Check this RF LDMOS out

High frequency, antennas

Check this RF LDMOS out

Postby johnf » Wed Dec 08, 2010 9:53 pm

This new FET pair is a major leap forward in LDMOS technology
more grunt than a pig farm and extremely rugged

Might be the ideal beast for Chris to play with in his torroidal machine

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Re: Check this RF LDMOS out

Postby Jerry » Thu Dec 09, 2010 12:56 am

At $270 a piece in Qty 1 I think he might pass!

Pretty cool part though.
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Re: Check this RF LDMOS out

Postby johnf » Thu Dec 09, 2010 1:08 am

Most good RF devices used to be US$0.50 per watt
I think that this is magic

I have a 2kW telecom 50 volt supply so when the funds can be got I might make a ham HF linear with a couple

all for now
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Re: Check this RF LDMOS out

Postby Doug Coulter » Thu Dec 09, 2010 9:46 am

Dunno, Chris seems to be into directly generated HV sq waves, though I think in his app that sines would probably do -- both have been used in quadrupole mass spectrometers with about equal results.

I'm not as sure in my app. I'm of course hoping I can do what I want with primarily sine-like waveforms, but not sure yet, so for the moment I'm thinking (multiple) 6DK6's for an output stage, which can tolerate 1-2kv DC plate supply no problems. However, once I get past stage 1a in this, I may find sines will do, and I'd use these fets for that in a heartbeat. I'd rather skip the hot filaments if I can (especially the big transmitting tubes). But on the other hand, it's real hard to just fry a tube while you're making mistakes, compared to most any semiconductor.

The issue being, of course, that it's really hard to get to kilovolts with a broadband kind of thing from a low voltage/high current input as these would provide -- the required magnetics are a pretty black art I've not yet learned. If I find I don't need instantaneous bandwidth (shaped pulses and such) then you bet -- the fets rule.

That's one of the PITA issues in research -- till you are really sure what you need/want, you need gear that "does it all" which is difficult and expensive. Then once you know, more often than not doing it becomes simple.

I think such is the case here. If narrowband sines are usable, then heck, tuned circuits handle getting to voltage no problems. But if you don't start out knowing the frequency....that's a major hassle to even do a sweep of the parameter space.

I believe this is one reason most mass specs are made as they are -- fixed F, sweep everything else. "The Book" says so, anyway.
That would be Peter Dawson's Quadrupole Mass Spectrometry and its Applications. ISBN 1-56396-455-4

I'm still trying to get a feel for the math there. In theory, it should let me predict the conditions I want, though they are pretty way out there compared to schlepping low energy ions around in a mass spec. In practice, the theory is abstruse enough so that I can't yet manage that, and even there it looks like a lot of empirical work had to be done to learn how to really make things like mass-selective ion traps with low space charge density -- my problem is a lot harder than that one. So for now, my best back of envelope guesses contain fudge factors in the small single digit integers, which doesn't narrow things down a heck of a lot. I know I need MHz and Kv....but for bunching in the presence of space charge a bunch of unrelated terms get into play, and even that fancy math can't handle it. They are perfectly happy that in the normal operation of a ion trap, ions all avoid one another due to space charge and spread out in space over time. I need better than that. What I want to do is gather and control a diffuse ion cloud in a kinda-predictable orbit, then bunch and accelerate the mess (while focusing) onto a spot. That means most of the time, the space charge can "do what it wants and I don't care much" and I can get some charge built up. Only for a very short time-space do I need or want them to all be together, which theoretically is a lot easier (but not simple to predict how I'll get there).

That price isn't so bad if you're not going to let the smoke out of them. Big transmitting tubes aren't cheap either.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Check this RF LDMOS out

Postby chrismb » Fri Dec 10, 2010 10:29 am

I am very appreciative that you would think of my experiment, John!

Though, Doug has it right. For my current application, the FETs I am using (500V x 3A, good to 6MHz) are more than enough because it is, indeed, square waves that I am after.

I originally thought sines might do as well, but I thought about it and I now think that; in the case of my experiment with ions rotating in orbits in a magnetic field, if one imagines rotating around synchronously with rotating sinusoidal fields, what you actually have is an electric field pattern that rotates with the rotating ions, rather than one that switches such that ions that drop out of synchrony see the full applied field. As I am aiming to do work on ions that may become asynchronous due to thermalising collisions, then it is much more effective [I think, and possibly even necessary] to have a switching field.

I'll try to give a generalised picture (so it doesn't just apply to mine, but also Doug's - and anyone else's - 'anti-thermalisation' idea) because I think it has general application; accelerating gangs of ions is quite unlike a rotor in a motor, for example, or even an antenna because if the ions are all pushed towards the same point in space for a given (peak) phase angle then we loose out due to space charge because they all push apart (whereas a motor rotor tends not to fall apart into constituent particles !!).

Having a switching acceleration voltage means that ions can take up a position, without preference, anywhere on the 'positive' side of that electric field force. I will try to illustrate by describing a square wave as a 'peak' between 0 deg and 180 deg phase, whereas a sine is a peak at 90 deg. In the former, the ions can fill up the first 180 deg of phase-space, whereas with a sine they'll all try to squash into the sector around 90 deg, with consequences of limited density.

Providing an ion is running around/back-forth [whatever] in a system at the rotational velocity of the applied signal , it doesn't matter if it is located in phase space anywhere between 0 and 180 for a square wave, but for a sine it has to try to run around at the rotational velocity and 90 deg phase locked, else it won't see the max field and will be either accelerated to keep up with the 90 deg position, or decelerated if in advance of it.

If an ion slips back in phase angle under the application of a square wave, then as it crosses, say, from 0 into -10deg, then it will see the full differential field that will do work on that ion to pull it back into the 0-180 phase-space, whereas under the influence of a sine it'd see a much weaker field gradient as it thermalises in phase-space away from 90 deg and, unlike the square wave, I think you will end up with a population of ions that will drift (thermalise) into all phase angles. (That might be OK for some applications, but it would not work with mine because the lower radial fields would mean the ions would take up the wrong orbit and crash.)

I believe this is not at all specific to just my experiment, but is the same for any system that seeks to 're-pump' ions with energy lost due to small thermalising losses.

PS.[I realise that I might be trying to describe something difficult to put into words so easily (even a picture is going to be a problem, I would have to make an animation of this if I'm not describing it very clearly) but let me know if it makes sense or otherwise and I will try to describe it better when I put it into the context of describing the detail of my own work in the near future. I find it difficult to tell whether this stuff is complicated or not to read, because I've been doing it for so long it seems all quite straight forward to me now!!]
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