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Messages - OgreVorbis

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16
The RF Workbench / Re: Class D V2.0
« on: June 26, 2019, 1111 UTC »
Nice. I haven't seen that one before. I found some little heatsinks for the drivers.
I am working on modifying the board now to be more like what you did here. I dropped the inverter chip and I am going to use the inverted inputs on the drivers. I replaced the long winding traces with a long bar that extends along each of them similar to what I have on the output of the fets, just smaller. I will drive with 13.8V or 15V instead of 18V because the efficiency didn't seem any better with that and they got hotter. For the decoupling caps, I switched to SMD. When I'm done I'll post the new board.

I notice you're not using the inverted inputs yourself. Are you driving it that way, or? I know it might be a little better to use an external circuit for that to do 40% duty, but the 50% duty seems to work fine for me.
Do those drivers get hot for you at all? I'm sure it's OK to get a little hot.

17
The RF Workbench / Re: Class D V2.0
« on: June 25, 2019, 0848 UTC »
Alright, I did a lot today.

What I found out is that, redhat, you were right about those clock traces. They were reducing the efficiency a bit and rounding the waves. I cut the traces to two of the drivers and inserted the clock directly into them. The waves were much more square after that. They do a little jumping around thing when the fets are powered and I'm not sure why, but they are very square. I switched from using my DDS module and used a signal generator instead cause my DDS got fried somehow.

I removed and replaced all the drivers and identified two broken fets, so I just cut them off the board. Now it seems to be working, but these drivers still get way too hot! I had to put little metal pieces on top of them and that helped, but I have to run them at 12V. If I run them at the 18V, they still get too hot.
Stretchy, you have actually tried these drivers, right? I made sure the waves are clean and measured the gate capacitance at 250pf w/o drivers attached. With them attached, it goes up to 2uF. I don't know if that's normal.

So I could make a new board with the clock traces improved, but these drivers seem no good, so I don't know what to do next. It's weird because the amfone guy used the same driver and fet combination.

18
The RF Workbench / Re: Class D V2.0
« on: June 24, 2019, 0333 UTC »
I'm not real fond of the meandering drive clock trace.  You could wind up with a propagation delay issue where the far end fets are being turned on and off slightly after the close in fets.  This can cause an overlap issue which will hurt your efficiency.

It appears as if your using through-hole parts in dead bug style construction for your bypass capacitors and loading resistors.  Is this correct?  I would use smt chip caps and resistors for this, less stray L, better bypassing and less opportunity for noise pickup.

+-RH

I see what you mean and if I ever do another board, I will try that.
But anyway, I don't think that is causing the problem.

My mind is really blown by this problem.

I tried direct driving the FETs with a signal generator and they seem to work. I guess they could be partially damaged or something, but I hooked up each pair and despite the signal generator not giving them their full drive, each pair made about 30W at 13.5V which makes me think they are probably not busted.

The condition when the problem happens is only with input drive to the drivers regardless of if the FETs are powered or not.
I looked at the scope and the waveforms entering the drivers are a little warped, but decent (probably just my cabling). The waves coming out of the drivers are not amped though; they are much less voltage (like 1V or so). They are clearly not working right. Now is it because my circuit is wrong or something with the FETs, I don't know.

My only thought at this point is to remove all the FETs (which will kill them in the process given my design) and power the drivers with no FETs attached. I really don't see what could be wrong with them though and why almost all of them have failed even though they are new.

None of this makes much sense. I have them wired directly to the FETs with no resistor, but I see stretchy and others do the same. I am using the same combination of FET and driver also. I don't see what else there is other than I just got bad parts. I see stretchy, you put a cap in series with the input pin of the driver. Could it be that?

19
The RF Workbench / Re: Class D V2.0
« on: June 22, 2019, 2224 UTC »
Ok, not sure wtf it could be then. You can switch those FETs with 12V, however 15V is marginally better, no need to go to 18V. You have decoupled the drivers VERY closely with 100nF's?

Str.

Yep, pretty close.

Have you looked at the drive waveforms on the gates to be sure they all look similar?

I had something similar happen recently.  For some reason, one of my rf fets got zinged and the gate had a breakdown.  This was causing excessive loading on the driver.  Replacement of the fet solved the problem.

I'll check it.

Here is the actual board layout in case you see anything wrong. . .


20
The RF Workbench / Re: Class D V2.0
« on: June 22, 2019, 2157 UTC »
Since your using the on fet drivers with dual input, you could feed the clock into the inverting inputs on one half of the bridge, and non inverting input on the other.  That would allow you to eliminate the 74HCT14.

Also, depending on power, one turn through the output transformer may exceed the flux density of the material.  I would probably do a two turn primary, four turn secondary, made out of coax.  This would also help control your impedances and keep your strays down.

+-RH

OK, I'll keep that in mind.

I'm in agreement with RH + parasitics can be a real issue with high power RF.

Start with a low Voltage on the FETs and monitor with a scope. You may have to set the trigger up so you capture short duration spikes.

Very layout dependant.

Try small Res in series with FET gates.

Do you have spec ani to monitor the RF O/P? Would help as you can see naughty things beginning to happen.

Need to see some scope traces...

Str.

I do have a spectrum analyzer and scope. I think the problem is likely something more simple than parasitics. The problem happens even with no voltage applied to the FETs. Voltage applied to the drivers is fine, but as soon as I input the square waves, the two drivers begin to heat.

It would be very hard for me to add the Res in series on my board. Plus the amfone guy that this board is based on didn't do that and he claims it works. Only thing I left out are those 330 ohm resistors on the driver's inputs. Not sure if that would help.

I am running the drivers at 18V btw.

Core saturation will be easy to detect with the trapazoid method under AM conditions.  Slowly increase tone modulation until you see non-linearity start to occur and this is most likely being caused by saturation.

I'm in the process of doing the same thing to my transmitter following a change in the output topology.

+-RH

OK, good to know. I'll try that when I get there.

21
The RF Workbench / Class D V2.0
« on: June 22, 2019, 2041 UTC »
So I have finished my new design with the NCP drivers and eight SiC FETs and I tested it. I am having a bit of a problem with it. Some of the NCPs are heating up and eventually blowing out. It is not happening on a single side of the amp. I currently have only four of the eight FETs along with four drivers populated and two of the four drivers keep blowing out.

Do they need to be balanced somehow? Maybe a current limiting resistor. Right now they are just running in parallel with the 5V square wave drive.

I left out R3 - R6
Here is the schematic:

22

i'm 24 and probably come here from a different path than most of you. i'm not into amateur radio and have no interest in transmitting. i got interested in radio as part of my greater interest in the electromagnetic spectrum. i'm an astronomy major and though i have always been fascinated by the sky as a girl scout we got a visit of a grad student from the local university's astronomy program. the way she enthusiastically talked about electromagnetic waves as distant info messengers sounded like magic! from that moment on i knew what i wanted to do. she sort of became my mentor and we exchanged a lot of emails! she's now a radio astronomer and has been to some pretty exotic places like the atacama large millimeter array in chile!

i experimented for awhile and learned the rtl dongle while good for vhf and above was not the best for HF so i bought an sdrplay rsp1a from ham radio outlet about a month ago. it was a definite upgrade and so now i'm looking to upgrade my antenna. i don't have a lot of space and learned there are some compact antennas like the mini whip and magnetic loops (maybe too expensive for me on a student budget). most of all i want to take down the wire in the house lol so any advice on how to replace it would be great. there is space to mount a compact antenna outside so that is my next step.

on a personal note i was reluctant to post here as most of the people here are old enough to be my dad or in many cases grand dad and i hadn't come across another female but i at least found a thread with some people closer to my age.

btw: we dont just listen to lady gaga and justin bieber, most of us listen to everything and have tons of spotify and/or pandora playlists.  don't hate me cause i like ariana grande and charlie puth ;)

My family has a background in astronomy and my dad's first major project was building an observatory for a university. I am into radio purely because of it's interesting properties. I've thought about connecting the two areas, but I'm more concentrated on my HF projects now.

Anyway, it really depends on exactly how much space you have, but a full size antenna is always best. There really is no good way of cheating the system and making something smaller that works better. You may find something decent, but not better.

It's fun to take advantage of the surrounding resources in a clever way. Like for example if there is a large metal fence or water main, you can use that for grounding a 1/4 wave vertical. Using a slingshot or crossbow to launch fishing line into a tree and then raising the antenna with that is another way.

If you don't have any grounding resources, then my opinion is to go for an inverted V. The cable goes up a tree to a branch and the two lengths of antenna slope down to the ground (it's a sloping version of a dipole.)

Good luck and don't feel intimidated to come back :)

23
Equipment / Looking to upgrade - SDR or traditional?
« on: June 15, 2019, 1038 UTC »
So I spend most my time building and experimenting with shortwave transmitters, but so far my experience in the receiver department has been limited. I currently have two receivers. The Tecsun PL-360 and this Chinese SDR: https://www.ebay.com/itm/RTL2832U-R820T2-chip-Set-100KHz-1-7GHz-UpConverter-HF-UHF-VHF-RTL-SDR-Receiver/132465613216

I'm looking for something with very good receive sensitivity. I am not sure whether I want a traditional radio or a better SDR. It is nice to have that spectrum analyzer type view, but my main focus is on sensitivity. I am not looking to spend very much < $300.

The Airspy HF Discovery looks good. What would you recommend?
How does an SDR perform compared to a Double or Triple conversion superhet?

These traditional receivers look good, but they are a little above my price point: Alinco DX-R8T or ICOM IC-R75

24
The RF Workbench / Re: Simple SiC TX
« on: June 15, 2019, 0524 UTC »
Same ones I'm currently using, C2M0080120D's and C2M0045170D's.  Of course its all theory until you build one... 

A big part of the problem in getting above 1KW is the thermal management.  Large bonded fin heatsinks and fan capacities over 200CFM become mandatory, especially for gear meant to survive operation in outdoor summer conditions.

It will be quite the feat when it is running.

+-RH

https://www.fischerelektronik.de/web_fischer/en_US/K%C3%BChlk%C3%B6rper/D/L%C3%BCfteraggregate/index.xhtml

These are a bit pricey, but it's a good company and they have everything.

25
The RF Workbench / Re: Simple SiC TX
« on: June 14, 2019, 0028 UTC »

Sometime in the future a new PA layout is in order...2 RF fets for 1.5KW, 4 for 2.5KW...carrier that is.  5KW in 5 rack units, for the power modules anyway....wow.

+-RH

What FETs are you talking about? What frequency? That's an insane amount of power. Is this some design you are currently working on?

26
The RF Workbench / Re: Simple SiC TX
« on: June 12, 2019, 2307 UTC »
HI folks

Ok, undestood. Thanks for the advice.

The main issue is not the size of the components but the fact that I cannot buy the new-and-faster ones here. They are unobtanium for me right now.

There are some ixys that are mentioned elsewhere in the forum that maybe I could get.

Cheers.

I know from personal experience that the 4422's are no good above 5 MHz. The NCPs I am about to test in a few days.
If you can't source those, then get the IXDN614's. I know these work at 7 MHz if the board is done right.

27
The RF Workbench / Re: Beginner class D design
« on: June 12, 2019, 2122 UTC »
I have resumed work on the project. It is very near completion. I just need a good heatsink for the FETs and I'm searching for one now. I want a cooling aggregate type.

I still don't know what to do to the transformer when I add another pair of FETs.

Do I change nothing?
Do I need to add another winding on the secondary?
Do I need to add more than four cores for eight FETs?

28
So I'm interested in finding out if there is anyone under 30 y.o. on this forum. It seems to me like the only people interested in this hobby are older (maybe 50-60).
If you are younger, how did you get into the hobby?

For me this all started with an interest in computers which then moved to electronics and then to radio.
I was somewhat interested in radio when I was only 15 y.o. but I didn't have the skills required to do anything really interesting.

The part where I think I differ from most people who are into radio is that I am not into it to make contacts or to do other ham-like activities and I am not into it even to play music (but that's part of it). There is just a huge excitement I get from putting out the waves and knowing that it's going very far. The physics of it just makes me really happy. Am I the only one who's like this?

29
The RF Workbench / Re: Beginner class D design
« on: April 15, 2019, 1734 UTC »

To drive a High side and Low side Devices I guess? Or tie them together for single FET. Just use a dual Inverter from the O/P of your Osc. Sine in and square out, inverted and double inverted to drive either side.

Str.

Sorry, I don't understand the term "high/low side device". Does this mean the devices on one side of a push-pull amplifier (like what I'm building)? So then I don't need an inverter. Just use the NCPs?

30
The RF Workbench / Re: Beginner class D design
« on: April 15, 2019, 1119 UTC »
See last two posts.

I realized after my design that it looks like the NCP drivers have an inverter built-in. The truth table in the datasheet makes it look like it won't work for this purpose though, but I don't know why else they would have such a feature. Is this to use in a push-pull amplifier with one square wave input? If so, then I don't even need the inverter :P

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