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THE AM BULLETIN BOARD => Class E Forum => Topic started by: VE3ELQ on February 09, 2017, 12:02:20 PM



Title: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ 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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: WB5IRI on February 09, 2017, 12:15:29 PM
Absolutely beautiful, Nigel. Hope to work you soon!

Doug


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: KQ6F 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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: steve_qix 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?


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ 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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: pw fallon on February 12, 2017, 08:50:53 PM
Hi Nigel,

Can you post a schematic of the transmitter?

Thanks,

Joe (aka PW Fallon)
WA1IWQ


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ 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.  :D
73s  Nigel


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: W1AEX 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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ 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 modulated 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.

Reduce the 27k pull up R on pin11 to 10K and connect it to +12V

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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ 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


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF 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.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ on April 08, 2017, 08:57:46 AM
Some updated improvements to my 3 band RF deck.  The drivers were changed to 6  NCP81074BDR2GOSCT-ND drivers one per FET, running at 9 volts 1 regulator per phase with 6V input drive. They will NOT toggle with 5V input. These are very fast drivers with matched delays which allow the FETs to operate at their full fast switching speed minimizing the transition time through the linear range thus improving efficiency and upper freq limit. I have not been able to accurately measure efficiency but at 350 watts on 40/80/160 M the deck runs cold in class D so its way up there.  At a $1.25 each plus $5 each for the FETs makes for an economical high performance configuration. Couple pics attached.

73s  Nigel

Edit:  20/05/2017  Suppliers are now stocking the "A" version which toggles at logic level input (5V) independent of Vdd making driving them a bit easier.  Part #  NCP81074ADR2GOSCT-ND


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: W3GMS on April 24, 2017, 06:33:46 PM
So great to work you on 3.705 with your new rig Nigel.   It sounded fantastic! 

73,
Joe-W3GMS 


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ on April 25, 2017, 10:07:48 AM
So great to work you on 3.705 with your new rig Nigel.   It sounded fantastic! 

73,
Joe-W3GMS 
Likewise Joe, thanks for a very nice QSO. Thoroughly enjoying the AM experience, builders, experimenters, restorers, all know which end of a soldering iron to hold. :)    Compliments appreciated.
73s  Nigel


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: ND8D on April 30, 2017, 01:31:39 PM
Nigel,
What are you using for an HV supply on this?


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ on May 01, 2017, 07:40:23 AM
Nigel,
What are you using for an HV supply on this?

Andy,
The B+ is from a 120V primary multi tapped isolation transformer that I was fortunate enough to acquire in trade from a friend VE3GSI. Am using the tap that gives +160V into the modulator. The filter caps are 6  1,000mfd 200V electrolytic. Note that 160V is only about 10V shy of rectified mains so am about 1/2 done on a new 80M mono band TX based on this design that will operate with "off line" B+ and get rid of that big, heavy, hard to find, expensive power transformer. It should work, will know in a couple weeks.
73s  Nigel


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: ND8D on May 01, 2017, 02:26:52 PM

Quote
"off line" B+

Similar to the K7DYY Class D transmitter, It would be convenient but I would be lying if I said I wouldn't be nervous about the safety aspects. What precautions need to be taken when rectifying straight out of the wall?


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: steve_qix on May 22, 2017, 05:16:36 PM
I built an "off line" power supply / modulator, and have it at the Rattlesnake Island shack.

However, this one is implemented in a manner similar to a switching power supply - there is a high frequency pulse transformer between the rectified/chopped AC and the load side.  This completely isolates the output from the AC mains.

The whole thing weighs about 10 pounds - no power transformer.  Definitely more complex than a modulator using a power transformer, but much lighter.

Here is the circuit:
(http://www.classeradio.com/off_line_pwm_output.jpg)







Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: KQ6F on May 22, 2017, 07:44:56 PM
Steve -

I've been toying with the notion of trying something like what you have done but have the same safety concerns expressed by ND8D.  Can you share with us how you went about playing with the circuitry on the "hot" side of the pulse transformer?  Temporary isolation transformer?  Floating scope?  One hand in the pocket? Or what?

Rod


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: steve_qix on May 23, 2017, 12:35:57 AM
For testing and development, I used an isolation transformer (1:1).  Once the development and testing was completed, I didn't need the isolation transformer anymore, and all has been well ever since (quite a few years at this point).

The circuit, as shown, works quite well.  I have also configured an overload shutdown board into the completed modulator, and use the standard PWM generator that I use for all of the class E rig modulators.

The PWM filter can be made entirely from toroids, which keeps the size down.  In fact, now I'm using filters made from only toroids at the KW level.  Makes for a much smaller filter for sure.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: VE3ELQ on May 23, 2017, 07:22:00 AM
Steve -

I've been toying with the notion of trying something like what you have done but have the same safety concerns expressed by ND8D.  Can you share with us how you went about playing with the circuitry on the "hot" side of the pulse transformer?  Temporary isolation transformer?  Floating scope?  One hand in the pocket? Or what?

Rod
Rod,
My new TX project is intended to run off a 240VAC source wired according to the 2011 electrical code.  It starts at the power company transformer outside as a center tapped secondary. White is the neutral center tap and black and red are the two 120V hot wires running 180 degrees out of phase. The neutral wire is at ground potential and is indeed grounded either at the breaker box or by the power company at the transformer. This can be confirmed with an AC voltmeter from neutral to the extra (for safety purposes) ground wire at the outlet, or the separate station ground, mine reads zero volts. Wiring in older houses prior to the 2011 code change is anyone's guess but probably the same.

Using two 50A rectifier diodes from both hot leads, black and red, feeding the filter cap with the white as ground, full wave 170VDC (depending online voltage), will be seen at the cap. An inrush current protector device and shorting relay should be included to protect the diodes.  That's the plan.  I have NOT yet done this. 

My project is in the final stages of bench testing and not yet in a box. Its currently powered from a 120V isolation transformer fed from a Variac as its prudent to bring things up slowly when testing new designs. I'm currently experimenting with different output transformers, different low pass filter designs, and a higher voltage modulator, and so far its all working FB. The last step will be the power supply as above.  The only stuff in my house using big iron 120V power transformers is my HAM gear and I hope to change that.

73s Nigel



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on April 17, 2018, 01:52:56 PM
Anyone else built Nigel's Modulator?


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: WD5BJT on April 17, 2018, 10:22:43 PM
Nigel,

Very sexey unit indeed. May I be placed in the queue for a unit?
Keep up the great work. You give hope to us construction  challenged
hams.
73,
Charlie
WD5BJT


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on April 30, 2018, 08:38:21 AM
I ask as I can't get the modulator to work and repeated mails to Nige have gone unanswered, NIGEL! ::)

It's behaving a bit wierd as I cant get the bottom FET to switch.

At least I think that's what's happening?

If you set the duty cycle to <50% it will NOT mod upto 100%.

I'm sure there's a mistake on the schematic and quite possibly a mistake in my layout altho' I have gone over it a 1000 times!

If I can get it to work properly I'll release the design so you can make your own PCB's (with Nigels permission of course!).

It's quite neat, about an inch square.

J.





Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: w9jsw on April 30, 2018, 10:07:01 AM
John,

As I mentioned in my email, I am pretty good with pcb work. I would be glad to take a look at your schematic and layout and give it a second set of eyes. Could be a translation error on inputing the schematic into whatever tool you use or could be a layout error as well.

Just before I saw your post, I was looking at the hand-drawn schematic and thinking how easy it would be to make a pcb for it. I sometimes find it easier to do the pcb first, build it, then revise rather than building it up dead-bug or on perf board...

John - W9JSW

on edit - did you see the change where pin 11 is changed to a 10K pullup to +12? Is the enable line being properly controlled on the IR2110?


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on April 30, 2018, 10:43:23 AM
Thanks John, well it's not 'my' schematic, it's Nigels.

Have attached a pic of the top of the pcb, then only connection on the bottom layer (in blue) is the 6V to the 'tune' switch.

I've used a SOIC version of the IR2110 so the pinout is different, otherwise the design is the same as the one in this post, minus the ripple null business.

Large electrolytic fitted across the VMod and GND and a small PWM VReg fiited (50V to 12V) in space to LHS. L06 is a 6V reg and Q1&2 are 2N7002's, D's are 1N4148's, C's are 0u1's.

The missing pullup to EN is external.

Let me know if it makes sense!

OK!


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: w9jsw on April 30, 2018, 05:52:24 PM
Didn't you have to create a copy of his schematic in the PCB program to create the components and nets to build the PCB Gerbers? That is the schematic I am referring to. I want to compare that one with the hand drawn one.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on April 30, 2018, 06:21:51 PM
No netlist, straight to PCB from schematic, only a handful of components. The layout is the same as the hand drawn schematic I trust, just repin the IR2110 and check thru'.

I'm hoping the thread will allow the author to comment...

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: w9jsw on April 30, 2018, 10:16:20 PM
Ok


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: KD6VXI on April 30, 2018, 10:26:08 PM
What software did you use to go straight to board?

This would save me SO much time with the cnc!

--Shane
KD6VXI


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on April 30, 2018, 11:53:18 PM
Hi

Just a couple of comments...
I use half bridges all the time and Nigels design seems fine and should work as is...
Although I would change the pullup resistor to pin 11 of the IR2110 to 4.7K .....from 27K...

I would check pin 11 to make sure that it pulls to ground to enable...
Also if there is a 5 volt pulse train on pins 10 and 12 180 degrees of of phase there should be output signals....when enabled....
Just for testing wouldn't apply 160 volts to the FETs just say 15 volts or so....
Although IRFP260 will work I prefer to use IRFP250s ... half the gate capacitance and makes life all round a bit easier  ;D

Wayne


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 01, 2018, 12:13:31 AM
Hope you don't mind these comments...

Is the Bootstrap diode installed...?
It has to be a fast switching type like a uf4007 diode......definitely not a 1N4148  :'(

If that is not installed then the upper FET will not have enough voltage to be held in the ON state...requires 12 volts between the gate and source if you know what I mean...

Wayne



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 01, 2018, 02:47:11 AM
Comments welcome (Nigel!)

I use EasyPC to layout, have since the DOS days, so somewhat familiar.

Ok circuit wise, EN is pulled low, there's no circuit around it and as mentioned the pull up is external,  currently hard wired low.

Hi and low logic levels present, as is the bootstrap, the 4148 is actually fine as I'm only using 48V, I've tried 3 different diodes, nothing amiss around the top FET.

I think that covers everything.

To be clear, I have the circuit working fine but ONLY at 50% (+) carrier. If you run at 40% it will not mod upto 100% as there's a gap in the pwm stream and the peaks flatten prematurely.

Most confusing and something very simple I have overlooked?

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 01, 2018, 07:39:40 AM
Hi John...

The bootstrap diode...what is it your using ?
I use uf4007 fast recovery diode.....

The gate resistors are they 4.7ohms and 16 ohms and not Kohms ?
If their Kohms then the gate capacitance of the FETs would not have time to charge/discharge...

The best Mark Space ratio is 60:40.....that is if you have 100volts on the upper FET drain than the carrier voltage or output voltage should be adjusted to 40volts..
In your case by the sound of it you have 48volts on the upper FET so your output should be adjusted to 20volts appox...

Can you do this ?


Wayne




Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 01, 2018, 07:54:14 AM
The bootstrap diode is the one that goes from 12 volts to Pin 6 of the IR2110 driver IC...
Also has a 10mF although I generally use 100mF Cap with negative terminal going to Pin 5...

If there isn't 12 volts between Pins 6 and 5 then 100% modulation would be hard to obtain...
That is the upper FET could not be turned hard on and there would be no headroom available or runout of headroom during modulation positive peaks...if you get what I mean....


Wayne



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: w9jsw on May 01, 2018, 08:31:15 AM
I found no errors on your board layout...


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 01, 2018, 08:33:28 AM
The bootstrap is working and a 4148 works fine, Yes I've used the UF4007 but it's thru' hole and I've actually used a smt equiv, cant remember what now but ALL 3 diodes give the same result!

11V4 across the BS cap...

Why anyone would drive power fets with KOhms res on the gates? VERY silly, it wouldn't work at all surely?

Yes 40:60, thats what I would like to acheive, however setting 40% would ONLY allow the mod to go to 90%, setting to 30% would ONLY allow 70% full mod, you get the idea?

J.

Thanks for checking it over John BTW :)


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 01, 2018, 08:59:40 AM
OK well I will ask another silly question......
The squarewaves from Pins 1 and 7 are they at 12 volts.....?

There is a reason I'm asking because I always drive the inputs with 12 volts and apply 12 volts to pins 9 and 3..
Nigel has dropped the Vdd voltage to 5 volts by that voltage divider there....and drives the inputs at 5 volts....
Have never done this myself so just wondering what the output amplitude voltage is from pins 1 and 7.....
Just finally before I goto bed....
Do you have 48volts on the upper FET and can you adjust the MarkSpace ratio so the output is 20volts...
Just checking..... :)
Are the negatives OK?

Wayne




Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 01, 2018, 09:34:36 AM
Hi Wayne, thanks for your interest too, apologies if I say question is silly but you never know what I may have done!

Yes the mark space ratio can be adjusted infinately, but if setting to 40:60 (or anything less than 50%) the the bottom of the waveform clips before the top reaches well...the top of the waveform.

So you just get a clipped sine if running less than 50%, I'm not measuring DC and inputing a 1KHz sine into the mod.

I'll check the gate drives.....

with 20V Vcc

Lo Side = 11V7 p-p

Hi Side 31V2 p-p, it's floating of course...

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 01, 2018, 07:53:54 PM
Hi John...

OK I've got it now...
It seems as though the problem is with the negative going part of your waveform.....clipping ....
Nigel uses a 16 Ohm resistor there to prevent shoot thru....
With the IRFP260 I have used both 4.7 Ohm upper and lower without any problems .....
A 16 ohm would just give you more dead time and not contribute to the problem you have....
So I feel like you do ...the output stage is working OK... :)

What do we do now....it gets a little tricky...

I'm not an expert here just a guy like you mucking around....but have built a few PWM and overtime I suppose learnt things...

The problem is the LTC6992......how can you check it ...

I don't use these ICs for 2 main reasons...
I like my reference oscillator to be crystal locked and prefer triangular waveforms then sawtooth...

So I use a 4060 chip with a say 8Mhz crystal and pick off 250Khz ... then into a triangular waveform generator ...then into a comparator LM311n...

Now I can check things.....I like probing around with my CRO..
The Comparator is very important part ....everything has to be setup correctly....
Also the Mark Space control goes into the Comparator .....

Look at what I've done ... rattled on and on and on without trying to give advise of what to do next....

What I would do is check that the audio stage is not clipping at pin 1 of the LTC6992...

Will print the specs of the 6992 and have a read up on it...

Sorry for the long post...


Wayne





Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 02, 2018, 12:00:14 AM
Hi John...

Just learnt something......
Been googling around and found this WEB site.....

https://www.nuffzedd.com/power-supplies-demystified/switching-basics/

Figure 2 ...... this might prove that the output from the 6992 is working OK.....certainly worth a try anyway and might identify or eliminate things etc:

Would leave Nigels frequency as is and disconnect the output to the driver and pin 10.....
Install the resistor and capacitor to ground and audio sweep the input to say 10Khz ..... just for a bit of fun....

Give it a try  ;D

Theres a slight change I would make to Nigels design but will comment on that later...


Wayne



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 02, 2018, 07:13:52 AM
Thats great info Wayne, sure I will have a look and report back.

Re gate res, I've actualy got mine the other way round, less ringing etc.

I'm building a sequencer also, has to be the same size as this pcb and theres a lot going on there too.

Delay circuit, LM3914 bargraph SWR indicator and fault O/P, 7KHz LPF, I limit etc etc, quite a busy board!

I'll have a look at the LTC O/P.

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 02, 2018, 10:38:54 AM
OK, It's the O/P from the LTC, so if you set the carrier to less than 50% the bottom of the sine will clip.

So I'm 'presuming' the circuit doesn't actually work correctly unless fixed at 50%?

Thanks for all your input chaps.

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: WD5JKO on May 02, 2018, 01:46:45 PM
OK, It's the O/P from the LTC, so if you set the carrier to less than 50% the bottom of the sine will clip.
So I'm 'presuming' the circuit doesn't actually work correctly unless fixed at 50%?

    Perhaps the intention is to allow for the male human voice being asymmetrical such that the negative going is limited to less than 100% and the positive going has the freedom to migrate up past 100% to perhaps 120% or more. This would require proper audio phasing. The QIX low level analog circuitry has a negative peak limiter as well prior to the PWM. If you are using a phase rotator (W3AM) than perhaps it's best to set carrier to 50% since the audio will be symmetrical.

Jim
Wd5JKO


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 02, 2018, 04:19:48 PM
As explained you can only set the carrier to 50%, then the + & - peaks will clip symmetrically. If you set the carrier to less than 50% then the negative will clip before the positive reaches the full voltage, i.e. it doesn't work for anything other than 'normal' 50% carrier.

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 02, 2018, 06:21:51 PM
Hi John..

When you connected the resistor and capacitor and applied audio did the output waveform show the negative clipped ?

Can you post a photo of that if possible...

Do you have Steves QIX PWM generator board ? just for testing purposes for the rest of the circuit.......


Wayne




Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 02, 2018, 10:26:49 PM
Hi John...

I don't believe there is anything wrong with Nigels LTC6992 design...
It should work fine and I feel it hasn't been setup right....

The last thing I want to say is that his circuit is no good....
All his work is top class and he has successfully got his modulator to go well etc:... :)

To set the DS to 40% 400mV has to applied to the MOD input Pin 1...
Your audio sits on top and peaks cannot exceed 1 volt otherwise the DS exceeds 100%.....

The LTC6992-1 seems to cause a cutoff of the internal oscillator if the DS exceeds 0 or 100% by a large amount.....the LTC6992-2 doesn't ....experimentation of that would be interesting...

So what I would do now is play around with the input levels being careful not to exceed 1 volt at pin 1.........

Again the last thing I want is for Nigel to feel I'm saying his design is faulty and doesn't work....


Wayne





Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 04, 2018, 08:14:40 AM
OK, here we go...

48V on Modulating FETs

Using 50:50 from the top of first image

1st image shows 50% duty cycle
2nd image shows sine in and out of modulator, nothing wrong here (ignore noise on waveforms, have used cursors for sine I/P)
3rd image shows symmetrical clipping when I/P is increased, OK, still 'normal'.

Using 40:60 from the top of second image

4th Image shows 40:60 duty cycle
5th image shows clipping of lower part of the O/P waveform.
6th image shows reducing the I/P to remove the clipping, the O/P clearly NOT making full V and therfore less than 100% mod.

The clipped O/P altho' taken from the filtered (unloaded hence the noisey waveform) PWM signal is present at the O/P of the LTC6992.
The FET driver circuit is fine but the LTC is NOT?

I'm at a loss...

I'd REALLY like the author to comment....

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 05, 2018, 10:06:45 PM
Hi John

Sorry have been away from my Computer for a couple of days....

I have decided to build Nigel's Modulator just for a bit of fun...
Have ordered a LTC6992-1 but will take 5-10 days ... have already completed LPF and board design...

Will post pictures as I go along....

Better earn my stripes before I make too many comments...

Will have a look at your waveforms and have a think....


Wayne





Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 05, 2018, 10:30:39 PM
Hmmm your right....
I quite like fault finding and probably muck around too much doing so.....
Having going TXs already takes the pressure off things a bit too when your R & D'ing.....

Well I would check both inputs into the IR2110 pins 10 and 12....
If the IR2110 is in a socket remove the IC.
Install the 2 poll filter as per that link....and check the waveforms particularly the lower one....
I'm looking for any clipping  ??? when you vary the Duty Cycle...

Looking at the Specs of the LTC6992 the output appears to be only 3.3 volts...that goes directly to pin 10...
The other input to pin 12 comes from the 2N7000 at 5 volts...
Cannot see this making any difference to the output goings on but things are getting desperate....
I use two 2N7000s driving the IR2110 ....each with its own pull up resistor ...one going to pin 10 the other pin 12.... so I know there at the same level....no reason I just do it that way...!!

Also again cannot see why it would make any difference but I drive the inputs at 12 volts with Pin 9 at 12 volts....as well....

Could you check the filtered waveforms at pins 10 and 12 and let us know how it goes....


Wayne



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 06, 2018, 05:27:50 AM
Added for further 'Clarity'

Nothing like LTSpice espec when you have the correct model!

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on May 06, 2018, 06:11:18 AM
Thats interesting..

The LTC6992-4 only goes to 100% positive and 5 % negative....
When at 50:50 DC: you.. I think....can see the top and the bottom of the sinewave rounding off a little.....
When at 60:40 DC you definitely only see the negative rounding off.....because the bias at the MOD input is less...

Can you run the spice program with a LTC6992-1......which is what Nigel has used...
You will probably see negative clipping when at 60:40 DC.....with the same drive level...
This is because the audio input is too high....for that DC...

Once the audio drive input has been set you should not have to adjust it again....that is for a given DC....
If you vary the RF output power level by say reducing / increasing the voltage at the drain of the Modulator stage ....you will not over modulate  :)
So you should not have to adjust your audio input .....
The recommended DC is 60:40....which gives you plenty of headroom for positives over 100%....


Wayne



Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on May 07, 2018, 11:03:06 AM
The modulator will not modulate from 40% to 100%.

It will from 50% to 100%.

I'm not willing to waste anymore time on this, there are better designs that will and I'll stick to those.

Thanks for your input.

I'd rather draw a line under this as have other technical issues to resolve.

I was hoping the author would comment, but has chosen not to.......

Cheers

J.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: g7klj on June 20, 2018, 10:54:00 PM
Quote
OK, It's the O/P from the LTC, so if you set the carrier to less than 50% the bottom of the sine will clip.

Sadly, you didn't DC couple your 'scope on the output waveform. If you do it will be obvious (? lol), or rather, its obvious to me after many years of faffing around with this crap that the modulator cannot ever output NEGATIVE DC (which is what you would require for *symmetrical* mod in the 60/40 case. Even if it could, your PA transistors won't thank you for supplying negative drain volts!)

It looks to me as if your circuit has the expected behaviour -- you say its not 100% positive modulated @ 60:40 because you are not increasing the input once you see the negative peaks clipping -- or are you?

If peak supply is 40V, and you set carrier @20V, now you will see "symmetrical" modulation -- and you do, right?

So now, for argument's sake (silly example) set the resting carrier voltage to 10V. The modulator will clip the neg peaks very soon (coz 0v is only 10V away) but the positive peaks should not clip until they hit 40V. It will "overmodulate" like mad in the negative direction (just to prove the point), but increasing drive should increase the pos peaks til they get to the supply volts, minus a little tiny bit.

Is this what you are seeing? If it is, then its expected behaviour, and its your outboard audio processing (or make a NEGATIVE ONLY peak clipper) to smooth off the negs just before the modulator hits 0v.

J, (is it John?) am I on the same page as you. Perhaps I missed something but I did take the time to look a couple of times at your posted images.

Summary: if you offset the carrier for more than 100% peak positive modulation, your audio chain has to take care to avoid the carrier getting shut off on the negative peaks. I do this with a peak limiter that only works on the neg peaks. Some people connect a diode to the drains from a very low voltage constant source. It still clips in the negatve direction on a sine, but the carrier can't shut off due to the diode conducting 2 or 3 volts to the drains when the modulator pinches carrier.

Anyhow, I hope you get some more responses because I have what I think should be a fairly common and logical statement on here for three weeks, and not one bugger has replied!

BTW: if you do have a modulator that takes care of this asymmetrical modulation automatically, I want to know about it :-)


Steve


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on June 21, 2018, 02:22:04 AM
Many thanks Steve, truly, for looking into this. The author still hasn't bothered to reply which is somewhat unacceptable. That and another thread has been started 'Detracting' from this.

I've got some time this weekend to muse over your thoughts and will spend some time in my lab.

It's with the LTC chip where the issue lies. I've built the same circuit as published and it doesn't work as expected.

Cheers.

JohnB.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on June 21, 2018, 07:31:01 AM
Hi John

Not sure why you are having troubles with this circuit....

Mine is disconnected at the moment but can set it up again fairly quickly if required....

It is an easy circuit really but could you do these things....

1.   set the duty cycle to 50:50 that's a DC voltage of 500mV to the input of the LTC...
2.   apply 10 volts to the drains and check the carrier / output from the modulator is 5 volts..thats
      a DC of 50:50
3.   Now apply an audio tone of whatever frequency say 1Khz at 1 volt PEP to the input of the LTC
      using your audio oscillator / generator ..... the output should be symmetrical....
4.   check both waveforms input and output using your CRO ...... both should not be clipped and
      should show a sinewave....one at 1 volt PEP and the other 10 volts PEP....
5.   Now change the Duty Cycle from 50:50 to 60:40 ... thats a DC voltage of 400mV...at the input
      of the LTC ....the output should be 4 volts .....
6.   both waveforms should be similar apart from the voltage levels.
      The negative going part of each waveform should be clipped....and the positive should not .....
      But now there is headroom....input has 100mV and the output has 1 volt.....
7.   Now if you further decrease the Duty Cycle you will see on both waveforms more clipping of
      the negative going part of the waveform....
8.   Going back to a Duty Cycle of 60:40
9.   Increase the audio level input to 1.1 volts PEP
10. Both waveforms will show more clipping of the negative going part of the waveform but....the
      positive part will reach the upper limit ... 1 volt for the input and 10 volts for the output...

If that's the case then all is going as it should...


Wayne

PS:  hope you understand what I have written ...
Maybe a change from 1 volt to 1.1 volts might be a bit fiddly.
Changing the DC from 60:40 to 70:30 or lower might be easier for you to adjust etc: ...
Anyway see how you go...

PS..PS.. Use a dummy load on the output of the Modulator not a TX




    


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on June 21, 2018, 08:19:03 AM
Thanks Wayne, I've got an Agilent E series sig gen with LF out so no problem adjusting any audio or RF level.

As I have explained in previous posts I'm unable to obtain the full O/P voltage unless I use 50/50 duty cycle.

Any other duty cycle will clip prematurely.

Surely whatever duty cycle you use the O/P waveform should still clip symmetrically, well at least it does on my UCC35702 PWM circuit that I've used for years.

I only built this one for its simplicity.

I will report back!

Regards

JohnB.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: vk3alk on June 21, 2018, 08:44:10 AM
OK John....

Could you do me a favor ...
Was reading your posts again and you had the upper and lower gate resistors back to front....
Do you mind just for my sake changing or checking those resistors again..
The upper FET has to have the 4.7 Ohm and the lower FET 16 Ohm......
Also the charge pump diode has to be a fast recovery like a uF4007..
That's the one going from 12+ to pin 6 of the IR2110....
You used a 1N4148 instead...
I know you said it worked for you but just for me could you change / check its a uF4007 or similar....


Thank you


Wayne






Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: g7klj on June 21, 2018, 10:18:55 AM
Thanks Wayne, I've got an Agilent E series sig gen with LF out so no problem adjusting any audio or RF level.

As I have explained in previous posts I'm unable to obtain the full O/P voltage unless I use 50/50 duty cycle.

Any other duty cycle will clip prematurely.

Surely whatever duty cycle you use the O/P waveform should still clip symmetrically, well at least it does on my UCC35702 PWM circuit that I've used for years.

I only built this one for its simplicity.

I will report back!

Regards

JohnB.

Hello again John!

No sweat. Take your time. Its damn frustrating when things don't work out as expected.

I also have one of the ucc-chip modulators and it does have the expected behaviour: when you set the carrier level less than 50:50, the neg peaks clip before the pos ones. As they should. Otherwise, the universe would explode from failing to obey the laws of physics.

LOL. Keep your end up. PM me if you need to.

Steve.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on June 23, 2018, 01:27:38 PM
Hi Steve, please forget about everything other than the LTC chip, this issue have nothing to do with the diode (4148 is fine at the voltages I'm using BTW) or the gate Res (just a cheap way of adj dead time).

As shown in my previous posts I clearly show the LTSPICE sims and it's clear that 60:40 won't go to 100% without clipping, sim it yourself and see.

I won't be using it and have reverted to my designs using the UCC device.

I'd rather not waste anymore time.

Cheers BTW.

JohnB.






Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: WBear2GCR on July 16, 2018, 07:20:09 PM
Might make sense to email Nigel - he's usually very ready to comment and help.
He well may be traveling now that it is summer?

Haven't heard him on the air in more than a month, fwiw.


                      Btw, I've heard him on the rig, and it sounds fine, and the modulation waveform is excellent.



                     _-_-bear


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: M0VRF on July 17, 2018, 03:41:25 AM
Sure it will sound fine but doesn't work as expected and sure I emailed him numerous times before even posting on here Bud.

I was trying to be polite but now fed up and have given up due to lack of help.

Cheers


JB.


Title: Re: 350W 3 band Class D FET AM Transmitter
Post by: WBear2GCR on July 20, 2018, 02:16:10 PM
Bud??

Hey, look, I was just trying to help.
I've spoken to Nigel on the air, and emailed him in the past.
He's been fine to deal with.

IF he's not responding then there is a good reason.
Perhaps one of them may be that your email gets "eaten" as a SPAM so he doesn't see it?
Possible.
Or, as I said, he is away.
Or, perhaps he is SK.

Also, this is not a KIT, and this is not a hand-holding, spoon feeding society - although people do try to help.

Regardless, take a pill and calm down.

                                  _-_-bear
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands