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Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Lots of harmonic radiation is definitely there -- the radio goes silent whenever I turn this on, regardless of what station, and regardless of what frequency I drive it at. =P
Hi BP, You are no doubt emitting lots of harmonics (for the FM radio, there will be harmonics at 108.48MHz and 94.92 MHz...both in the FM broadcast band).. You are evidently overloading the frontend of the radio receiver, rendering it insensitive to any station.
I wasn't so worried about the heatsink having RF on it... but at this stage, I'll try putting some mica and grease on. Do I dare ground the heatsink?
Yes! Ground that heatsink! If you make another PCB, use a double sided one. Keep one side as the ground plane and connections on the other. I usually mount the components on the circuit trace side. This minimises drilling. If you have ground traces on the circuit side, put lots of vias (1 every 2 cms or so) to the ground plane on the other side. Keep all connections as short as possible and keep power leads away from air-core coils (where they can couple easily). Mount heatsinks directly to the ground-plane side of the circuit board to minimise RF drops across inductive paths (keeping the heatsink at RF ground). The mica washer transistor mount will add a little capacitance, but if you are careful about eliminating inductive paths to PCB ground plane, there should be no troublesome VHF/UHF resonances.
I'm beginning to feel that 1uF of decoupling isn't enough, and I should be using a variety of decouplers, like ceramic, film and electrolytic all in parallel.
Beware of self-resonances. Try to keep decoupling caps in the range of 0.5-1 ohm reactance at your operating frequency. 0.01uF should be plenty for your frequency range. Keep the cap leads SHORT!! 1 ohm inductive reactance is produced with 10nH...if this inductance is present in the 0.01uF cap leads and the cap itself, it will resonate, giving unpredictable results... I would suggest the smaller cap.
Does your setup look something like this:?
How do you calculate or estimate the value of the RF choke, Lmatch and Cblocking? Or is the RF choke uncritical as it'll just allow more/less current through? Have I even got the arrangement of Ls and Cs correct? :P
Yep, it looks similar to that circuit. I also have a variable capacitor across the primary for tuning.
The choice of Lmatch and Cblocking is a bit hit and miss because the it is difficult to calculate the loading from the arc. It generally has to experimentally determined. I started off assuming thet the resonator Q was about 10-20 with arc loading. Assuming that I wanted the real part of the feedpoint impedance to be around 50 ohms (chosen partly for "sentimental" reasons as well as allowing me to use a smaller variable capacitor for tuning). Generally the total inductive reactance of the output circuit should be about 5-8 times the load-line impedance of the transistor. (My case: 100V, 2.2A -> load-line resistance is about 0.5 * 100 / 2.2 = 22 ohms) Including the coil primary and Lmatch, total inductive reactance is about 100 ohms... In practice, my coil Lmatch gives best performance with about 60-70 ohms reactance at my operating frequency. This can be adjusted with the variable cap connected across the primary anyway. The variable cap also assists in transforming the real part of the impedance to a suitable value....when you vary the spark power level, the feedpoint resistance changes.. match can be (almost) restored by adjusting the variable cap. In short, settle on some starting values for Lmatch and Cblocking, Ctune and make an estimate of the primary feedpoint inductance. (I had the benefit of using an advanced simulation to make my estimates.)
The RF choke is simple... A good rule of thumb is to keep its reactance about 10-20 times the load-line resistance (500 ohms or so). 5 or 6 uH should be enough in your case. Don't overdo it tho'.. You don't want your choke to resonate. Use a powdered iron RF toroid core (no ferrite, Amidon mix 2 or 7 or equivalent should work fine).
There is nothing wrong with your topology, as long as you can get a reasonably good match and the transistor switching timing is correct. The only disadvantage is you have the DC supply present on the primary...this could make tuning difficult if you wanted to add a parallel tuning capacitor to the primary.
Often when I need to split a signal, the logic isn't fast enough, or my oscilloscope is too crap to accurately draw two waveforms in sync (as in I twiddle the trigger and dual traces move left/right in opposite directions), I've really got no way of adding delay to compensate for slow logic. So what I do is make a little transformer like so
At your frequency...in the absence of advanced driver chips...transformer splitting is a great idea. Just make sure the driving circuit can supply enough power to prevent gate-drive "sag" when operating the final PA at full power.
Since I'm adding 50-63pF of my own capacitance across DS, do I presume then that there's no way I can use FETs with an output capacitance larger than, say ~200pF? Or does that change when I use a proper network to couple the primary?
The drain cap affects the coupling to the output circuit. Larger drain cap = lower coupling to output. Hence, by scaling the impedance looking into your matching network, you can counteract somewhat the effect of extra capacitance in the drain circuit. You may lose good class-E operation, but as long as your transistor runs reasonably efficiency and stays cool, who cares. Once you get proper class-E operation in one situation, you lose it if you change power level or change toploads. I found that as long as the transistor switches when it is subjected to Vdrain < 0.15 * VDD, it continues to run fairly cool and generates good power output. In fact, I can run what looks like class-B for a few seconds and get about 350 watts. Operating in its "near class-e" mode, it generates about 220 watts and will run all day in CW mode with no problems...
Hope my long-winded discourse is of some help. In short, by using ground-planes, metal enclosures and fine wire-mesh faraday cages, you will improve TC performance as well as reducing drastically "EM pollution" in your lab.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Thanks for the great tutorial WaveRider!
BP: As you go up in frequency, you eventually reach a point where the MOSFET's own output capacitance is as much as you need, and ultimately more than you'd like, for proper Class-E operation. So you don't need the external drain cap any longer. The capacitance between the FET's drain tab and the heatsink through the insulating washer starts to be a problem about then. One that I noticed you elegantly solved by leaving the whole heatsink live with RF
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
WaveRider, you have my gratitude. Your "long winded discourse" goes into wonderful detail -- I feel very lucky to get such personal advice, thankyou.
I may be off-forum for a few days, but the first thing I do will be building a small RF enclosure.
Thanks, sincerely indeed, again. I'll update you guys on how I go.
*** mod edit, combined double posts. We're serious about the double posting rule! ***
After tearing apart a large monitor today, I found the perfect base for my RF cage -- 0.5mm steel with perhaps 5% 1mm perforations. I've made two walls, two ends and a top for the cage from 1mm steel 1" pitch square weldmesh.
Quarter-wavelength at f=13.56MHz is ~5.5 metres.. will 1" pitch be OK, or should I find money to spend on finer mesh? :P
I regret that my camera is ~3000km away atm, will take pics next week.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
I use 1cm steel mesh at 4.5 MHz. The evanescent field outside the shield decays to about 39% at about 10 cm from the shield. Larger holes will mean more field (slower decay) just outside the shield. Since your frequency is higher, smaller holes would be better. I'd go for the 1cm hardware mesh. The field containment will be better (i.e. less problem with coupling to cables lying around close to your "cage"). Plus, standard hardware mesh is cheap and readily available at hardware and garden shops..
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
I bit the bullet, put all my loose change together (AUD$5.20) and cycled to the big hardware store. There was some nice fine aluminium flymesh at $10.50/1m. 1M minimum. But I smiled at the very pretty girl on staff and she cut 0.6m, and only charged me 0.45cm, at her employer's expense.
This mesh is closer to 2mm pitch, and I'm overlaying it on the 1"-pitch steel wire. Can't wait to see how things improve with the animal restrained. Should I feed power in/out of the cage with coax?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Your mesh should be good.. Aluminium can be hard to solder, tho! All joints must be electrically well "sealed" Use a coax to enter the cage (with the coax shield electrically well connected to the mesh cage).
Registered Member #87
Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191
Besides the obvious troubles of driving one, any good reason a tube couldn't be used for a high frequency coil? I geuss what im thinking is that i see relativly cheap tubes on ebay rated for some hundreds of watts at 40+mhz, more than enough for a small coil.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No reason whatsoever, except that it wouldn't be a Class-E SSTC any more, so it would belong in a different thread.
Tubes are much easier to drive at high frequencies than switching MOSFETs, you can just hook them up to self-oscillate with some variation on the classic Armstrong VTTC circuit. The classic VTTC tubes like the 833 should all be good up to 30MHz.
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Firkragg wrote ...
Tubes can also be set to woork in class E, and even driven externally if needed (it would still be class E but VTTC rather that SSTC)
Regarding soldering aluminium with normal solder, anyone actually managed to do this?
I've read some time ago on an italian electronics newsgroup something about soldering aluminium using some oil / grease.
Normally, when aluminium is soldered using TIG or MIG welding systems, the area being soldered is protected by some gas like nitrogen or argon, to avoid the really fast oxidation of the alluminium, so to be able to solder it we must protect it someway from oxidation!
this is the translated process, tomorrow I'll try with a little piece of alu and normal soldering wire.
1st: clean very well the aluminium bar you want to solder, maybe removing phisically the oxid from it with some sanding paper, then put on it some grease or oil, to protect the surface from oxidizing.
2nd: pick a big soldering iron, maybe the "gun style" ones, rated at 100w or such, and heat up well the alu, and let some soldering alloy to get to the heated alu, that is still protected by the oil. Now, the tin/lead alloy should coat easily with the aluminium bar, since its not oxided on the contact area.
I'll try it tomorrow, using a small aluminium bar well sanded and heated up with a hot air gun, then soldering iron.
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smaller coils (and a 12MHz target)
