The AM Forum

I was getting ready for the AM rally by actually turning on my 75 meter class E rig.  Everything worked fine.  I even ran the carrier back to 300 watts to be cautious since I previously blew a FET pair running it too hard.  I don't have overload protection in my transmitter.  Just meters for me to view parameters.

I also had prepared a whiz-bang antenna for the weekend.  I wanted to try a full wave loop at 75 meters since I had never done that before.  It barely fit in my backyard, but unfortunately most of the antenna was literally 6 feet off the ground.  As you'd expect, it didn't work very well.  I watched my signal on both web SDR receivers out this way, and even at 2am the signals were pretty lousy.  So, back to the original dipole.

I listen to my transmitted audio by sampling the RF output and feeding it directly to my headphones.  All sounded crystal clear on the loop antenna.  When I switched to the dipole, the audio had lots of hash in it.  Like a good ham, I kept transmitting to try to figure out what the problem was.  The only change was the antenna.  Well pop-bang-fire!  I blew the same FET pair on phase B as last time. 

I believe it's an RF problem getting into that phase.  That phase is physically located nearest the output of the transmitter and always ran slightly less clean that the other phase as seen on a scope.  You can see the two blown FETs in the picture. 

I've built the transmitter heat sink assembly such that I can rebuild it quicker than a bubba can swap out a transmission, but I'm looking for the cause of the problem. 

It could also be parasitics rising from local RF, and Steve, I know you mentioned the 300 ohm resistor trick... but what is that little resistance going to do for a device with super high input impedance? 

Jon 

just my opinion get rid of the doorknobs, if you must we have used 5% silver micas 500v not the best but they will work. I would also try turning the output transformer 90 degrees from the tuning network. I cant see the tank coil but i would also keep that 90 degrees off the tuning network.

I just thought of this too:

The front chassis of the transmitter has BNC jacks to accept the output of the DDS VFO.  From there, it's twisted pair 20" long to the back of the transmitter and connects to the IXDD's.  It runs smack dab through the output tuning network.  Amazingly, it runs as clean as it does (especially phase 'A').

I just slow-boated a couple 24" BNC-to-SMA cables and I'll extend the coax run right up to the IXDD inputs.  I'm not a betting person, but...

Jon

And if I'm wrong, oh well.  I guess I'll spend another $10 in junk box parts for a couple FETs and a driver.

Again, Get rid of the Doorknobs thats old news.

Right-o. I have some ATC’s on the way. That’s a quick fix.

Jon

If the twisted pair connects to the BNC jack then that's the problem. The jack is an unbalanced component, twisted pair is balanced. You have a 20" antenna in a high RF field.

Same thing as grounding one lead of an open wire line feeder.

Use coax.

That’s what I was thinking, and I’m amazed it worked as long as it did. Basically like you said, it’s an oscillator power level signal trying to get through a 400 watt carrier field. I’ll make both changes and see what happens.

Jon

I don't know how you're getting the signal from the frequency source to the drivers, but you _must_ put a 300 ohm resistor directly in series between the driver input pin and everything else.  Then, the "feedline" connecting from the frequency source must be terminated in its characteristic impedance.

Shielded cable is the absolute best.  The cable can be quite long.  I've run 15 feet of cable between the VFO output (12V signal) and the drivers with no issue - this is with thin coax.

The problem you had does sound like a parasitic.

The signal from the frequency source should be substantial - 12V, 0V referenced (AKA, the signal goes from 0V when off, to 12V when on.  This improves noise immunity.

My existing configuration is too complicated to describe without eyes glazing over, other than to say it's wrong.  It worked, but it's wrong.  I'll run 50 ohm coax directly to the drivers and use the 300 ohm resistor inline. 

I was using a sine wave but not near 12volts.  I think it was around 7.5 volts pk/pk.  I can't go any higher because the duty cycle will get too high coming out of the drivers.

Anyway, I think this will greatly improve things.  Thanks!

Jon

Ok, I see what's going on.  Terminate the coax in 50 ohms.  Analog drive to the driver ICs is unpredictable, and I don't know how the drivers will react to any sort of negative going input signal.  This type of operation may be undefined (you'd have to look at the documentation), or it may be that the signal is dc-shifted up due to clamper action within the driver input.

Apparently something like that is going on because it does work. I could easily throw a buffer inline to square it up, but I don’t think it’s needed.

Jon

It definitely will work.  It's just that the behavior is not defined.  The series resistors and the terminating resistors at the end of the feeding coax may clear up the parasitics completely.

Let us know !

I've got coax coming, ATC capacitors, and 300 ohm resistors.  I'll adjust things one at a time until I get a clean wave.

And to be clear, it will run "dirty."  I monitor the audio directly and when things are fine, the audio is clear.  Only when I get hash in the audio do things go to hell.  It'll be very simple for me to determine what the fix is (after I replace 2 blown FETs and a driver).

Jon

I did some experimenting with my VFO and discovered that I did have it set without an offset, meaning that half the wave was a negative going pulse.  I set the position for maximum and now everything is above zero DC going out.

Also, my current transmitter configuration has resistors totaling somewhere around 500 ohms at the driver input.  I did that back in the day so as not to drag down the VFO.  That was a mistake.  

I tested again with just the VFO and an o-scope and terminated the VFO with a 50 ohm load.  For about a 43%-ish duty cycle, it's going to take between 11 and 12 volts output out of the VFO.  No problem - it'll do that.  So, after I re-terminate the drivers with 50 ohm inputs and feed it with coax, things should behave better with a 'stiffer' driving current that's more immune to RFI.

Jon

sounds good!  Just do make sure that there is a resistor between the driver input pin and everything else.  500 ohms is also just fine on 75 meters.  It's a non-critical value.

Regards, Steve

Ok, thank you.

Jon

Steve,
I'm building your 400w 80m transmitter; I note on the schematic on the class e site you don't have those 300 Ohm resistors shown between the input of the driver FET. I have been following the circuit pretty much to the letter and have not put them in,but as yet I'm not in a position to test as there's still work to be done on the RF deck.

Justin

Hi Justin,

From what was stated by QIX Steve  in the   ...   Starting 24 FET ...   Thread,   there is an UPDATED schematic that does show these resistors:
http://classeradio.com/24_fet.htm

Here is the Link to the other discussion:
http://amfone.net/Amforum/index.php?topic=43529.0

FWIW,   GL,   Vic

Cheers Vic, that explains it. Will also put them in

The mood finally hit me to start tearing into this transmitter.  I mounted two SMA connectors near each phase's driver input.  There was already a tapped 4-40 hole on each side of the heat sink for them.  It's a sign.  It was meant to be.  Ordained from God.  Angels descended and sang as I tapped them in.

I still need to replace the blown side of phase B and add the 50 ohm resistors to the SMA connectors.  AND, that slow boat from China is still chugging its way over here with my two coaxial jumpers.

Jon

Surgery is complete.  Testing tomorrow.

(just so I said it... stuff is laying on the output network only for the gate drive testing.  Full output testing will be done a little more prudently)

Jon

Ok, I think I may have found the problem.  

Firstly, the transmitter is working once again.  I spoke into the microphone at 300 watts carrier and the TX audio in my headphones was clear.  

When I dekey the drain relay, I'm drawing an arc.  I can hear the arc as well as see it on my drain DC meter.  The meter is located at the transmitter post-relay, and it jumps about when I hear the sizzling.  I was experimenting with increasing the duty cycle of the gate drive and found that as I increased the duty cycle, the arc got worse.  The longest duration lasted several seconds, so long that I had to jump up and run over to the drain power supply and turn it off.  

I had inserted across the relay an RC combination to mitigate this when I noticed the arc before, but it doesn't totally work.

I guess I could just put in a vacuum relay.  That's probably the simplest and best solution.

Jon


*** And I just realized what I'm doing wrong.  For this new setup, I'm not cutting off drive to the gates and the FETs are always 'on'.  There's a low resistance path to ground on the opposite side of the relay from the DC supply.  It continues to try like hell to pass current.  I'm going to install a sequential relay to cut off the 12 volts DC to the IXDD's and see if that helps.

*DON'T* cut off the drain drive until AFTER the DC input to the RF amplifier has fallen to zero.

This is a sequencing problem, and it is absolutely necessary to solve it or there will be failures.

So, when going from transmit to receive, the modulator needs to be put into a state where it generates no output first.  The drive stays on.  Then some number of milliseconds after that (maybe 200ms or so - give the modulator plenty of time for the output to fall to 0), stop the drive to the gates of the RF amplifier.

I don't remember (or maybe I didn't ever know) what you are using for a modulator.  If you're using a PWM type of modulator, stop the pulse train.  That will reduce the modulator output to 0V real fast.

If you're using something else such as an analog modulator, you could use a couple of IRFP260Ns in parallel as an electronic switch.  The devices are put into saturation when you're operating, so the loss is negligible, and put into cutoff when you're in receive.

There are different ways to handle it, but the bottom line is - the drive STAYS ON until the DC fed to the RF amplifier is at 0V.

Regards,  Steve

I'm using a Heising modulator for this transmitter.  The audio amp is a commercial unit which provides 8 (or 4, I forget) ohms to the Heising circuit. 

Everything is sequenced using a mechanical sequencer.  Timing is dependent upon hand motion.  Ha, ha. It goes IXDD 12 VDC and antenna relay, drain voltage, then modulator cut-in.  It goes in reverse order from TX to RX.

Sequencing is there, but during my testing phase this time, I forgot that I kept the drive on all the time.  That gives the drain power supply ample incentive to draw an arc and keep it going.  I've now put the IXDD power supply in the sequence. 

I'll test everything again later on today when I won't propagate (if I get back from going out in time).  If not, I'll just blast over a QSO.  Just kidding, I won't.

Jon

The transmitter is functional again.  The arcing is gone with the gate driver power supply sequenced and the audio is clean in the headphones.  In the picture is the gate waveforms for a little over 300 watts carrier.  This was transmitting into a dummy load, and it's not tuned exactly as it should be, but it's close.

Next is to try the dipole again.  I need to do some work on my tuner before I do that test, but at least it's generating RF again (rather than smoke and fire).

Jon

Jon,

    Very good on the class E rig, and repairs done.

I see your Hantek scope. I have the 100Mhz version otherwise just like yours. There is a Hack to make these scopes 200Mhz:

http://www.eevblog.com/forum/testgear/hantek-tekway-dso-hack-get-200mhz-bw-for-free/msg448551/#msg448551
and
https://www.youtube.com/watch?v=Mtt-g5putLY

I have not tried this, but just passing it on.

Jim
Wd5JKO

I raised my dipole ends and my tuner is now good to go.  Tomorrow I'll make tuning adjustments during the daytime.  

I don't think I'm going to touch the innards of my scope.  It's my first digital one, and I like them much better than the analog ones. I don't want to risk breaking it.  Besides, my highest measurements are at 7 MHz so it'll definitely handle that.    

One final thought.  In this new transmitter configuration, the VFO sits on top of the transmitter.  Since the 'enclosure' is made out of plexiglass, I thought that the RF fields might wreak havoc on the high quality Chinese construction of the VFO.  So, I put a copper plate underneath it and grounded the four corners to the threaded rods.  It seems to work well.  I didn't even want to try it without any shielding.  I'd probably have 8 blown FETs had I done so.

Jon

There's a lot to be said for good housekeeping with wiring.  I thought that my coax run was as it should be, but things were switched everywhere and I couldn't tell what went where.  Throw in an MFJ tuner with a bad antenna selector switch, and a coaxial open and it got weird.  I couldn't get a good match with my new dipole, and the E rig drain waveforms wouldn't come into spec. 

I pulled all coaxes and started from scratch and I now get a 1:1 match on my dipole at 3.960 MHz.  AM is frowned upon up there, so I need to adjust my coax to move back down with 'you guys.'  Joke.

Jon

Well John, theres a lot to be said about the " zoro beat " when answering a CQ or checking into a net. Lots of fun, ' specially when one has a thick skin.

KLC

True, I can zero beat and they’d probably never know. It’s an interesting fact that just by adding 2 feet of wire, IQ goes up by a factor of 10. Skin, eh, that’s another story.

Jon

Yo Jon, I read this thread from top to bottom. I think that since your are always building something maybe a new title for you would be something like;

"Class E Adventures With Jon!"

I also like the way you build and layout your class E stuff and the tube transmitters that you have build in the past. You get my Blue Ribbon Award :D :D :D

 

I now have a good match but the waveforms aren't quite there yet.  I've had them darn near perfect with this rig before, so something's up.  

Skimming the book of Proverbs again recently, it caught my eye that you can't have thin wire as a gate bus.  On my old configuration, I used short pieces of #12 solid copper.  This go around I used a piece of #18-ish gauge wire for the 2-FETs per gate bus.  I'm thinking that's where the problem is coming from.  

This time I'm going to add on flat copper to connect the two gates.  

In the meantime, here's a link for you plate modulated guys:

https://www.youtube.com/watch?v=C-nqMcWeTO4 (https://www.youtube.com/watch?v=C-nqMcWeTO4)

Jon

I finally got around to it. I put 1/4” x 1” copper strips in between each gate pair.

I’ll test a couple weeks from now. I’m redoing the shack with a new desk. I’m going to use Panduit as a way of wire management rather than the rats nest method.

More desk space, neatly routed cables... holy cow!

Jon

Here's the start of the shack re-do.  I'm trying to simplify and make it a cleaner look.  As a result, I'm doing the office desk thing with a "class E corner" (otherwise known as Plexiglass City; like the song https://www.youtube.com/watch?v=Rbm6GXllBiw (https://www.youtube.com/watch?v=Rbm6GXllBiw).

The rack stays the same with the DX-60/3-500 linear.  

There's an Alinco DX-SR8 with the remote mountable front panel on the desk under the lamp.  

On the other side of the room is a 3' high pile-o-wires that I removed from the original setup.  I'm trying to put back in as few as possible.

Jon

Neat oh, you should be on the cover of The Rolling Stone, er, I mean QST  ;D ;D As much as I love the old clunkers, I think I'll build a 25 watt class E rig one day. Why 25 watts, I'm listed in Dummy's 101 book with this stuff, but as QIX had said or I think he may have said that it's easier, to build a class E rig over a tuber radideyoo..........

He's wrong.  No offense.  Tubes can take blasting and destroying until you get it right.  Mosfets, not so much and the tiny soldering spaces are a bother. 

BUT, and this is the best part, the smug factor is out-of-this-world!  I literally walk around sniffing my own farts after I built the 8-FET rig.  And now I'm doing the modern desk thingy, like I live in California. "Yeah dude... and it's got an E-rig."

That would make a good QST cover:  Sniff your own farts! 

I'm joking, but really, this E stuff is where it's at.

Jon

Transistors have ratings
Tubes have guidelines!


My Use net tag line, circa Y2K

Pretty much true, even today.

--Shane
KD6VXI

I've literally built over 100 transmitters in my life - both tube and solid state, and hands down, it's easier (at least for me) to build a class E rig than an _equivalent_ tube rig - for sure - once you get over a hundred watts or so of carrier power.

Tube rigs are great and all, but there just are a lot more things to deal with - powering up the cathode/heater, the high voltage involved, possible forced air cooling, lots of metal work, special and often large components, the cost of parts, etc. etc. etc.

Even with experimental class E transmitters, failures are very rare if you pay attention to the details such as proper sequencing, good gate drive, no parasitics, etc.  and live by the discipline that you do not proceed to the next level of testing until all of the underlying, dependent tests meet the design criteria and pass.

I have built transmitters upon which I performed more than a week of testing and verifying every parameter (and bringing things up very, very slowly - rechecking everything along the way) before reaching full power.  All of this testing - because the design is experimental or unproven - and absolutely every possibility must be considered, verified, and even the smallest anomaly corrected before moving on to the next level or step.

This attention to details really does pay off, but doing so does require a lot of personal restraint - there is always SUCH a temptation to skip the tests, and go for all of the marbles right off the bat  :D   In my experience, this almost always results in disaster.  I follow the same procedure with tube rigs - I've had plenty of failures on the tube side as well - when I got careless, but great successes when I refrained from moving ahead too fast.

Yea, it's a pretty conservative approach, but I'm an old person - what can I say ?   ;)

Making the gate connections with flatter, heavier gauge copper seemed to have helped.  I made the change and finally got the rig wired up again on the new desk and the waveforms look better.  Here's both drains at 400 watts carrier.  I'm going to run it at 300 watts just to be a little more conservative (ha).  I'm tired of rebuilding the heatsink assembly whenever I do something that causes a catastrophe.  

I changed my dipole antenna to include a 1:1 current balun at the feed point.  I'm not the core winding expert, so I chose to go with the good people at Palstar.  Their 4000 watt rated balun (B4000-1:1) sells for $60 at HRO.  I mounted it to a teflon cutting board, and drilled holes for the wire and mount supports.  

Jon

One thing that may be of great value is a very good overload shutdown system.  This will save *lots* of time when you're experimenting.

Also make darn sure there is no connection between gates of devices - that is, if a gate to drain short occurs, that there is NO WAY for any gate voltage to get to other gates - like through drivers, back to the driver power supply and to other drivers, etc.  All driver power should connect to a common point, and that point should be protected with a transzorb.

Another thing about an overload system is that it shuts things down very quickly if there is an overload.  The idea is to shut things down faster than any device can fail.  For standard MOSFETs, this is about 100uS

I do have a driver power bus, so adding a Transzorb definitely is a good idea.

I’ve looked at your overload protection board to see if it could be applied to a Heising modulated system, and it can. Now that I have my shack reconfigured exactly how I want it, I’ll probably get one from you at some point this year. I just need to add it to my to-do list (not all ham radio related).

Jon

You'll need a series element that will carry a lot of current, and will switch quickly.  I have used IRFP260N MOSFETs for this purpose, connected as a source follower - between the modulator output and the RF amplifier.

At some time, I designed a switch circuit for adding to existing analog modulators, to provide circuit protection for both the modulator and the RF amplifier.

I'll see if I can find it.  Been quite a while!

That would be awesome!

I can make a source follower for the modulator. But Steve, can’t you do that using tubes?  :)

Jon

What is ham radio without experimentation (and failures)? 

I heard George in Tucson call CQ on 5 watts last night.  I couldn't help but reply.  I spoke lightly so that my modulation peaks wouldn't go over a KW and everything was working fine for several rounds.  I'm not happy with the phase 'b' waveform and something is still amiss with it.  George was operating a Heil pine board transmitter that he built.  It sounds really good and I could hear him pretty well in Phoenix for 5 watts carrier.

Anyway, my 4 year old son doesn't understand moderate voice level.  He got excited talking into the mic, and on my last go around he did a close-talk shout.  You know the routine - fire shoots from a FET on phase 'b.'  George, if you're reading this, I had a crap out.  That's why I didn't come back to you.

My plan is to go back to the original design of the rig.  I was able to give my 2KW wattmeter a good test without any problem back then.  Each FET had it's own driver.  I had 8 FETs and 8 drivers. 

I'm also going to add an aluminum shield between the output network and the heat sink assembly.  I think that's causing some problems as well since 'b' side is closest to the transmitter output. 

Jon

My advice to new class E builders is to make the heat sink assembly removable.  It has paid off immensely for me.  This is the second time that I've decided to do a total rebuild of the solid state area, and all it took was to unsolder a few wires and unbolt two nuts.  It was an afterthought when I built the rig.  Turns out it was an epiphany greater than the flux capacitor. 

Jon

The thing is, you shouldn't have failures.

Failures such as you are having almost always point to a parasitic, assuming the devices are rated for the job of course - voltage wise and current wise.

For standard MOSFETs, at carrier, the voltage should peak up to no more than 1/4 of the device's voltage rating.  At carrier, the current should be no more than about 1/10 the device's current rating.

Assuming all of this is met, look for parasitics.  Most readily observed by watching the drain waveform under (particularly very low frequency) modulation.  The parasitic will show up as "grass" rising above the otherwise normal class E waveform.  The parasitics show up better when observing the drain waveform as an audio "envelope" , rather than individual RF peaks.

Sounds like your audio limiter needs attention

I've been able to hit 2kw peaks with this rig, so the audio chain isn't the problem.  It is the waveform on side B.  With reconstruction and adding the shield from the output network, I think I'll be back in business. 

I'm building the rig the way I did the first time:  8 drivers/8 FETs, driver bus (even though I've heard they're bad), ground bus in between the driver bus and the drain bus. 

I had some planned building time today for this transmitter, but I ran out of mica insulators.  ...something so simple.  Anyway, I'll order more.

Jon

I just finished the heat sink assembly this morning. I’ll install it back in the transmitter sometime soon.

Jon

Installed and ready for testing!

https://www.youtube.com/watch?v=YBC1Qob27sM (https://www.youtube.com/watch?v=YBC1Qob27sM)

Jon

I did my first test at 13.8 vdc on the drain and here's the waveforms at around 50 watts carrier.  Side B (the blue trace) looks cleaner than before. 

Jon

Jon,
Just a suggestion.  Plug a flash drive into the USB port and then perform a "save to USB" function, the button just left of the trigger level control. Set both traces to the same Zero point and vertical Gain for about 2/3 scale of the traces for best comparison.  Convert file from bitmap to .JPG and post it. Then we can have a critical look at it and make some suggestions.

73s  Nigel

Thanks, will do!  I guess that would be better than a fuzzy, zoom-in picture. 

I'll be in Vegas for a few days so hopefully I'll get to it on Saturday. 


Jon

Here's an image of the drains at 13.8 vdc and I had to modify the drive from the VFO to clean it up to look like this.  I think the amplitudes are actually the same height.  I'm using mismatched probes.  I took the channel 2 probe and moved it over to the other side, and it was in fact at the same amplitude.

The other picture is a snippet from my Vegas trip.

Jon

don't want to say what I like more, your scope pic or the other one :)

https://www.pinterest.com/pin/553590979188484905/ (https://www.pinterest.com/pin/553590979188484905/)

Jon's wireless station :D :D ;D ;D

I though what happened in Vegas stayed in Vegas?

The vegas pic has been viewed a lot more than the scope pic ??? ::)

I'm just posting the G rated stuff, like this one.  The other stuff stays in Vegas. 

I got new matching probes and the amplitudes are the same.  This shot was taken also at 13.8 vdc on the drains.  I'll do an atmospheric dummy load (75 meters at noon) test sometime soon at full power. 

Jon

Scope pic quality looks great Jon, much better. Can you post another with the same zero point for better comparison between phases.  They should be identical in width and amplitude and exactly 180 degrees in phase which may take some minor tweaking to achieve. But they look fine to me and should be OK at higher power. Bring it up slowly and watch for abnormalities.  It could be you got some RF into the VFO buffer/driver chain when modulating at high peak levels.  That can cause blown FETs if both phases are driven "ON" simultaneously.

73s  Nigel

Here are the two phases in comparison to each other.  During my explosions, I didn't have the DDS VFO set correctly at all.  It wasn't even in the ballpark.  After zooming in to look at the phases, I got it to where it is right now.  Maybe a little more tweaking is in order?

Jon

Jon,  Looks good to me, should be OK for higher power testing. BUT I have no practical experience with that design so defer further comment to those who do.

73s  Nigel

Here's 400 watts carrier into the dipole.  It looks a little noisy.  I'm not sure if that the proximity of the scope probes to the RF or the real drain waveforms. 

Jon

May be probe ground wire inductance, Try to ground the probe VERY short

Hi Jon...

Will throw in my 20 cents if you like...Steve is the best one...
Also noticed Bruce KZ1F is around...he has build many Class E TXs..
Have not seen any Gate Waveforms so will take it there OK and your phasing between modules are 180 degrees apart....
Looking at the waveforms I don't feel your at maximum efficiency as there is overlap...
This is important as Class E was developed because of the horrible FETs at the time to remove the Miller Capacitance which acts against the Gate drive etc:
I would reduce the Power Level to appox half and get all this right before applying operating power....
Again do all your testing at a lower power and also check your efficiency which should be 90% + when tuned correctly....
Remember test test test to get everything right..or it might go bang ...  :o
Is your main series output capacitor 500pF...as what Steve has on his WEB site...

I notice too that your still using door knobs for your shunts capacitors....don't think there quite suitable for the task .....

I will hang back for them to comment...


Wayne

Hey Wayne!  Good to hear from you.

You're right, and I haven't looked at the gates under drive.  There could be an issue there.  The waveforms look absolutely clean at 150 watts.  Around 200 watts and above they start to get dirty.  Also, at 400 watts my home security system starts to freak out.  It's very loud and every few seconds will alarm while I'm transmitting.  I think that's due to the system being in close proximity to the antenna though.  We'll see...

I'm hoping that the drain bus isn't coupling to the drive bus at high power levels.  I put a ground bus in between to prevent that.  It worked the last time.  It's a similar concept as a grounded grid linear... no neutralization required!  Looking at both a drain and gate waveform at the same time should tell me as I increase power.

I will test more at lower power levels and see.

Jon

Hi Jon...

Looks like Vegas is the place to go to clear the brain and have a cuppa  :P

OK Class E TX....

Could you post a photo of your Gate Waveforms without any voltage on the drains of the FETs....both phases one on ch 1 and the other ch 2...
You need to check the waveforms on all FETs...they all should be similar....
Have attached a photo of one of mine some time ago...

The DC should be appox 43% at the 10 volt mark to ground as per the picture...
The waveforms should be similar to mine...nice and clean..

Also the 12 volt supply to the IXDD drivers....what type of supply are you using .... analog / SMPS ?
They need to be bypassed with a say 10uF and a good quality .1uF right at the Vcc pins of the IXDDs......


Wayne

Also too check that the IXDD are working as a team  :)

Place your thumb hard down on each IC feeling the temperatures are all the same.....
None warmer or colder then the others....all the same....


Wayne

Here's the gate drives (for two FETs) with no drain voltage applied.  The duty cycle looks acceptable, but the noise is present here as well.  I do have some commercial 80m low pass filters that I can throw inline with the DDS VFO output to the transmitter.  Maybe that'll clean it up a bit?

Jon

Try again with a VERY short ground lead of the scope probe. May be resonance in the scope leads. I use fuse clips a lot directly soldered to GND where the ground of the scope probe fits in, so no ground lead at all.

Here is a nice vid,    Oscilloscope Probe Ground lead length affects on signal quality

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

klc

Hi Jon...

The DC is different on both.....one quite low maybe just under 40%....

What type of power supply are you using to the IXDD...
Can you place a probe right at the input pin to them and take a photo and post...

There is often high frequency switching ripple a bit like a sawtooth waveform present..
If the ripple exceeds 1 volt PEP thats too high and has to be filtered better...

Steve uses LM2576 IC quite close to the IXDD....there good regulators....

For testing purposes a small motorbike or car battery will remove that ripple and give you a better idea as to what it should be....


Wayne

I'm using a cheap, Chinese regulated 12 volt power supply that you get off of eBay.  I never thought, but that could be an issue. 

Here's another screenshot of the gates.  I put the 20MHz band limit on the scope and it cleaned up a little nicer.

Jon

...and actually, here's the gates again using a 20 amp Radio Shack switching 13.8 vdc power supply.  It seems to look even better with this than the Chinese power supply. 

I did notice that the Chinese power supply dropped from 12 to 10 volts under load whereas my Radio Shack power supply is rock solid.

Jon

...and here's the drains at 400 watts carrier.  Thanks Wayne!

Jon

https://www.youtube.com/watch?v=L09qnRfZY-k (https://www.youtube.com/watch?v=L09qnRfZY-k)

Sounds like your chicom supply is folding back.

--Shane
KD6VXI

Hi Jon...

Looks so much better....

The Power Supply regulation to the IXDDs cannot be understated.....it has to be perfect really....
And has to be checked at the Vcc pin of the IXDD not the output from the Power Supply...

So your testing hasn't finished yet .... sorry  ;D

You have 8 drivers and 8 FETs ..... all need checking .... you know waveforms etc: etc:

If one is loafing check it out.......

Also can you tune the output for maximum efficiency .... you have never talked or mentioned about that.....
Steve has a efficiency meter probably made up because some could not find the spot....
Its important too not because it gives more power at less current but cancels out the Miller capacitance that acts against the input waveform causing DC change etc:

Its worth the time to go over Steves WEB site to refresh things etc:

Well hopefully its all OK now...

Ohhh what is the efficiency now ?   are you getting 90% + appox .....


Wayne

I just looked at your gate waveforms again ... the one with the 13.8 volt SMPS....

The volts per division seems at 10 ..... this would mean that you have 20 volts on the gates....

A little high but probably OK being a pulsed wave  .....

12 volts seems to be the way to go.....

Anyway I'm sure your aware of that....


Wayne

I did mess with the capacitance adjustment on one probe so that's not reading amplitude to spec.  That may be the phase you're looking at.

Anyway, I will do an efficiency measurement.  It's only as accurate as my analog Daiwa wattmeter reading.  Everything else I can measure with a multimeter and is accurate to several digits.  But, it'll be in the ballpark.

But, not tonight... the wife and I are attending a Russian dinner party.  Priorities.

Jon

The drain waveforms look good for a too lightly loaded RF amplifier.  You should have a little "back porch" on the drain waveform ideally.  This usually indicates the best efficiency as well.

In other words, run with the least amount of loading you can get away with before the efficiency starts to drop off.  The loading should "peak" the RF output at proper loading.

This will also help keep the drain waveforms lower.

Otherwise, everything looks quite good !!!

Thanks!  Running the rig at 300 watts (my intended operational level), I'm getting 90% efficiency.  That's with the loading capacitor fully unmeshed.  I have a fixed amount of capacitance in there with a doorknob, and can't go any lower.

90% is good enough for me, and here's the waveform at that level.  Any more and it's just bragging rights.  Who cares about that?

Jon

Hi Jon and Steve...

Yes everything does look quite respectable  :)

Been a large total post really ... 4 pages and many viewings.....


Wayne

The main reason for the back porch (proper loading) is not just efficiency, but also control of the drain waveform.  A heavier loaded amplifier has a lower amplitude drain waveform, which is always a good thing  ;)

Hi Steve..

Actually Jon's waveform is a little high at 200 volts with 48 volts carrier......

What I would do now since your still at this testing stage is make it 100%....or as close to it...
Replace those shunt capacitors and play around with the loading capacitor to obtain the efficiency spot etc:

One day you might miss tune or your antenna could fall off  :o and things happen fast at that power level....

Anyway its going better now...


Wayne

I need more inspiration on what to do next.  I'm heading back to Vegas tomorrow for a few nights. 

Jon

Ok, so efficiency does matter.  I'm getting distortion on voice peaks, so I think I need to get them there waveforms a little smaller. 

I was never able to judge where my loading capacitor was set since it didn't have a dial.  Wandering Walmart, I found a compass and tore it apart.  It'll do the job.

So now begins the painstakingly long process of tuning for maximum efficiency.  Nudge the loading capacitor, tune for the power output I want, calculate everything, then redo a bunch of times.  I did this for my single FET 40 meter transmitter.  It too was getting distorted voice peaks.  It wasn't tuned properly, so I did this process and it fixed it.  Now I can blast away into the mic.  How I got both of these transmitters working the first time is beyond me.  I'm learning much more now about operating class E rigs than I did the first time.  Beginner's luck.

Jon

Hi Jon...

Is the audio OK now or do you still have a problem....
Finding the efficiency point is sort of similar to learning to drive a manual car....getting the clutch and accelerator working together nice and smoothly....
If you don't get it right its called kangaroo hopping over here....but that's another story...

When you figure it out you always seem to remember how its done... ;D

Not sure if not being at the efficiency point of your transmitter would effect the audio though  :-\

Wayne

I’ll find the efficiency sweet spot this weekend using my antenna and the 75 meter dummy load (the noontime sun). I just stopped at Sears and picked up an additional two digital multimeters for the process. Two will measure phase current and the third will measure drain supply. It’ll just make things go faster.

I also ordered an audio compressor to limit high excursions. My son gets excited and wants to scream into the microphone, so I need to minimize the shock to my FETs.

After all is said and done, I’ll do an audio check. Hopefully my kangaroo hopping is gone. We don’t have any nifty saying over here like that, other than it’s AFU.

Jon

Hi Jon...

Our whole language is full of " nifty sayings "
We call it Australian Slang or Lingo ......come to think of it those words themselves would not be in the dictionary.... :o
As an example...I would never say " I am going to drive the car now "
I would say " I'm gunna hit the road now " .... make sense of that....
Don't know where it all started from but for me from birth.. ;D

Back to AM stuff I suppose.....

FETs live happily as long as they operate within their specifications...although they can tolerate excess current to a point....
If your little son there screams into the microphone your TX should still be happy unless something exceeds things....
My guess would be your modulator ........ and would be worth running some checks....
You could get your son to scream into the mic again and set your DSO to trigger at some high voltage point as an example.....just a bit of mucking around really....

The thing with PWMs is if you set your Mark Space ratio to 60:40 you can never exceed 150% modulation peaks...so now you can calculate things....
But like what Steve said about that efficiency point too reduces or keeps under control the Class E waveform.....if it goes too high and with peaky modulation applied could exceed the 900 volt rating of the 11N90.....
Just guessing thats all......hope what I said makes sense.. :)

Maybe purchasing Steves PWM kit would fix all this up .....


Wayne

Wowsers..I didn't know that Ray Charles was an Aussie.


KLC

What about "Stone the flaming' crows!"   ;D

or "boiling the billy"

Thanks for all the comments everyone...

Was a bit worried I said the wrong thing.. :)

Ray Charles did have a song " hit the road again" ... I'm sure...no it was "on the road again"


Wayne

My analog meter board has convenient sampling points for the digital meters.  

I wish there were an easier way to calculate efficiency.  

Jon

Hi Jon...

Hopefully your output circuit is the same as Steves....that is the large coil / 500pF tuning cap and the loading cap of 3000pF......
I have never used all the meters that you have installed trying to calculate etc:

What I did by experimenting a while ago now was to use a low power level like 100 Watts....

So tune up for power output of 100 watts  ...... the tuning cap was just off maximum capacitance and the loading cap was nearly fully meshed.....

Then I looked at the power meter reading say the 100 watts and the current meter.....
By decreasing the tuning 500pF capacitor and decreasing the loading capacitor was able to giggle around until I noticed that the power output remained at 100 watts but the current dropped...

I won't carry on any more it might confuse things more for you.....

What I was trying to achieve was same power out with less current..

Steve does have an efficiency meter but have never used it......

If you keep giggling ... for me anyway it just happened and never forgot  ;D

I am sure he will pop up at some stage and add some comments that might shred more light on things....

Give it a try.......if successful increase your power level and repeat again....


Wayne

I bought an audio compressor and did a more accurate efficiency reading.  I'm getting 93.7% efficiency.  I don't have a digital wattmeter, but the Daiwa CN-801 is a pretty good meter.  I made my adjustments with the dial right on the 200 watt mark. 

I still had audio distortion and a little hum though.  Long story short, it was from the audio amp that I use to drive my Heising modulator.  Side A has distortion, but side B is crystal clear.  I'm guessing that something in the audio amp took a hit during one of my explosions.  Anyway, I'll just use side B and run everything very conservatively at 200 watts. 

Clean waveforms, crystal clear audio... 4 pages later and I'm calling this one good. 

As a parting note, this picture was taken at the wax museum in Vegas over the weekend.  It's a little cleaner than my other pictures, just like my audio!

Jon