2 FET RF Deck for 75 Meters

[ka1tdq]

Ok.  In my spare time from building my new amp (huh), I've built this 2 FET RF deck for 75 meters.  I'm getting 135 watts out of it (into a dummy load), but efficiency isn't that great at 58%.  Each FET (FQA11N90's) are dissipating 47 watts.  Vdrain is regulated at 48 volts. Total drain current draw is 4.78 amps.

I don't have a schematic for any of it, but I'll do a quick summary:

Crystal oscillator for 3870 Khz (Square wave output)

I use an octal inverter to make two phases 180 degrees out of phase (duty cycle is about 45% going into each IXDD614)

Push pull FETs

Output core is the same as Steve's published Class E rig, except I halve the number of total cores.

Output filter is a 7-element 50 ohm design


I think the power dissipation is too great to just modulate it right now.  The FETs will probably get too warm, even with the large heat sink I have.  I think if I leave it as-is, I can just lower the drain voltage to an acceptable level and go from there.  My goal was to have a 50-ish watt carrier rig for portable/mobile use.

Jon
KA1TDQ

[WBear2GCR]

You have there a linear amplifier?

Class what?

Class E stuff does >90%, but Class AB or C does not.

[ka1tdq]

Yes, this is operating in a class other than class E.  I could change the output from just a filter to a resonant Class E circuit, but I can live with the inefficiency.  The engineering is simpler and I don't need to tune anything (or sequence drive).

I use a very similar setup for my 40 meter transmitter, and everyone tells me that it sounds very good.

The empty space on the chassis will house the modified Heising stuff.

Jon

[ka1tdq]

I tried a junk box transformer and 32 vdc gives me 66 watts carrier.  That's in the ballpark of what I was looking at for power output.  Efficiency is still the same, but everything runs cool at this power level.  

R-modulation works out to 9.44 ohms (very close to 8 ohms).  I'll use a 1:1 toroidal isolation transformer for the Heising component.  

I can't run 75 meters at home because the band is wiped out by power line noise at my QTH.  Luckily though, my workplace is setting up a Ham station and I'll be allowed to bring this transmitter in.  It'll be nice.  The dipole will be on top of a 3+ story building.  

Jon

[steve_qix]

We need waveforms    Waveforms tell all.  Need gate and drain waveforms and shown at the same time.  Also need gate waveforms for both phases shown at the same time.

If the duty cycle going _INTO_ the IXDD414s is 45%, it won't be 45% at the other end.  The IXDD414s introduce a certain degree of both propagation delay and they stretch out the waveform to some extent.  What is the duty cycle at the GATES?

[ka1tdq]

I've never measured the duty cycle at the gates (probably a good place to start). I'll buy another scope probe after work today so that I can see two waveforms at the same time.

I'll post some scope pictures in a little bit...

Jon

[ka1tdq]

Here's what the pictures are:

IMG_1742 = FET #1 (gate on top / drain on bottom)

IMG_1743 = FET #2 (gate on top / drain on bottom)

IMG _1744 = Both gates with drain voltage applied

...more on next post

[ka1tdq]

This shows both gate waveforms with no drain voltage applied.

I just decided to have a change of plans with this transmitter though.  I added a DDS VFO and I'm going to use it on 40 meter CW instead.  135 watts CW isn't too shabby.  My old transmitter is 60 watts, so this'll be a nice improvement. 

Jon

[n1ps]

Do you use a circuit to adjust the pulse width?  It seems you still may have some crossover.  Cant tell from your gate wforms.  I personally think you should have a pulse width adjustment for an E TX.

BTW...does anyone have a copy of Frank GFZ QEX article written in the Sept issue? Would like t read it  PM me or my call at roadrunner.com  TNX....~ps

[steve_qix]

Hmmmmmmmmmmmm.... the gate waveforms show a lot of overlap - duty cycle approaching 60%, I'd guess.

The two phase converter circuit would help a lot with this.  You could adjust the waveforms.  I believe this schematic is posted in some other post about this subject.  I can post it again if requested.

The drain waveform is pretty nasty    Somewhat akin to class D, but with either ringing or major harmonics showing on the waveform.

It may or may not clear up with a lower duty cycle.  40 meter class E is fairly easy to do.  The tuned circuit is quite a bit smaller than that of a 75 meter class E, and the efficiency is very good when all is set up and adjusted properly.  Might be the way to go here...

[ka1tdq]

I just finished converting this to a CW transmitter when I checked in and read everyone's post. 

No, my previous circuit didn't have a way for me to change the duty cycle.  Since converting it to a CW transmitter, I've added a DDS VFO.  It can adjust the duty cycle and amplitude (very useful).  I'm still using the inverter circuit from the original though, so there is still the overlap problem.

However, things do look promising initially.  I adjusted the waveforms pre-IXDD to where they looked about even.  Then, I applied drive with only 13.8 vdc on the drains for safety.  Power output was 25 watts at 3.00 amps total drain current.  That works out to 60% efficiency.  Not good, but I haven't changed the output filter from 75 meters to 40 meters (I'm transmitting on 40 meters).  Once the parts come in from DigiKey and I make the filter change, power output should go up along with efficiency. 

FYI:  For the keying circuit, I'm keying DC voltage to the IXDD chips along with the inverter circuit.  Drain voltage is always there.

Jon

[ka1tdq]

This is turning out to be a good learning exercise.  I'm getting 160 watts out on 40 meters, but efficiency is atrocious!  Worse than it was on 75 meters.  This is good though because I can 'feel' where the problem is.  One FET stays cool while the other gets warm really fast.  I think it's due to my drive circuit.  There isn't a total 180 degree phase shift since I'm using just an inverter chip.  I'm going to get a flip flop and drive it at 2x the frequency.  I have a 14060 KHz crystal and that'll put me at 7030 KHz.  I'll add a DDS VFO later on after I get everything all figger'd out. 

I'm sure I'll still need to adjust the duty cycle even after I get 180 degrees phase difference.  Not a big deal... I'll just steal Steve's idea for adjusting the duty cycle after the flip flop.

Quite honestly though, I'm not all that concerned about efficiency.  If I can get it to 160-ish watts out and the FETs aren't going to burn up on a typical CW QSO, I'm happy. 

Jon

[steve_qix]

FYI:  For the keying circuit, I'm keying DC voltage to the IXDD chips along with the inverter circuit.  Drain voltage is always there.

Jon

EEK !  That is risky - possibility of parasitics as the DC voltage falls and the FETs are in the analog region for a small amount of time..... but the biggest problem is that you will likely generate bad key clicks.  If you severely overshape the drive with VERY long rise and fall times  _AND_ if the amplifier is 100% stable (unlikely), you could key the drive.  This was an old trick used "back in the days of vacuum tubes" (professor Wadsworth, WPI) to reduce key clicks with keyed-drive non-linear (like class C, D, E, F) amplifiers, but it was not always very effective..  You need to have a linear region of sufficient length to avoid key clicks.  The rise and fall times must be controlled.    Thought I'd mention it....

Regards, Steve

[ka1tdq]

I didn't mention it in my last thread, but I had to remove the IXDD's from the keying circuit.  Efficiency went to around 15%.  Come to find out, the IXDD's were only getting around 7.5vdc.  They must be power hungry little beasts.  Anyway, I put them back to their continuous 13.8 vdc supply and everything was happy again.

So now, the only thing that'll be keyed is the flip flop.  I know, that's in the drive circuit too, but I'll try it for now and for testing. 

BUT, my last 40 meter CW rig which used a single FET/IXDD didn't sound bad.  I got many compliments on the air for a clean sounding homebrew rig.  But hey, when was the last time you heard someone give an honest opinion on the air about someone's audio?  They could be using a carbon microphone, using a stock Viking II and driving a 3-811 linear with 100 watts carrier and someone would still sing their praises. 

Off topic, but that happened recently.  Someone gave a guy a report that he sounded great.  He didn't.  I offered to record him using my SDR software and email him the wav file.  I did and he immediately reported back that his audio did in fact sound... shall we say... yell-oh-wee. 

But point taken about the drive.

Jon

[ka1tdq]

I just finished putting in the flip-flop.  With no duty cycle compensation (50% going into the IXDD's), I'm getting 150 watts out.  The keying sounds pretty good on my local receiver.  Efficiency is tolerable, but nothing to write home about.  Again, 48 volts on the drains and total drain current is 5.73 amps.  That works out to burning 275 watts to get 150 watts, or 55% efficiency.  Each FET is absorbing about 63 watts.  My heat sink is able to make it all work, but I'm going to build (eventually) Steve's duty cycle adjuster.  I just ordered the parts from DigiKey.

I also ordered a DDS VFO so that I won't be rock bound.  In the picture, the circuit board on top of the heat sink will go away.  That's the crystal oscillator.

Jon