It should look pretty cool.

I just ordered the FETS and that was $75 alone!  By the way, DigiKey's supply of FQA11N90's is running low.

A PWM I'm sure will be a requirement for this rig.  I don't know if an audio amp big enough to drive this transmitter even exists.  If it did, I'm sure I couldn't afford it.  I'll get a working RF deck and then worry about the PWM later.  

Jon
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steve_qix
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« Reply #4 on: January 18, 2017, 04:12:22 PM »

Yeah the PWM is by far the best particularly when you start getting into high power.  The power supply requirements for your transmitter - with PWM - are quite reasonable:  125VDC to 130VDC at about 8 or 9 amps depending on your power input.  You should be able to fit the entire modulator/power supply in a 7 inch rack space 15 inches deep (minus the power transformer which should just sit on the floor behind the unit).

$75 for all the MOSFETs is not all that bad.. considering, where are you going to find something that will handle that kind of power (1kW CCS and 200% positive modulation) for that little money  Cheesy

If Digikey ever runs out of FQA11N90s, I still have lots of them in my stock.  But, Fairchild is still making them, so I expect they will be with us for quite some time !

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ka1tdq
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« Reply #5 on: January 18, 2017, 04:30:21 PM »

The whole thing off of a 120 volts circuit?  That's great!  I was envisioning running a new 220 volt line into the shack.  I currently run my 8 FET rig off two 120 volt circuits... one for the carrier supply and the other to run my audio amp.  I probably don't need to, but do to be cautious.

If I can do a 1kw rig off a single 120 line, then heck yeah!  PWM is the way to go.

I should have all the parts for the RF deck by the end of the month.  Once I size everything up and get exact dimensions, then I'll start welding the frame.  The heat sink will be the bear.  I'll have to drill and tap around 50 4-40 holes.  Too bad my son isn't old enough.  I'd have him do it. 

Jon
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steve_qix
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« Reply #6 on: January 18, 2017, 04:53:12 PM »

You could run it off 120VAC but 240 is better.  At 240V, you're only pulling about 5A off the line at carrier - not much at all !  at 120V you're pulling about twice that amount and of course you have to add in other equipment such as receivers, lights, test equipment, etc.

But, it can be done.

Ideally, a Signal Corp DU2 is the best power transformer for the application, but any transformer that's big enough and has the correct secondary (104V to 110V - in that range) will work.  I have used standard isolation transformers with a smaller "buck" transformer in the primary to cut the voltage back to 104V.
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W2NBC
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« Reply #7 on: January 18, 2017, 06:01:26 PM »

http://www.ebay.com/itm/SIGNAL-DU-2-Isolation-Transformer-Step-Up-Down-Lot-575-/141041385024?hash=item20d6b8be40:g:UJ8AAOxyLNpSEiIa
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ka1tdq
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« Reply #8 on: January 18, 2017, 06:52:43 PM »

Nice but pricey.  Antek sells a 55V @1000VA transformer for $100.  The output is specified at 9.1 amps.  I don't know if that's per 55 volt section or if that means 18.2 amps in parallel.  With enough filtering, two of these should be enough, fed in phase.

http://www.antekinc.com/content/AN-10455.pdf

Parts so far... (coil not cut to length)


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steve_qix
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« Reply #9 on: January 18, 2017, 08:06:44 PM »

You  would need more VA than a single Antek, so you would need to get 2 of them (at $100 each ?).  At that price, the DU2 is actually better.  It will take up less room and be easier to configure.

For whatever reason, the DU2s are expensive.  I've been able to secure SU2s which have the same VA rating and taps on one of the windings, but require 240V - bought for around $60.00 from Ebay.  Currently, there are no SU2s available on Ebay.

You may be able to find a standard isolation transformer and buck the line a little.  Sometimes regular isolation transformers are fairly inexpensive.
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steve_qix
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« Reply #10 on: January 20, 2017, 07:55:20 AM »

I made a small change to the documentation for this project.

The schematic and pictures show 4 individual small switchers supplying the 12V dc for the drivers for each of the 4 modules in the transmitter.

You can certainly use a single 12V power supply for everything.  12V @ 15A should be more than sufficient.  It is not necessary to switch the driver DC supply on and off during transmit/receive cycles.  It is perfectly fine to leave the 12V DC on the drivers all the time.  The current draw falls to near 0 when there is no RF fed to the drivers.
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ka1tdq
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« Reply #11 on: January 20, 2017, 09:26:52 AM »

I leave the 12 volts on all the time with my 8-FETter, and I'll do the same this time. 

Here's the 12 volt transformer that I'm going to use for this rig.  It's salvaged from a 12 volt outside lighting transformer box.  I'm planning on just using the supply unregulated.  When the drive comes on, the supply will fall to around 12 volts, and there's about a second for this to happen before the drain voltage comes on. 

I probably make my class E stuff too simple, but it works.

Jon


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ka1tdq
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« Reply #12 on: January 20, 2017, 05:13:51 PM »

The heat sink and FETs arrived. Everything seems like it will fit. One phase on top and one phase on bottom. I'm going to use one IXDD to drive each 6 IXDD's per phase. One will be mounted on top center of the heat sink, and one bottom center. The lead length for the coax going to each IXDD will be about 5 inches.

Jon


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KD6VXI
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« Reply #13 on: January 20, 2017, 06:36:59 PM »

Where did you find that seckzi heat sink!

--Shane
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ka1tdq
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« Reply #14 on: January 20, 2017, 06:42:53 PM »

I found the heat sink on eBay.  It's, well, big...

It seems that I can't break away from my typical transmitter layout.  This one will be no different than my past 3 rigs.  I wanted to fit everything on an 18" x 18" sheet of steel, but it looks like 2' x 2' will be the winner.

Jon


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« Reply #15 on: January 21, 2017, 09:00:07 AM »

I like that heat sink. About the layout of the ferrites and FETs, I like the proposed new one better if only because it makes all the leads more the same, perhaps, while also moving the ferrite cores closer together rather than in a line, so all the leads from them are also closer to the same length. I don';t really know how impretant that is with your kind of project.
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ka1tdq
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« Reply #16 on: January 21, 2017, 01:48:00 PM »

I think it'll work out ok.  This morning I found a piece of remnant steel that fits perfectly on top of the cabinet.  The cabinet will obviously house the PWM and power supply.  

I'm going to order a beefier loading capacitor.  That's the only component throwing a red flag in my mind on power handling capability.

...the shack's starting to get a little full.

Jon


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ka1tdq
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« Reply #17 on: January 23, 2017, 03:20:51 AM »

I calculated what I'd need for a Heising choke for this rig and it would be around 10mH.  Hammond sells one that'll do 50 amps, but it goes for $500.  Add to that the audio amp.  I found a 2500 watt Crown class D amp on Sweetwater.com and they want $5000. 

I'd like to do Heising because it's so simple (see attached photo for for current transmitter), but it's almost cost prohibitive.  But hey, I am over 40 and I'm due for a mid-life crisis.  Instead of the Corvette I could buy a beastly modulator setup.

Jon


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pa0ast
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« Reply #18 on: January 23, 2017, 05:44:59 AM »

Al ready tried a currentcoil from an old scrap  tig/mig welder ? Can easily handle 50 Amps. Some times even adjustable, and cheap on a scrapyard.
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« Reply #19 on: January 23, 2017, 09:42:38 AM »


Jon,

   You could always skip the AM modulator, and try 75m FM.

With FM, you only need ~ 3db signal above the background to silence the noise. With AM, it is more like 30db.

FM is legal below 29Mhz so long as the bandwidth is no wider than am AM signal of equal frequency response.

There is a fellow in New Mexico who operates QRO SSB on 3892 (94?) who is Hell bent on the destruction of AM'ers who dare operate on 3890.

Your location, and 24 pills might just take care of that.  Grin

Jim
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ka1tdq
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« Reply #20 on: January 23, 2017, 12:32:24 PM »

I'm not too put off by the $500 price tag of the Heising coil, it's the $5,000 audio amp that is the problem.  I'm not even going to joke to my wife about buying that!

QRM is a problem here in the southwest.  When the AMI net is on, SSB'ers in the Midwest/Texas believe that it's their frequency too and we have to talk over one another.  California stations don't seem to mind as much, but here in Arizona it gets to be a little more of a problem.  Especially when trying to listen to weaker stations. 

I hate to say this, but if we all had 24-FET rigs, this wouldn't be an issue.

Jon
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« Reply #21 on: January 23, 2017, 02:37:37 PM »

Building a pulse width modulator is really not that difficult.  I find the modulator easier to build than the RF deck.

The nice thing about PWM is that the power supply requirements are quite modest for the given power.  For instance, you need something like a 130V 8A supply (or thereabouts) for your transmitter.  That's not bad.  Easily obtained components for such a thing.

The modulator itself is made easier with PC boards and kits of parts (if you want to use them).  Certainly takes a lot of the tedious work out of the mix for sure.

The filter can be toroids or air core inductors.  Air core is less expensive to make, but takes up more room.  But, both work and I personally use both in my implementations.

At Rattlesnake Island I have such a modulator, and it occupies a 7 inch high rack space that's about 15 inches deep.  Everything is contained within except the power transformer (a DU2), which sits behind the rack on the floor.
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KD6VXI
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« Reply #22 on: January 23, 2017, 09:06:55 PM »

Jon,

Your rf decks so far have been SO much harder than the actual pwm.   I've built one,  of my own design,  based upon reading a lot of Steves info,  and others on the net.   If I can do it.......

When your npl operates at audio level frequencies,  it's great!

If you do decide to go pwm,  there is a reason for an air core first inductor in your pwm filter.   Steve can fill in why.   I did both methods,  and didn't notice a difference,  fwiw.   The air core first inductor was huge in my design...   I did use 12 gauge wire,  due to length.

--Shane
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« Reply #23 on: January 23, 2017, 09:32:01 PM »

As Shane says, air core for the first inductor can make the construction easier.  The first inductor is subject to the full PWM switching frequency and switching voltage, so the magnetic stress on any core would be the greatest in this inductor.

For a core based first inductor, the design dictates a good, high flux core.  I have found for the 24 FET RF amplifiers and accompanying modulators, the Micrometals T300-TD core will work just fine for the first inductor.  I stack 2 of them for use in the first inductor.  The subsequent inductors in my design use the Magnetics X-Flux material for the cores.  I stack 5 cores for each of the next 2 inductors in the filter (there are 3 inductors).  Other materials may be used such as hi-flux and its equivalents.

These PWM modulators are pretty much cookbook these days.  Everything is predictable and calculable, which is great!
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KA6MOK
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« Reply #24 on: January 30, 2017, 06:38:04 PM »

I'm not too put off by the $500 price tag of the Heising coil, it's the $5,000 audio amp that is the problem.  I'm not even going to joke to my wife about buying that!

Jon

Maybe there's alternatives...
https://www.parts-express.com/behringer-nu3000dsp-inuke-3000-watt-power-amplifier-with-dsp--248-6706

Not only would this do 1500-3000W in bridged mode,  you also get a programmable DSP audio front end to play with as well.

Not sure what you need, inductor value wise,  but they have a variety here, some in low enough DCR that they could live with 10A flowing and no smoke...  not sure about linearity and saturation concerns.  Air cores are available, but maybe not in large enough diameter wire and large enough values...

http://www.parts-express.com/cat/crossover-inductors/296

This is a site I've frequented for years,  for a different hobby, of DIY Speaker building.  Large, low loss inductors and caps are needed for the crossover filters, in values you don't tend to see elsewhere.  If you ever need me to get talking,  ask me about it sometime...  Wink

Jon

And, I'm not meaning to turn you off from your first PWM project... Wink  just mentioning other items...
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