RF 'echoes'

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Linda Haile
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RF 'echoes'

Post by Linda Haile »

I hope I've posted this in the correct forum. Can anyone throw any light on the mechanism by which so called automatch units virtually eliminate RF echoes by tricking the RF supply into thinking there is a 50 ohm impedance in the circuit and reducing the RF echo to between two and four percent of the signal? This is something I've come accross during my research into ion sources but I've not been able to find out much about it. Thanks.
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter »

The real trouble is that I can easily say more than this forum can hold on that subject. So I will try for brief here.

Most of these can look at forward and reflected power via a mechanism you can find in most older ARRL handbooks -- that part is easy if you know the range of power and frequency you are going to be operating in. Some fancier ones can see phase of volts vs current as well. Most pro ion source RF units or auto tuners have this in some form.

They also have what amounts to a variable transformer, only it's usually done the same as in a ham transmitter, as a Pi network. This usually has a variable cap to ground on the input, one on the output, and a coil in series. By adjusting the caps you can get to a place where you have it all resonant at the desired frequency, while having a given ratio of the two capacitors, which is what determines the impedance ratio input to output. They do this with some opamps looking at the power sensors, and little motors on the caps to change them. I have here a unit designed for 3kw at 13.59 mhz, it's fairly impressive. It also looks at the final tube plate current so it can see the dip there at resonance.

This tech is from WWII in figher planes and such, a lot of it was done by Collins, then translated to the less advanced physics world for guys who don't "get" RF. In some of the fancier avionics, they used a motor driven rotary inductor to get resonance, as the basic impedance ratios were well known to match the amp to the antenna from first principles, so on the inductor needed tuning when the frequency changed. You can find this kind of thing at many hamfests if you know what to look for.
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Linda Haile
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Re: RF 'echoes'

Post by Linda Haile »

I can follow the part about tuning the two variable caps to minimize the current, but where do you start when designing an ion source? Assuming a fixed frequency.
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter »

Well, in this case I have a bit of an axe to grind. This thread details what *I* did myself, and I'm still very happy with it.

viewtopic.php?f=12&t=5022#p32330

If you want to use more power and lower frequency for some reason, I think Carl had something nice worked up and maybe he'll chime in with a link to that version.

Due some very careful use of overdesign, my thing doesn't care about the issues of impedance matching so much, it just works if duplicated as described. So there's a start, all you have to do is make that one, or Carl's. Or for that matter, the one Andrew Seltzman did, which I've not tried here yet, as mine gave no indication I needed improvement , but it looks good (and nicely simple if you're a master machinist) too.
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Linda Haile
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Re: RF 'echoes'

Post by Linda Haile »

I have read those threads but found no mention of eliminating echoes. I was just thinking that in the interests of improving efficiency it may be advantageous to address this issue.
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Re: RF 'echoes'

Post by Doug Coulter »

It's not so much efficiency in this part of things, it's making the power tube or coax and connectors live, and things not arc over where you don't want them to. This is all very basic transmission line concept stuff, in all basic books that exist that mention transmission lines (tons of them but check ARRL handbooks from the earlier days for a lot of detail, or any "radio engineer's handbook" whatever).

So, if you want to know more, do a little bit of homework, this stuff is very easy to find cheap or free, but you do have to go for it at all (hint, find used bookstores). You will almost certainly find something of interest here:

http://www.pmillett.com/tecnical_books_online.htm

(and so should a lot of other people here, and that's just one link of many).

Here's one on standing wave ratio, the standard term for what you're calling echoes. Both are actually correct, depending on what you are concentrating about thinking on.

http://en.wikipedia.org/wiki/Standing_wave_ratio


I prefer Fredrick Termans Radio Engineer's hanbook, 1943, over all others so far, but the ARRL stuff is also good if you get one from the years when hams were into building things themselves.
We just found a couple of those Termans by searching online, and got another one for about $15, and it's a piece of solid gold for anyone needing to learn the basics and even some pretty advanced things. I would pay far more for one and be very happy.

For example, it covers particle beam optics (electrons, but you can do the scaling for fusor ions), along with "what is an inductor", and so forth. From how much resistance is there on some size wire, to parasitic capacity to getting good figure of merit in a video amp, to complex filter design basics, it's all there.

The reason some lower frequency ion sources (particularly for sputtering at decent power levels) use an auto tuner rather than just one manual adjustment is partly because the load changes during use as the thing can be lit up, not lit, or the gas pressure etc may change so the load isn't constant.
The other reason is that most people out there can't actually handle two interacting adjustments if their life depended on it, and will fry things as they spend a lot of time in the wrong settings and the power tube, already near limits is burning up. That's why a lot of ham transmitters have a "tune" setting that reduces the power while someone not too good at it learns how.

The reason I don't mention SWR in my design, is that with a tube designed for 600 watts plus output, running at 20 watts DC input into high voltage hard-line coax, well, even if all the unused energy reflects back into the driver, no big deal at all -- the tube gets very slightly warmer (after all the filament alone is 30w and is pure heat before any DC comes in) -- and anyway, I have low SWR because I designed it right with that in mind, the coax is a 1/4 wave matching stub -- it's all taken care of already. The only impedance matching issue in that one is not having too high an impedance in the cavity feed point, so things don't arc in there at the desired power input levels and eventually melt solder joints in the cavity. I think Andrew's version takes the cake in low power in vs ions out, not so sure it makes the best ratio in monatomic vs diatomic ions though -- because mine worked fine, and I had other things to move on to at that point, problems all solved there.
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Linda Haile
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Re: RF 'echoes'

Post by Linda Haile »

I've come accross SWR meters. It's starting to make sense now. Surely the reflected energy is wasted, though? Thanks for the link, it looks very interesting from what I've seen so far.
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Re: RF 'echoes'

Post by Starfire »

Great link Doug - thanks
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Re: RF 'echoes'

Post by Doug Coulter »

The reflected energy is often wasted, but not always. In "echo thinking" it's used to help switch the lines in all PCI bus applications, actually -- most of us own computers that have that bus, and I will refrain from editorializing too much on other aspects of that which I don't like so much. The bus logic levels are only reached after the reflection off the unterminated end comes back and adds to the original drive....a fairly cool idea that saves power in the drivers. What I don't like is the latency of the design for master sharing and burst initiation, and the patent/licensing lockout that keeps small companies from being able to innovate and make products for this bus that are competitive in price. Big step backwards from ISA on that one. Put a company of mine out of business (would have more than tripled the product parts and license costs to convert the design), so I guess I do have the right to whine a bit.

The echo model is mostly used for broadband signals like pulses and sq waves, and is the best way to think usually when those are what you're working with. Motorola's old ECL handbook has a ton of good stuff on that, as most digital engineers then and now don't understand this set of phenomena.
That one is worth picking up if you find one and are interested in more, the explanations in there are very clear.

When you're thinking in impedance mismatch (SWR) kinds of terms you are most often working with a single frequency (narrow line on a spectra or FFT). As some of those links show, this will result in peaks and dips of voltage and current on a transmission line at lengths corresponding to 1/4 or 1/2 wavelengths of the frequency on that line (which will usually have a velocity factor less than C).
The peaks in voltage in a standing wave can cause arcing and increased dielectric losses. The peaks in current can cause additional I^2 * R losses. This can be a curse or a blessing, depending. For example if reflected energy is a problem for your source, and you have plenty source power, you might just want some loss in a long piece of coax to attenuate that reflected energy.

If tuned for proper match and resonance, reflected power to an RF source need not be wasted, though it will add a tiny amount due to the things mentioned above. The key here is that an arbitrary length of say 50 ohm line that has a not-50 ohm load on one end, will not present an impedance of 50 ohms to the other end either, but whether it's higher or lower than nominal depends on line length, as well as the full complex impedance presented to both ends of the line, not just the resistive component of the source and load.

For example, a 1/4 wave line shorted at one end has infinite impedance at the other, and this kind of thing is the basis for many cavity resonators and waveguides. A 1/2 wave line shows the same impedance at both ends no matter what impedance the coax is! (This is all disregarding coax losses, which can indeed be so low as to not matter). Multiples of 1/2 wave lines have this property as well, but will have peaks and nodes along the length were either current or voltage peak, which can cause more loss or arcing. Odd multiples of a 1/4 wave line act like a 1/4 wave line -- think of it as a quarter wave line in series with a half wave line (or multiple), and it's obvious.

Best of all, if a quarter wave line has a characteristic impedance that is the square root of the multiplied input and output impedances, the match is then perfect -- it's a transformer, not reactive at that one frequency. Sweet trick for narrowband applications.
This is what I did in my uWave ion source to match the tube output to the cavity input, and that's why I took some pains to publish the right dimensions for all that -- small changes affect this a lot.

Lines slightly shorter or longer than 1/4 wave with one end shorted act like high Q impedances either inductive or capacitive, depending on which direction the error is in. For a line a little on the short side, a variable capacitor can be tied across the high impedance end to tune it to a frequency desired, which is much easier to do than change the length in some construction methods. This effect is also used in my uWave ion source in the main cavity. In that, the tubing with gas in it acts like a capacitive load due to the dielectric constant of the tubing itself, with a resistor (a non linear one) due to the energy pumped into the gas to ionize it. So in that case, the load seen there is a capacitor in parallel with a resistor that changes with gas conditions, and in some cases this can require re tuning for both cavity resonance and gas loading. This particular design is overkill enough to not need that, though.

The big risk in bad impedance matching is having it happen with a source that is operating within an inch of its life when matched, seeing higher voltages or currents due to the mismatch, and getting fried. The answer there is of course to go ahead and match things up. Rarely is it a good idea to brute force by having a larger output device in the source as regards efficiency. The larger device will be harder to drive (more input drive power is a waste) will be slower (transit time admittances), and will usually draw more power for the same output RF.

I get away with this in my source design because I already have a magnetron designed for high power dissipation, but in CW mode have to run it well below its normal ratings anyway, for other reasons such as space charge accumulation in a tube designed only for pulsing. So there's plenty of ways to get rid of any excess heat, it was designed to dissipate far more than it does in that design even with 100% reflection of power when there's no load.

This power level is in the noise for a conventional fusor which is using 400w of DC HV input power anyway, and only would become significant when a much higher Q mode is happening than that.
Eg, it's smarter to work on the 400w of almost total losses first, then worry that last couple tens of watts, don't you think?

In the new pulse mode I'm running in, where the HV power is in the 5 watt region, yes, the ion source is using more power than anything else is, for now. I am also ditching about 99% or more of the resulting ions by not having enough ion extraction field to pull them out of the source against the curve of the magnetic field lines used to get ECR effects in it -- because I'm right now more interested in that high Q pulse mode and improving that further than in the total system power (which would include vacuum pumps and all the rest as well, a bigger factor yet, especially for those using mech and diffusion pumps running full out all the time, vs a 20w average turbo system that only runs the fore-pump intermittently). It is kind of like having a 500 hp car to go to church with -- you just don't push on the gas as hard in that mode, vs on a racetrack, and accept that it's nice to have that there is you need it, wasting a little gas to get to church. Once I establish that this vehicle is only going to church, I can substitute my ~5 hp go kart and save some gasoline, but I can't take that on the track or on the roads (legally ;~) for long distance travel either. If you could own just one car, well, the choice is fairly obvious, I think.

Now when making ions as the main task, like for sputtering or something like that, the efficiency becomes more important to be sure, but for a fusor, it's not that big a deal. For now, the main worries are not melting things, and you can tolerate some losses. You can always optimize post facto once the other parts are nailed down.

As for figuring out how to make an RF ion source -- the trouble there is that the effective load at the ionizer is not often calculated until after the fact -- easier to just lash it up and measure that. Then you can do matching all day long with some confidence to tweak things. And even know how to size the RF source correctly, which will improve efficiency further in most cases.

Premature optimization is often a waste of time on things like this (this is a famous saying in software).
At first, you don't even know what's going to be most important, and wasting a little so as to be able to evaluate the system as a whole first is most often a wiser move.

Hope I don't get too much crap about posting this link and picture here....

Go kart:

http://www.coultersmithing.com/kart.html

and see pic below for when I'm not going to church, though it gets used for that too.
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Linda Haile
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Re: RF 'echoes'

Post by Linda Haile »

Some interesting background there, Doug. You are correct in that it was while reading up on the type of ion sources used for sputtering/deposition that I came accross the 'echoes', thanks for pointing out that this is the same as SWR losses. While I appreciate your comments relating to power levels associated with normal fusor use (and ion sources are not usually required here), I have been considering ion sources for high gas feed/extraction rates as a way to keep the ratio of fusable ions/neutrals at a maximum and also maximize the ratio of monatomic ions/diatomic ions. Again, it may be simpler/more effective to just increase the power input but there comes a point where this becomes less practical than increasing efficiency.

Continuing your analogy, if you were to travel to church in a 1000HP car, you may have problems controlling all that power, especially in the wet, and could wind up wiping out some of the congregation that walk to church, which would not be a very desirable outcome, to say the least.
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter »

You will be happy to know that the uWave source does indeed handle crazy-fast gas flows fine. It's just loafing at normal flow rates and pressures. I started with mid range rates, and have been going down from there ever since, and getting better results as I do. Some of the byproducts are best kept in the tank, it seems. I don't think gas flow in general is a big limit with any of the ion sources I've yet seen, anyway. Some I've tried wouldn't go low enough for me, this one will, and also too fast, just turn the knob for instant response, very nice indeed.

And it's not like D is real cheap to just blow through the system fast or that the approach is "efficient" by any definition at all. I'd concentrate on keeping them from recombining instead, if possible, and keep purity high via good vacuum technique, which is only a one time cost, not a flow rate. if you look at some ion traps, or even a quadrapole mass spectrometer, they can keep ions happy for long periods without losing them. So it's possible in practice at least under some conditions.

I would very much deny that most fusors don't need or couldn't be improved by an ion source -- just give both ways an honest try and you'll know that too. Here that's not what I see experimentally at all, even running the "conventional" modes. At the point a fusor is its own best ion source, the gas pressure is often too high, the current rises without apparent limit (some of this perhaps due to a lot of wasted tank volume in my setup that provides a lot of surface for recombinations and a lot of spare gas to reionize over and over), and the Q is terrible compared to other parameters I run much more successfully with here, all the time. I can reproduce all the results reported here by others on this system though, and have already as my baseline for improvements. It's just a scramble to reduce the gas after the thing lights off to a point where even a 2kw supply can get the volts up high enough for fusion again, without letting it go out and having to start over.

My point was simply, why go after better efficiency in the *lowest* power draw part of the system before you even get anything working? Doesn't make sense to me as an engineer or as a scientist.

It's too soon in the scientific process to deliberately limit your ability to search the multidimensional parameter space by limiting this and that too much in a misguided search for 50% better efficiency when many more orders of magnitude are what's needed to get to gain. All that stuff is simple (re)engineering once you have nailed down what you really need. For now, you can't say you know that. No one can. Got net gain, anybody? I don't see any raised hands, yet. No one is within a factor of magnitudes you can count on a hand even.

Believe it or not, btw, the hot rod is actually far more controllable, due to vastly better traction and an anti-everything-bad computer it contains (among other things like very nice tires and suspension), which neither my truck or Buick do -- I got some pleasant surprises in the recent ice storms here when I went out and deliberately tested that. I just keep it in first.

I do walk or take the Buick or kart when it's nasty as there is also a cliff to fall over and replacement cost for the old Buick is a lot less if the worst happens. Where I live, btw, no one walks to church, or nearly so (in my one case, it's practically in my front yard). Too far, all our land holdings dictate that one - we're all way too far apart for walking there in farm country, particularly the old and frail.

This ion source is also very controllable. Due to magnetic field line curvature outside it from the ECR magnets, you can have ion rates of very near zero if the extraction field isn't strong enough to pull the particles past that. It is extremely controllable even though it's also powerful. True, it's not efficient if you only want 100ua of ions (eg camaro at idle). I don't yet care about that, and neither should you.
And BTW, I'm off the grid and on solar or gas generator power, so in general, I DO care about what power things need to run. Compared to all the other things, this just isn't on the radar as yet.

I too think while in my armchair that pure monatomic ions would be best, it makes sense. Get rid of all electrons, and many (but not all) of the loss mechanisms go with them. for example, and those who think they don't have to deal with space charge regardless are very provably in utter error with the results I now have here. And the old math that also backs that statement up.

However, this new mode I'm running in with super high Q compared to all else here, about ~300x, there's only enough ions at the start to get a pulse triggered, rest are neutrals (most of them, in other words) and at gas pressures so low a fusor won't quite trigger itself without an ion source. So much for even trained intuition with a ton of real experience behind it (eg mine) -- the armchair fails again as a source of progress -- hindsight and standard model backed up by real experiment wins again.

Hopefully, it feeds the armchair part of me and now I make better predictions, and we wash, rinse, repeat. In some active drive mode with pure ions, yes, that may be better-- hope so, there's a lotta orders of magnitude left to go, so more better is needed. But I found the mode without that. In fact, with nearly pure ions, I can't get this pulse mode that has the high Q so far at all, but I'm getting up to active pulse drives rather than ballasted DC to continue to try that, as my armchair does think that will be best in the end.

I do now have a theory of all this that's passed the Feynman test a few times, so I will share that on the appropriate thread soon - I am in running fusion and gathering data to back it up further mode just now.

Easier to resist the firestorm of flames I know I'll get from the skeptics when I am utterly sure I'm right (already, 20 runs replicated with changes matching predicted results thereof) and can by golly prove it in any way I'm asked to, which is almost "right now". Just a little more data collection to go at this point.
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Re: RF 'echoes'

Post by Linda Haile »

So SWR or echoes is not something we need to consider at this stage? I'll start another thread soon on inductively coupled plasma. I'm considering a transistor driven HF tank circuit rather than a magnetron to start with. I've achieved CCP before using a similar circuit with argon at STP, so I think I have a starting point now, especially if I don't need to worry about SWR.
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Re: RF 'echoes'

Post by Doug Coulter »

Not with a magnetron, no, no worries. With solid state, yes, definately! SS ability to handle any peaks of any sort is insanely terrible compared to any tube thing. Much solid state homebrew class gear will go up in smoke in a second or two with SWR at 2::1, so you'd need to test and match at lower power, or provide a lot more available power in the SS system than you need so you can still light it off at something under half power -- opps, that's not efficient! Sometimes what is best isn't as obvious as you'd think.

Though why you'd use an expensive SS thing compared to a free dumpster magnetron I haven't a clue unless you're running it on batteries or something up on the HV terminal. From reports I've seen, though it doesn't make a heck of a lot of sense, people are using much higher powers at the lower frequencies (13.59 mhz) than at the Ghz, maybe it takes more there, but it's not obvious to me why that would be. I chose the 2.5 ghz as that makes the magnetic field for ECR (which is a huge improvement in efficiency and running range of gas pressure) a decent number -- not too big to make easily, and not so small that fields in the lab would affect it. When you get down to the few gauss range, that becomes a problem itself. ANd if you're not doing ECR, you're not caring about efficiency -- that's a factor of about 3-5 better with than without in testing here, not to mention running a lot bigger range of gas pressures with no other changes, which is merely handy. With the magnets on my thing it will stay lit to below e-6 mbar, which is kind of nice sometimes.
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Re: RF 'echoes'

Post by Linda Haile »

Very interesting, Doug. You ask why transistors? I wouldn't say I'm out to re-invent the wheel, Maybe the wheelwright's apprentice, though. I want to start from first principles, or at least with what I already know, and work up from there. I'm doing this to learn about ICP from the experimental viewpoint.
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Re: RF 'echoes'

Post by Doug Coulter »

In that case, you should look into some of the designs the hams are using for solid state linear amps, many are fairly extra rugged and moderately cheap as well. Now, a linear amp won't be very efficient (about 50% if lucky) but it would be a good thing to have in the lab anyway, and those only take a couple watts to drive. Many of the ones I've seen run fine on 12 volt dc, making it possible to do battery operation should you need to go off ground as well. Then get some ideas from the design (many are written up in detail in ham publications) and make your own if you like. You'll find the reasons this or that pcb track gets wide here and narrow there for close control over capacity to ground at some point and a lot of other cool and necessary tweaks that make them rugged and broadband. Many will cover the range of interest here.
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Re: RF 'echoes'

Post by Linda Haile »

Thanks for the advice Doug. I plan to do some experimenting soon starting with my CCP circuit but modified for ICP. As I blow things up I'll modify it with an aim to increase frequency. I'll be using argon to start with as it seems easy to ionize and it is cheap.You can't beat hands on experimentation for learning rather than just copying another design. I find your posts and Chris Bradley's quite interesting but I'm not sure where Chris's experiment is going.
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Re: RF 'echoes'

Post by Doug Coulter »

You are most welcome, thanks for putting up with my sometimes excess verbiage.
Hard to beat hands on, and yes, I use argon too. I found N2 does some interesting but mainly nasty extra strange effect things -- I will be putting up some movies about weird sudden ion recombinations/large N polymer decompositions soon on you tube. Till then, Ar or He are good, as Ne is expensive. Though, I must say, really impressive in an ionizer, blinding bright when you get it right.
You could also fool with H2 from electrolyzed water, ala Seltzman's thing. But be careful with that so an accident doesn't pull water into your system.

I do know where Chris is headed, but I don't know if I'm supposed to tell. His theory is fantastic and a real breakthrough, but it seems he got caught up some in some pretty glows short of pushing to make it real according to plan. I think this is fine, as everyone does that at some point before moving on the the real deal Basically, he's trying to make a novel cyclotron, wherein more than one set of beam going roundy round in the same H field "rub" at the edges, and has a way to re-focus with low energy loss those particles that don't fuse but do scatter, as well as handling the monkey-motion along the H field axis, the idea is pretty slick, actually (and patent pending IIRC). Now if we can just get him to start running real conditions and trying to make his idea into actual reality.....My analysis (under NDA) shows it will actually make fusion practical, or a heck of a lot closer than we are now (even including my recent 300x improvements). Could be good, stay tuned. If I could talk him across the pond to use my lab etc, I would.
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Re: RF 'echoes'

Post by Linda Haile »

Maybe he has limited resources, but I think we're going off topic here, although this thread has served it's purpose as far as I'm concerned.
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Re: RF 'echoes'

Post by John Futter »

Lyn

Please drop the term echos it grates the nerves to an Rf professional
The correct term is return loss or mismatch loss. A slang of this used commonly by amatuer radio enthusiates is SWR. shortened from VSWR or Voltage Standing Wave Ratio.

My companies website has a little discussion on why bipolar and mosfet RF amps are susceptable to imperfect loads and getting more power to the load by using attenuators (Iknow this sounds counbterintuative)
This probably is the best way for you to use a solid stae PA to drive your ICP or CCP without damaging your PA. Using an SWR meter between the attenuator and your ICP load will then allow you to tune without damaging the PA or have it rapidly turn down the power

Link
http://www.iel-rf.com/AttenuatorsDescription.html
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VSWR...return loss.....mismatch loss....

Post by Linda Haile »

John, Thankyou for your input, I appreciate it. 'echoes' is a term I came accross while researching RF sources. Doug explained that this is the same as SWR, which I have come accross before. I also came across the term 'auto-match unit', as used in commercial plasma sources. I agree that I should have dropped the term 'echoes' earlier in this thread because, as you point out, it is a slang term. Thanks for the link, I will explore it in greater depth in the near future. I'm still learning the nuances of RF sources and any useful pointers are most appreciated. I believe I now have a rudimentary understanding of what was meant by the term. I admit I do find VSWR a lot easier to comprehend than 'echoes'.
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Re: RF 'echoes'

Post by Dustinit »

To get a better handle on the reflected power due to mismatch
I found this program that may help and interactively explains it beautifully.
It can be found here

http://www2.fh-rosenheim.de/diegelmann/ ... nglish.htm
Look for the title
Reflection and Transmission at an Impedance Mismatch.

At the bottom z1 is source impedance z2 is load.
green line is voltage into load or termination
red is source voltage blue is reflected
black is the sum of source and reflected and is what is seen at the source.
echo terminology grates because the wave is continuous but would be relevant
for pulses shorter than the transmission line length.
Hope this helps .
Dustin
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter »

Guys, I know some of you are "RF pros" for sure, as am I. I am also expert in fields where the term echos is more appropriate as well, when for example digital switching is involved where lines are a fraction of a wavelength for some frequency components, and many waves long for others. This term is used in the ISO PCI bus spec and many other places in digital land, as it makes way more sense than return loss or SWR there. Lyn wasn't all wrong to use it.

SWR and return loss are fine for narrow band, steady state where the results of a lot of cycles are added and smoothed out into the nice peaks and nodes. Now handle the case of a single step function that way -- echo makes *far* more sense in that context, not to be all that pedantic about it.

Obviously, it's the same root phenomenon either way, but one way talks far more intuitively about single events, the other for narrowband steady state situations. Step outside your narrow specialty! There are no "standing" waves in a single step or impulse event, it all flies back and forth on the transmission lines precisely as an acoustic echo goes back and forth between reflecting walls, so it makes sense to think of that way, and the RF concepts don't work as well for that at all.

Jargon and overspecialization, don't get me started! Both obfuscate and make things harder to understand when getting a little outside the box. And this forum is about just that --
Why guess when you can know? Measure!
John Futter
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Re: RF 'echoes'

Post by John Futter »

Doug
Iwas under the impression that lyn was going to drive his ICP/ CCP with CW RF not pulses.
I agree Echo is acceptable for pulse work whether it is RF, audio, electronic but not for CW
Try a web search for eliminating RF echos and it will not give the info Lyn wants --unless he is really trying to build an NMR which in this case is using pulsed RF.

Use the correct terms and then searching will provide the appropriate answers
From memory I think Lyn has a solid state ENr RF amp around the kilowatt level his ICP should only require 20 -80 watts so a 6 to 10 dB attenuator should keep the amp more than happy ie a return loss of 12 -20 dB presented to the amp output.
A cheap wideband attenuator is a 100m roll of RG58 5dB attenuation @11MHz, 10dB attenuation @ 22MHz etc. At this power level I would keep the roll of cable in a bucket of water for extended runs.
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter »

Oh agreed. I was just saying the term isn't categorically in error. Even standing waves don't stand, it's just a superposition of echoes, well explained in say, the ECL handbook.
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Linda Haile
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Re: RF 'echoes'

Post by Linda Haile »

John, I don't have an amp at present, maybe you have mistaken me for someone else. I'm planning to try different circuits including some built from scratch starting in the 20-80 watt range as you suggest. I'm starting from the pretext that an ICP source is a transformer with the plasma as the secondary as I believe this description is the simplest definition of an ICP source. My previous experience in this field is with air core transformers in Tesla tank circuits and I'm planning to build on this experience to develop an ICP source from first principles. I'm planning to experiment with tank circuits, linear amps and ferrites initially, rather than magnetrons. I've no idea which of these approaches will work (or work best), hence the experimental approach.
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