General > General Technical Chat
Magnetron insulators
antenna:
--- Quote from: T3sl4co1l on December 31, 2021, 03:53:17 am ---They're FM to applied current or something like that, so they're spread spectrum at line frequency. Delightful, eh? :P
IIRC, phase noise at DC is alright, like most any LC oscillator you'd expect; drifty, not like you'd get from a crystal controlled synth of course, so, not really suitable given typical channel size/BW up there, but just doable. And for WBFM voice or something like that, yeah, works out. Oh, uh I suppose you'd normally have a PLL servoing DC, then modulate on top of that to get reasonable performance. But the PLL only works so fast, it can clean up close-in phase noise but misses farther-out noise, or added modulation obviously. And drift can be helped by keeping temp stable, or regulating the magnets with an external servo coil or something. Since those have a... negative tempco usually, isn't it?
Tim
--- End quote ---
I'm lucky to make an audio amplifier work decently so I'll pass on making an FM transmitter out of my pizza reheater, I was just shocked at the level of power getting outside the oven and the bandwidth it covered. What you say sounds logical though, but honestly, it's way above me. I don't think I'll be trying to stabilize my microwave oven, its bandwidth is probably a benefit when it comes to cooking. But with that first sentence you wrote, am I correct to assume that if I remove the 60Hz ripple on the HV side with better filtering, it wouldn't do that? I thought the frequency was controlled by the geometry of those little cavities the electrons zip past? I guess I need to hit up wikipedia and get a crash course on magnetrons, klystrons etc.
T3sl4co1l:
Heheh, yeah. Certainly wouldn't recommend you do so; but, have seen an amateur build such a transmitter before -- more just to say it's possible, and how well (or badly) it works.
As an oscillator, it depends on cavity dimensions yeah, but also whatever's coupled to the load (so, what's going on in the chamber), and various other conditions. It has some natural bandwidth, of which it mostly oscillates in the middle, but can be perturbed to one side or the other -- this is what gives the tuning range. It's only a 100MHz or so, so the fractional bandwidth is pretty low -- as you'd expect -- but compared to the band it operates in, still a pretty wide range.
Just like, you can pull a crystal, but only by a few kHz; you just can't couple to it strongly enough to push it around further. And indeed, with good reason, else it wouldn't be a good frequency reference, hehe.
I forget why exactly current causes FM. It causes AM too of course; the V(I) curve is not like a tube diode, but a zener diode -- the magnetic field causes a threshold effect, below which the electrons spiral uselessly back to the cathode. So of course, up at that threshold voltage, more current means more electrons blowing the cavities like a whistle, more power output; but it turns out it also modulates frequency. (FM receivers are designed to reject AM, so the AM doesn't matter in practice.)
It would be a bit of doing to put a filter on there, but that is indeed the correct reading, at least as far as I know. You'd need a few more capacitors (at least one more, but probably of a much larger value in order to get useful filtering effect), and another diode, and maybe an active circuit (to regulate the current, saving on filter size). Not recommended, of course; the voltages are nontrivial. :)
Tim
SeanB:
Yes, allowable leakage for a microwave oven is pretty high, much higher than for intentional radiators like WiFI AP's, and the associated units. 0.5W is the limit in most cases for the microwave, and more for the larger commercial units, while your AP can only dream of ever getting to 0.01W right at the antenna port.
Guess which one people are concerned about.......
tom66:
Most Wi-Fi routers and user devices implement "microwave oven avoidance". Either they use the region code or the RF measurements to determine when the microwave is not actively transmitting and transmit in that period. Most microwave ovens use simple capacitive voltage doublers which leads to a quiet period at 50 or 60Hz during which time transmission can be scheduled.
I do wonder how this applies to ovens that use inverters, as they use a power-factor correction circuit that should ensure current through the magnetron is a sine wave at 100Hz, which should mean overall utilisation of the 2.4GHz band is closer to something like 70-80% rather than the 40-50% of a traditional microwave.
amyk:
I have troubleshooted and worked around problems with dropped connections while the microwave oven is on by changing the WiFi channel to one or the other end of the range.
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