Beyond 45/48m

[Stretchyman]

The bands should start to open up soon and with that in mind I've been looking at higher frequency designs.

GaN is the way to go as they're very easy to drive and a pair will give a 60W carrier @ 24V, so 240W pep @ 100% mod.

I seem to have nailed a decent design that covers upto 16MHz and is on a single PCB with built in PWM modulator

Of course I can no longer advertise or sell anything on here coz as we know that is against the law.

However if anyone would like any further 'information' please email me at;

stretchyman2k@googlemail.com

Cheers

Stretchy

Earlier version wi CREE SiC shown, by using class D gives a 5-7.5MHz B/W, Nice!

[Charlie_Dont_Surf]

Some folks on this side of the ocean have been using 30-31 meters (9500-9900 KHz) lately with some reasonable success. I have been thinking about transmitters for 22 meters at the low end of the broadcast allocation, and staying above the North American beacons in the ISM allocation (~13550 KHz). Propagation predictions showed that this band would have been quite useful for transcontinental and perhaps transoceanic use earlier this year, maybe not quite as much now in northern hemisphere summer.

I typically see significantly better bandwidth in an optimized CMCD configuration than I do with Class E. In order to get reasonable losses and lower transistor dissipation in Class E, I end up running loaded Qs in the 25-50 range. The simple LC tank circuits that I have been using for CMCD probably have not been that high yet still work well and are very efficient. I guess that I could go to higher Q but the circuit optimization algorithm has been leaving off elsewhere.

After all your cheerleading for Class E, I'm surprised to see a CMCD from you. Also... STOP THE PRESSES. It looks like a Cardinal Components crystal oscillator! Well, well, well. We've gone from "crystals are unnecessary" (paraphrasing your words) to "crystals are in use" in a short amount of time.

[Stretchyman]

CMCD was more of a happy accident.

I don't use Xtals!

You are however correct that's it's a CPP from Cardinal. All frequency sources need a reference, even a DDS has a xtal does it not?

I have a programmer and 100+ oscillators, simply done, can even reprog it (once more only!) when you've got it wrong/changed you mind.

They use an internal PLL are well screened and O/P 5V, no brainer for a simple single channel Tx.

Thanks for your comments.

Str.

[moof]

What's this thing run on?  Power-wise.  Ya know like a car battrey?  Does it require an oscilloscope to tune coz I don't have one.  LOL.  Is it just a board with stuff soddered on?  So I need to put it inside a proper radio?
Sorry to seem thick, I just see tiny surface mounted things, bondage wiring, and a capacitor big as....well.  Does it go inside a radio you sell?  I don't know what to make of the pictures.  Christal?

[Stretchyman]

Hi Moof, 48V is the usual voltage, altho it will work from 18-60V. Not sure what you mean by, putting it inside a proper radio?

It's a complete transmitter in a single PCB.

Yes you would normally put it in a case, if that's what you mean?

Like any electronic circuit, if you build it from scratch you will need some test equipment to make sure it's working correctly!

Of course you can get someone else to do that for you!

 

Str.

[redhat]

In the modern era of 56V power tools, higher voltage operation is no longer a challenge.  My Ego trimmer has enough battery on it to run a 50 watt transmitter for over two hours, and recharges in 15 minutes!

I too have had reasonable success with CMCD since 2017 and have been running with Cree 1200V devices.  They have even survived a few moments of stupidity with VSWR easily over 2:1 at rated power.  I am now eying Microsemi as they have 1700V devices at reasonable cost, and in stock.  This should facilitate 2KW carrier or greater in one PA with two devices.  Time will tell, as prototypes are in the works.

+-RH

[Stretchyman]

Thanks RH.

'In stock', don't hear much of that these days! Prices have shot up too.

Could you let us know what model of FET you've been using?

Cheers

Stretchy.

[redhat]

The current transmitter has C2M0080120D's in it I believe.  Mouser has them in stock again, but the price has gone up.  Microsemi/Microchip have comparable ones are a little cheaper (half) and have lower gate capacity.  I have not tried them in a circuit though.

+-RH

[Stretchyman]

Yes, the name of the microsemi parts?

I'll give them a test.

Str

[redhat]

MSC080SMA120B

+-RH

[Stretchyman]

Are you sure?

The Qg = 68nC

The C3M0280090 =9.7nC

Somewhat easier to drive!

Are you using 'bolt down' drivers? I'm using SOIC 8's.

I normally use Transphorm GaN for above 5MHz as Qg = 6nC and 6V is plenty to switch them.

That Qg is too high for me!

Str.

[Charlie_Dont_Surf]

Are you sure?

The Qg = 68nC

That Qg is too high for me!

Str.

It's actually 64 nC, not 68, but yeah, I noted the same thing.

Keep in mind that with higher power, Qg tends to go up. It has to because in order to meet the required current density demand and preserve other relationships, they make these by replicating unit cell transistors in parallel, meaning there are multiple smaller gates in parallel, which means multiple Qgd and Qgs in parallel too. It adds up. The other thing that matters is the breakdown voltage. The higher the BV, typically/often the input capacitance and Qg increases too. The FET channel will be longer, which means larger surface area of the gate capacitance, which likely means larger Qg.

The Cree C3M0280090 is small (900 V, continuous drain current 6.8 A continuous at 100 C) compared to that Microsemi FET (1200 V, 26 A continuous at 100C), so of course the Cree is more "nimble". Side note: the Qg of these transistors are at different conditions so not exactly comparing apples to apples but still...

I'm about ready to populate a board with the Transform TP65H035G4WS: only 650 V but Qg = 22 and Id continuous at 100 C = 29.5 A. Potentially not the best solution for me but at least it is available right now, which is more than I can say for some other choices!

In any case, if he was trying to run that MicroSemi FET at 15 MHz (picking a number out of the air), it might be low(er) efficiency compared to other solutions due to the higher Qg. (Maybe.) On the other hand, I'm only aware of him using 6285 and 4185 KHz, so maybe he doesn't care about anything above 6.3 MHz.

Also, at some point it may make sense to use RF transistors - as opposed to switching transistors - to meet the performance goals you want, particularly at higher frequencies. (That's why they exist.) Personally, I'm nowhere near that point but I have thought about it.

[redhat]

Your not reading again stretchy.  I am talking about 1700v devices with 80mOhm RDSon...not 900v devices at 280mOhm.

The current rig has run up to 10 Mhz.  I have little interest in anything much above that as I don't operate during the day.  Bolt on fet drivers are used to deal with the gate capacity.  I will explore GaN when the price performance ratio gets more attractive.  For now Sic makes more sense in my topologies and power levels.  If they come out with comparable high voltage GaN devices at a reasonable price, I'll consider it.

I like my current approach as it affords high power with a simple low cost PA architecture.  There are ways to get similar power with lower voltages, but circuit complexity, the headache of ferrite, high current losses, etc start to make things unnatractive for my needs.

SiC devices have made what I'm doing possible at much lower cost, and simpler design than was possible just a handful of years ago.  GaN for the moment, does not hold the same advantages.

+-RH

[Stretchyman]

Ok Both, All understood!

Regards.

Stretchy.

[Charlie_Dont_Surf]

In the modern era of 56V power tools, higher voltage operation is no longer a challenge.  My Ego trimmer has enough battery on it to run a 50 watt transmitter for over two hours, and recharges in 15 minutes!

It's funny that you brought this up since that same day that you wrote this my new Ego leaf blower arrived. I bought it without the battery since I already had one with my Ego weed trimmer; you can share batteries among their equipment, saving ~$100 or so.

In any case, I saw the amp-hour capacity of those batteries a few weeks ago and thought, "you could run a transmitter off this bad boy."