X band cavity design and microwave safety - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2020, 06:09:56 pm
content: Hi!I'm slowly assembling an electron paramagnetic resonance (EPR) spectrometer, which involves coupling X-band microwave energy into a resomnator inside a strong magnetic field.Setup is going to be something like[Gunn diode oscillator]---->[magic tee/circulator]---->llVllV[cavity/resonator]So basically the resonator is the only X-band part needing homebrewing.The iris is commonly just a nylon screw with a small metal (ferrite?) slug at the end that can be moved in and out in front of a smaller-than-waveguide hole.The resonant mode on commercial solutions usually TE102. There needs to be a hole through the cavity to allow for the insertion of a 6mm sample tube.Here's the actual question:A) Could one just cut a lenght of copper waveguide with flange, to the desired lenght of the cavity (and what would that be?), solder the other end shut with copper foil, drill a hole through the center of the H plane and be done with it?B) Can the iris just be copper shim stock inserted between two flanges, the non-cavity end having a ferrite tipped screw?C) How serios should microwave safety be taken with an x-band gunn diode? From what I can read, it's a fairly low powered device in a band that doesn't really resonate with human tissue.This is what a commercial X-band EPR probe looks like, for reference:Thank you in advance!

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 22, 2020, 07:02:28 pm
content: With copper/brass waveguide tubing, yes you can just solder a plate across the ends and have a nice cavity. You can model it in something like openEMS to verify the dimensions.Commercial Gunn modules have the diode in its own cavity with an iris for coupling. The iris has a tuning screw some distance away to control the coupling factor, but they are just regular stainless screws.As for the safety aspect, Gunn oscillators are in the 10-20 dBm range, so won't pose a general heating risk. However, avoid staring into open waveguide ends or horns to avoid exposure to your corneas. Heating does not correspond to resonance - pretty much any band will cause heating in soft tissue. Corneas are high-risk because they lack blood flow.Edit:Just curious - why Gunn oscillators, when low-power amplifiers arecheapandcheerful, as are low-cost synthesized sources?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 22, 2020, 08:14:02 pm
content: Thank you very much, that's good info!I'll look into openEMS, I assume a waveguide cavity should be a whole number of half wavelenghts long?Gunn oscillators simply because that's what I have - a little Toshiba X-RX15 module. It doesn't have any visible tuning screws by the way.Also most of these EPR spectrometers uses either Gunn oscillators or Klystrons.Aiming for something like this:

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 23, 2020, 12:00:13 am
content: Yup, half wavelength if you want a TE010cavity. If you're lazy like me and didn't want to derive the formulas I forgot since grad school,hereis a reference (see section 3.5).I did a bit of reading on EPR, gave me the same headache as trying to wrap my head around MRIIt looks like frequency stability of the source is less important since you sweep the magnetic field and observe the resonant peaks, right?The diagram shows an isolator at the output of the source, this is done in low-cost Gunn sources by controlling the coupling factor from the Gunn cavity with a screw (and is therefore not a true isolator). Additional screws are provided to control the cavity size and hence the diode's output frequency. The one I have here at home also includes a varactor for electronic tuning. If you look on E-bay you may be able to score a circulator from an old marine X-band radar, maybe even the detector modules they use.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 23, 2020, 01:08:32 am
content: Good reference! That one goes in the folder!--Even lazier cavity design: Couldn't I just make a waveguide 'slide whistle' - have a close fitting copper plate (backed by some sponge or coax braid) on a stick and move it in and out of a lenght of waveguide to find resonant lenghts? Basically a super simplified version of a cavity wavemeterThanks!

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 23, 2020, 07:41:26 pm
content: You could do this with a regular cavity, plus a couple of tuning screws, which would give you much better control over the cavity size.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 24, 2020, 07:18:23 am
content: The problem is the cavity is very constrained in width, as it needs to be placed between the poles of a powerful magnet, and that space also needs to accommodate some sweep and modulation coils. Which is why using the H plane of some WR90 would be ideal.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 24, 2020, 09:58:00 am
content: Another safety concern I've been considering: Is waveguide ever made from beryllium copper? I know some flexible waveguides are.Machining, drilling and filing beryllium copper is a BAD idea, but this fear may be unfounded, as I can't find any references of standard waveguides being made of anything but copper, brass, aluminium or, rarely, stainless.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 24, 2020, 03:17:07 pm
content: The only time I've seen warnings about Beryllium are on BeO windows (on high-power tubes) and BeO insulators used on (older) high-power semiconductors. Both are rare. Never seen anything about BeCu stuff. Maybe someone with an EMC testing background can chip in?

author: coppercone2
author_rank: Super Contributor
timestamp: August 25, 2020, 08:25:35 am
content: I heard its nasty and i heard from people that work no spark jobs using beryllium copper hammers that their a big hazard if you decide to clean it up with a sander. They use them because their non sparking, but when you use copper like a steel hammer head its going to deform no matter what kind of alloy you use.With a normal copper hammer you can clean it up on a belt sander or whatever with little risk, but if you do it with a beryllium copper hammer its a bad thing.https://www.amazon.co.uk/Ega-Master-Beryllium-Copper-Hammer-Sparking/dp/B07MSFNBLBBut in general brass dust is bad too. Yes normal brass if pretty bad if you sand it (often wet sanded). You don't hear much about sanding brass because it polishes up really nicely with brasso or whatever.As a general case, anything you read about 'metal fume fever' applies to the base metal powder, since basically under normal conditions all metal fume exposures are actually particle exposures (it condenses, you don't actually get any metal in gaseous form), based on what OSHA says. So if there is any air float component, the toxicity is pretty much the same.The only EMC BeCu thing I heard about is RF fingers, in addition to the connectors pins that everyone knows about.The only waveguide type I could think of would be the corrugated kind.https://patents.google.com/patent/US4429290A/enThis catalog claims its available to order:https://www.microtech-inc.com/Products/WRD-350-Waveguide-Flexible-Double-Ridged/p3s25/I don't think it makes sense to make a BeCu wave guide that's not a weird flexible one because the safety controls for manufacturing are expensive and wave guide is the type of component someone might actually modify in a lab, so you don't want to sell something you know can poison someone. Unless its ancient stuff where no one cared. The safety behind BeCu fabrication is taken seriously in the modern world, which makes it expensive.Obviously be careful sanding brass because that would be my go to for making a waveguide, you can do very nice sanding and cutting and soldering work on brass and make a precision assembly without manufacturing difficulties (you can use a chisel even, or is this trick for bronze? never tried anyway). I recommend you do as much work as possible with tools that do not make fine dust or fumes, then do as much finishing work as possible wet/ventilated. If you are new to shop work, you will find that copper is irritating to work with, while brass is very pleasant.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2020, 08:59:50 am
content: Thanks for the insight! I agree in that it wouldnt make much sense for it to be BeCu but almost all 2nd hand x band waveguide parts I come across are 1950s and onwards vintage radar/marine/aircraft parts, so you never know.The waveguide piece I have looks to be brass. I dont plan on grinding it with power tools, I think I can get close enough by hacksawing to rough lenght, hand file to within 0.2 mm or so, then wet sanding flat on a glass plate to 1000 grit,, then solder closed with copper sheet.I didnt know metal fume fever was a risk without heating/grinding! But yeah it makes sense that it's not actually "fumes". Vapor pressure at room temp is through the floor for most metals.

author: coppercone2
author_rank: Super Contributor
timestamp: August 25, 2020, 09:03:38 am
content: i think its that the amount of product you get that you can inhale is small compared to the amount you vaporize during weldingI don't think I would be too scared of brass, unless you plan on doing it for a career. I meant don't get medieval just throwing squares of metal on a belt sander to make the shape you want (with a fast sander its tempting, you can do it with steel). (the metal eating sanders seem like very popular shop projects on youtube, in lieu of using jeweler saw, plasma cutter, coping saw, etc). I see some equipment knife makers made where it hardly even makes sense to do anything but just sand the whole thing down when it has a long fast sharp belt on it. (maybe you call this a belt grinder not a sander). Seriously those 72 inch sanders are pretty crazy in terms of how fast they devour metal.Your work plan is pretty good. All I can think of is that if you had a precise saw (adjusted band saw or cold cut metal saw with vise) you could make a closer cut to reduce filing(its probobly a negligible amount of work to file a precise hack saw cut anyway). The hard part will be precision soldering, as not to have a fillet (I have seen very skilled solder workers do this some how, but they have a extremely good feel for how solder flows, along the lines of stained glass work).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2020, 10:35:37 am
content: An inside fillet is an acceptable deviation from the resonator shape, I think. I plan on oversizing the copper end plate, standing the waveguide on it, then blowtorch some electronics solder all the way around (higher flux:solder ratio than thick plumbers stuff). I REALLY should silver solder something like this, but that's not going to happen.The best solution would to solder on a brand new WR-90 flange in the correct lenght and then just bolting on an end plate, that way the end will be perfectly plane.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2020, 11:49:55 am
content: Oh, and to answer my own question, I found a paper "Improved rectangular cavity for EPR in TE102 mode" in which the cavity is WR90 dimensions and then 4.15 cm long - which they call one wavelenght for some reason. I don't get that, but 4.15 cm might be a good lenght.

author: coppercone2
author_rank: Super Contributor
timestamp: August 25, 2020, 05:56:41 pm
content: you might not be too happy with silver solder because its a high temperature process and without nitrogen purge, the cleaning is more difficult then solder cleaning is, not to mention when the temperatures get high enough (red heating) you get might still get RF surface finish problems because of copper 'separating out'. I think it would require internal repolishing. Not sure what brass will do.i also calculated a while back that the effect of fillet on impedance in the waveguide is small until frequencies are high. But I still don't like it, makes the project feel super half assed.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2020, 06:47:09 pm
content: I agree, I don't have all the oxy-fuel torches and stuff to do silver soldering, but I'm pretty sure that's "the" correct way of attaching waveguide flanges to waveguides. I think there's a US signal corps waveguide 'plumbing' handbook, and I think that's where I've read it. Soft solder should be decent enough, if the waveguide isn't supporting anything heavy.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 25, 2020, 06:49:30 pm
content: I had the fortune to be allowed to examine a commercial EPR spectrometer cavity today at my university. Taking off the side caps, it has a potted coil on each side, these are the modulation coils, but look at the white substrate below it. The cavity is silver plated on the inside, and I think the sides must be made from either some ceramic pcb or Rogers substrate. The point is the sides that are normal to the magnetic field must be thin walled to reduce eddy currents from the 100 khz modulation coils. The silver plated ceramic/teflon sides are just edge soldered onto the rest of the cavity. It might be worth making a cavity like that; cut/grind off the edge and solder some silver plated pcb onto there. The hole in which the test tube is placed is quite wide bore (under the black screws top and bottom), I didn't have calipers with me but I'd say 10-12 mm.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 30, 2020, 08:20:32 pm
content: I've received some X-band parts! Among those this strange homebuilt cavity thing. No idea what he used it for, but it shows a very simple way of fabricating an iris.It's difficult to see, but the waveguide has been cut about 12mm deep all the way through the H plane symmetrically on either side. In the slots, brass shims has been soldered to form an iris. A tapped hole with a brass screw forms the 'shutter' or coupling adjust.Not sure I can reuse this particular piece of waveguide, as it has 3 consecutive iris' 1,5 cm apart but who knows. What do you think? And does it matter if the iris narrows the E or the H plane?After the 3 irises, the cavity has a plunger, setting the cavity lenght from 83 mm to 96 mm, or from 5.2 to 6 half wavelenghts at 9.4 GHz. I'm wondering if it's reusable as a first cavity. Would save me some trouble.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 30, 2020, 09:00:02 pm
content: Also, a more concrete question: when a magic teeis drawn as it is in the schematic above, which port goes to what? port naming and numbering is NOT consistent at all. And most references I find uses magic tees for either combining or splitting signals, not as a reflection isolator.Thanks for your interest!

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2020, 10:33:38 pm
content: there are arrowsAlso that wave guide is in serious need of cleaning. I would put it on the buffing wheel before working on it (contamination) (plug the holes). I have one ancient big high power filter that looked like that. Not a lovely patina. However when I cut one of them apart when I was very curious as to how the insides looked like (it was capped with coaxial feeds), the inside was nice looking.Also how old is that university equipment? It looks like the plating is failing on it. I have not worked with bare waveguide, only sections with coaxial feeds, but I wonder if you should grease sections before mating them with deoxit grease or something. The old pieces I have looked like brass bells.I think I will try to electroclean the inside of this open piece of WR90 i have. Also on the interior of mine, there is no solder spill through, the solder joint looks flush.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 30, 2020, 10:50:10 pm
content: There most definitely are not arrows on the actual tee, unfortunately. I think the two colinear arms should be signal source and reflection detector respectively, but whether the cavity should be at the sum or difference port I can't really wrap my head around.I agree, waveguide looks abysmal, insides looks much better though. And the university probe is pretty new, no more than 30 years I'd guess. It's not the plating, it's solder I think.

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2020, 10:53:35 pm
content: btw what exciting uses does this machine have, I have been looking for a engaging microwave project for a long time, they don't really excite me

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 30, 2020, 11:03:44 pm
content: It's called an EPR spectrometer - "electron paramagnetic resonance" - you sweep a magnetic field (around 3500 gauss for X-band) through the sample in the cavity, and once you hit the resonance field strenght of an unpaired electron (paramagnetic ions, organic radicals etc) the electron is kicked up in a higher spin state, absorbing some energy. This causes a dip in energy (kinda like hitting resonance on a grid dip meter) and you can plot that as a spectrum.Different spin systems will give different sets of spectral peaks, and these peaks split up (instrument resolution willing) into very fine peaks called hyperfine coupling - stemming from electron being spin-coupled to other atom nuclei (especially hydrogen).It's a more obscure technique than the related MR/ NMR, but decent resolution can be achieved without a superconducting magnet so that's my motivation.as well as, like you, finding an interesting microwave project that's NOT a 144 mhz down/upconverter transciever. I'm a bit bored with HAM stuff tbh.

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2020, 11:17:16 pm
content: so do you sweep frequency?I.e. waveguide has a stable pass bandoops, 3500 gauss is not that much magnetic field. thought you said 3500 teslaSo its like S parameters with the sample in h field flux?I made one good antenna and I figured out how many evangalists were on the air and gave up with reception lolWhat interesting materials parameters can you study?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 30, 2020, 11:37:40 pm
content: Sweeping frequency is tough to do with good repeatability. Sweeping a DC electromagnets current with a slow (minutes to hours) ramp gen is the option people has gone for. More recently, short bursts of RF energy and then fourier transforming the decaying echo. EPR is still usually CW swept field.It has been used to fingerprint authentic dyes and pigments in ceramic glazes and artwork to detect forgeries, medical imaging, and investigating chemical reactions. Reactions involving radical mechanisms can be monitored and the shape of the radical molecule can be deduced from the spectrum. It is pretty neat actually.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2020, 12:29:16 am
content: how do you make a reactor in there ? like if you need a condenser, gas flow, adding chemicals, mixer, etc?Or do you mean measure the result of a chemical reaction.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 31, 2020, 07:00:51 am
content: Well usually it's as simple as mixing reagents in an EPR tube - 3-4 mm dia quartz test tube - but some cavity also have a quarz window so mixtures can be irradiated with UV, or visible light for the study of photocatalyzed reactions. But I've also seen high pressure hi temp probes, as well as liquid helium cooled cryoprobes.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2020, 03:31:13 am
content: But can you make the geometry so you can study a fed continuous reaction?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 01, 2020, 08:27:34 am
content: You mean like a flow probe so reaction mixture is continuously siphoned from a reaction vessel, through the cavity and back in a loop? Yeah most likely. It is commonly used with many other kinds of spectroscopy at least.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 01, 2020, 03:56:28 pm
content: Cavity Coming along nicely! Unsure in which plane to put the coupling screw.

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2020, 07:33:45 am
content: Quote from: ChristofferB on September 01, 2020, 08:27:34 amI was thinking more like a reaction occurring inside of it, to study what happens when chemical reactions happen (being careful for the reaction not to cause a gas or precipitate to form (avoid phase change), if liquid-liquid reactions like this even exist?), or to vary the temperature of a fluid loop etc

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 02, 2020, 08:50:17 am
content: That is very possible. In fact that's the standard way of doing it.The curing of epoxy resin proceeds via a radical mechanism, and that reaction should be fairly followable on EPR.

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2020, 09:10:54 am
content: thats an interesting way to verify if the epoxy you have is good, see if cure time ends when they say it does. Tensile strength test might not tell you everything (might be something going on the side that it clues you into even if tensile came out good).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 02, 2020, 09:27:00 am
content: One could even set up software to monitor the integral of a range in the EPR spectrum over time, seeing the reaction rate decaying. I'm actually not sure if the polymer industry uses EPR at all, but maybe they should.

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2020, 01:51:09 am
content: if you can build it, it seems like a good QC for critical applications where you have it shipped climate controlled and have to keep it in a factory in a refrigerator. Cheap

author: StuartA
author_rank: Regular Contributor
timestamp: September 05, 2020, 12:29:25 am
content: EPR is a new one on me, but I'm not telling you how long ago I did my chemistry degree! Wiki says it was first observed back in 1944, but I wonder if it had to wait for better electronics and computing power to become useful?Just curious, what does an EPR spectrometer cost?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 05, 2020, 12:00:16 pm
content: I think it was one of those things that was researched, but not used for stanard analysis for a long time. the electronics was ready almost from the get-go, since all the X-band wrangling could be done with surplus radar parts (of which there was a LOT from the 1950s and onwards). I think the first commercial EPR is from 1959 but I may be wrong.I'd suspect a new, modern dual-freq. one costs north of 500.000$. The magnet is a ton-heavy iron chunk, with watercooled copper coils run by a DC PSU like an arc welder.

author: StuartA
author_rank: Regular Contributor
timestamp: September 05, 2020, 08:08:11 pm
content: Quote from: ChristofferB on September 05, 2020, 12:00:16 pmThanks for that info. Some time ago, I worked for a company which was exploring the limits of what you could do in terms of 'chemical analysis' by making measurements of permittivity; not a new idea, but it came as a spin-off from some other work.I seem to recall that there was a German company offering quite a low-cost device (<<$500,000) which followed the curing of polymer formulations by watching the change in its permittivity. Unfortunately, our Management decided to abandon the work after getting just one product on the market (an oil 'analyser').

author: LaserSteve
author_rank: Super Contributor
timestamp: September 09, 2020, 05:25:11 pm
content: Sounds like a good use for those X-Band Stellex Yig Oscillators that are all over Ebay.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 10, 2020, 02:26:13 am
content: it sounds like temperature stability is of the upmost importance with this design.i think the best option has to be selected here, I am not sure Yig is the best choice. Its good for sweeping but he said multi-frequency response and calibration is extremely difficult.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 10, 2020, 01:10:29 pm
content: The Stellex yigs are quite stable, and a YIG/Synthesizer board pair is ~100-150$. I own a pair.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 11, 2020, 01:26:12 am
content: what does the stability of the various oscillators look like on a theoretical level? yig vs DRO vs CROor others.if there was a table of expected behaviors it would be easy to make a confident decision

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 11, 2020, 11:12:39 am
content: I've never heard of YIG's being used for this application. I think the industry (Bruker and Varian, really. only commercial EPR companies AFAIK) went directly from surplus radar tech (Klystron sources, later Gunn oscilators) to DDS.

author: coppercone2
author_rank: Super Contributor
timestamp: September 12, 2020, 02:49:53 am
content: 10 GHZ dds for a single frequency application?you mean like one of these?https://www.analog.com/en/about-adi/news-room/press-releases/2017/5-2-2017-analog-devices-wideband-rf-synthesizers-size-reduction-versatility-performance-to-13-6-ghz.htmlthat one only needs 600MHz clock, 40$ for the IC*thought you meant AWG

author: coppercone2
author_rank: Super Contributor
timestamp: September 12, 2020, 09:49:44 am
content: I was re-reading the thread and I noticed you have one misconception ,you can silver solder fine with a normal mapp torch, or maybe even propane. You just need to insulate it. Also air-acetylene might be something you are interested in, i.e. smith silver torch (this is a jewelers air-acetylene torch, as opposed to the smith little torch, which is a oxy-acetylene torch. they look similar and are made by the same company but two very diff pieces of equipment). The silver smith torch would be 200$ with regulator + ~$80 for the bottle and 30$ for the fill for a few hours run time, if all gotten new. If you don;'t mind getting a non premium large-size torch, it would come out cheaper used then the 'gold standard' smith set. Add another $100 for the silver rods and flux, cleaning brush, etc. Expect to pay 20% for the chinese knockoffs on ebay, but obviously you need a garage, I think 10cU of acetylene would still be hazardous in an apartment. The newer mapp torches with the 'spiral' flame would help, and you can preheat using a torch then use one of those small oxy-mapp sets for $50 to actually direct silver solder flow. BTW you will be impressed by the fillet size/fluidity of actual high silver solder, it tends to 'plate' things after the gap is filled if there is excess, it does not pool up like solder does. The highest fluidity rating of commonly sold braze is the 56% silver type. So it will look like a 'stain' of pure silver that is very thin, rather then globs you get from excess solder, so long you heat it, think low surface tension fast spreading, higher quality I think, the geometry will be more stable. If you silver plate the whole thing afterwards with the cool-amp powder, it might be not detectable?And if you want a real number on run time with a 10cubic foot acetylene tank (MC) on a smith torch, I can extrapolate from the manual (and this will be lower if you use insulation)brass: 2 inch diameter, torch life between 1hr to 30 minutes using a #4 or #3 tip respectively (depends on how fast you need the heat, if you fixture good you can use the smaller tip that has a 10cfh flow rate, instead of the larger 20cfh flow rate). I expect heating time to be 5 minutes or less.My calculations on fillet size effecting the waveguide were based on microwaves 101 specification of common waveguide dimensions being 5%.I am thinking about this because the waveguide random pieces I have happen to be the correct frequency range and I have not found a use for them in over four years.The signal power necessary is under 1W (you wrote the bare gunn oscillator without a amplifier is adequate at 100mW 50ohm)? In that case I have all the equipment other then the apparatus itself. I think I just need to fix one amplifier control card (a few transistors I never bothered ordering, the PCB has been sitting on my stereo for 2 years now).Does the test experience any power dependence from 100mW-10W ?And what is the expected measurement noise level from this equipment? How sensitive does the pickup need to be (LNA requirement)?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 12, 2020, 10:06:17 am
content: Quote from: ChristofferB on August 30, 2020, 11:37:40 pmI'm curious why sweeping frequency is tough to do? The frequency of a signal is one of the things we can measure and control most precisely of all physical properties - we can generate tones in the lab that are accurate to ten decimal places or more quite 'trivially', but we need entire industries and years of expertise to create a voltage or current with have as many significant digits.I'm not an expert on the topic, but it would seem to me that you can use a coupler (with 20 dB or 30 dB or more attenuation) to take the oscillator frequency and feed it back into a classic PLL architecture or similar, and sweep it with very high frequency, or even just linearly sweep the control voltage and just measure the frequency at the same time as you measure the detected power and store the power-vs-frequency with very high precision?(it might be easier to just sweep field strength, as I said, I am not familiar with the details of the topic, but the statement that you can't sweep frequency repeatably just seems an odd one to me)

author: coppercone2
author_rank: Super Contributor
timestamp: September 12, 2020, 10:21:09 am
content: Quote from: TheUnnamedNewbie on September 12, 2020, 10:06:17 amI guess it needs high power and receiver linearity. You are saying to connect a VNA to it. All I can think of is that the sample itself is nonlinear with power, even if you compensate with measurement. I think low signal VNA test subject is supposed to not be effected by power in terms of its response. The most powerful VNA I have seen myself is 150mW as a special feature, and this is only for a 300MHz unit.The regime transition point between small signal and large signal S parameters might be much more different with a weird circuit element (i.e. epoxy) then it is with a microwave filter made of microwave components. In literature large-signal S-parameters are treated differently and are considered a more difficult, expensive test. The VNA cheats with measurement by assuming power invariance, which is OK for highly linear stuff and low power levels. And it might be bad for business if you put a whole bunch of clauses in the use of analytic equipment (it causes confusion if you say 'these things are ok with these settings, these settings are required for this, it might give you weird outputs for this', generally a simple measurement tool produces better results and prevents bad data from being published, if someone did not read a manual, and a company is more likely to buy it if they think its bullet proof. They want to say 'we put it in this machine and there are no doubts'. If it has huge adoption and everyone knows about it, then you can start putting complex features in it, but this sounds unheard of.. they want the market calm.**** microwaves 101 ** just a hunch, not sure.Types of S-parametersWhen we are talking about networks that can be described with S-parameters, we are usually talking about single-frequency networks. Receivers and mixers aren't referred to as having S-parameters, although you can certainly measure the reflection coefficients at each port and refer to these parameters as S-parameters. The trouble comes when you wish to describe the frequency-conversion properties, this is not possible using S-parameters.Small signal S-parameters are what we are talking about 99% of the time. By small signal, we mean that the signals have only linear effects on the network, small enough so that gain compression or other non-linear effects do not take place. For passive networks, small-signal is all you have to worry about, because they act linearly at any power level (at least until you blow them up).Large signal S-parameters are more complicated. In this case, the S-matrix will vary depending upon the input signal strength. Measuring and modeling large signal S-parameters will not be described on this page (perhaps we will get into that someday)Mixed-mode S-parameters refer to a special case of analyzing balanced circuits. We're not going to get into that either!Pulsed S-parameters are measured on power devices so that an accurate representation is captured before the device heats up. This is a tricky measurement, and not something we're gonna tackle yet.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2020, 06:35:23 pm
content: For the oscillator I would have a look at the local oscillators for satellite TV receivers / LNAs (the part mounted at the dish). AFAIK they are some kind of dielectric resonators, somewhat similar to YIG, but more fixed frequency. The specialty with YIG is that they are tunable over a wide range and still high Q and thus low phase noise.In more complex systems they may use some kind of DDS for modulation, if they want to do odd thinks like pulse echo and similar things. So the DDS part would be much lower frequency and likely not the usual sine but more like some arb generator / pattern generator.

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2020, 08:20:14 am
content: called a DRO, dielectric resonance oscillator, if that is what you meanhttps://en.wikipedia.org/wiki/Dielectric_resonator

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 10:13:45 am
content: Whoa, I'm way behind on this thread I can see!With regards to the solddering: A proper acetylene set is out of the picture. Too expensive and I don't want to store the fuel. The only air-fuel set that's a real choice is this kind with disposible canisters, oxy-propane:With regards to the swept frequency: I don't think the cavity can be made wideband enough that you wouldn't see a curved spectral baseline, as resonance changes.I'm unsure. The only thing I know is that in 70 years of magnetic resonance, nobody ever uses swept freq. As I don't know all the theory, and are just eyeballing quite a few aspects of this project, I'd rather not go completely against established systems.I also think the sample (and nature of the cavity) is quite nonlinear. Not to mention the two 10mm holes I need to drill in the cavity to let in the test tube.I just got my lock-in amplifier, so signal detection will be trough a WR90 1N23 'crystal' detector mount, fed to the lock-in, the magnetic field (or frequency) modulated at something like 10 KHz.I'm handing in my MSc thesis in 2 weeks, so I don't have the greatest amount of time for a while, but I'll be back once I make progress!

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2020, 10:27:47 am
content: those tend to make a super pin point flame, if you get one of those stock kits with a thin nozzle, you won't make big lap joints, thats like if you want to do copper welding, not brazing (the idea is with a copper weld, which is really like brass, you use a alloy that does not flow so well but offers strength from the actual bead), or very small area silver solder. i think mapp gas air would be better for that (plumbers torch). It will do it I think.I have one of those torches and I am disappointed in the oxygen bottles (they seem to leak for some reason), and I personally think for indoor storage, the fucking container those gases comes in is less safe then the acetylene MC tank. I caught those benzomatic cylinders leaking more then a few times. Replace when old and do not use the youtube trick of refilling them with wet rags and the freezer, the seal is too low quality to be reused.When you make your holes, get yourself a 10mm reamer, start the hole out with a narrow step drill, and when you mark the hole, use calipers to scratch (not sure if you are familiar with how to use them, their carbide tipped so you actually extend it to the measurement you want, then drag it on the edge so it leaves a scribe line to the right dimension. You make 2 scribe lines from 2 faces and you get a + sign, that's very accurate. Then if you want precision, you drag a sharp hole indicator punch on it till it gets stuck on the scratch line intersection point, then you mark it, then you use a stiff drill (I like a sharp carbide countersink) to make a deeper hole, then you drill it with a pilot, then you use a step drill, then you use a reamer.This should get you extremely precise holes in dimension and location in thin material with a hand drill.https://www.mcmaster.com/8851A25/- reamer 10mmhttps://www.mcmaster.com/8841A21/- step drillhttps://www.mcmaster.com/2915A13/- countersinkand I assume you have a caliper and normal drill bits.That should reduce your error there.

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2020, 10:31:05 am
content: the problem is the test tube is likely way less accurate then the hole I told you how to make hereif you want it to fit really nice you would need to take the test tube and grind it on a diamond wheel on a rotary surface grinding machine. Might not be hard to do if you are working with the university and they have a machine/glass shop. If they help you, when you buy parts, buy extra and give it to them (i.e. extra test tube), often they like the weird artifacts they get to keep from jobs. It makes it more cool/thoughtful then a normal machine shop if they are doing scientific things. This job is likely going to make them think a little because its non standard, you probobly need to offer some intradepartment cooperation and resources. For some reason I don't think the chemistry department is going to have this machine, unless maybe they are making their own cuvettes (little extremely precise expensive piece of glass rectangle test tube for photospectroscopy machines).Also, if you want to put a test tube into a thin metal box and align it, you need to glue on some bushings to help align the tube (fat precise washer you can buy on mcmaster that has a precise internal diameter), or you can make the waveguide wall thicker before you drill/ream it, so you don't need two steps. Otherwise the long test tube will try to align with 1mm thick waveguide body, rather then a 1cm thick bushing. (i.e. get yourself two 1cm thick chunks of brass, braze them on the tube, then drill it, but then you need a stiff drilling machine. If you buy and glue bushings, you can make your hole first then glue the bushing in place over a 10mm rod that you insert through the waveguide before hand, with a bit of grease on it, so the glue does not stick there. Be sure to take out the center when the glue is not completely cured because I had stuff get stuck despite applying vaseline). This will be the tricky magical not enough resources moment, to add bushings to the hole.Likely it will be hard to align the two holes, you need a machine to do it right. ask them for helpAlso, aircraft trick : if you are sure the waveguide is good, like rectangular, after you drill your top hole, you can glue a BIG tall bushing to the top with super glue, aligned over a 10mm pin, let it cure, then drill down from the top with a long drill bit to make your other hole. You can tesslate bushings, to make a properly centered but different diameter hole on the other side, so you can make both holes reamed, rather then drilling a nasty properly centered but crappy edge quality hole on the other side with a 10mm drill bit. It would mean you need to glue a bushing into a bushing after you glue a aligned bushing on. Then you take it apart with fire, you would need to make sure the two bushings fit together. But then you can do it with a hand drill pretty good. And you might need a long length drill bit if you are drilling a thin hole with a drill bit aligned by a bunch of bushings. so get a 10mm ID bushing, center and glue it over a 10mm hole with a 10mm pin, then glue a 5mm OD bushing inside of the 10mm bushing, then drill through with a 5mm drill, then do the other side like you did the first side. you can make all sorts of structures with the bushing to make it more stiff, like put a long one in there.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 11:08:13 am
content: My university fired all glass blowers and specialized technicians half a decade a go, because just buying from external companies would be "better"I have a precision drill press, my plan was to drill a 3mm pilot hole with a carbide stub drill, drill out to 10mm with a step drill (last step very very slowly, maybe turning by hand with a tap holder).I'm going to reference lenghtwise off of the waveguide flange, as I'm pretty sure that is perpendicular to the waveguide, and then drill from each side.The test tube is actually only 6mm, my plan was to make a plastic bushing, slightly eccentric, so turning the entire bushing would spin the tube around 0.5-1 mm to be able to allign it, that would allow me to calibrate around any mechanical sloppyness.The test tubes are going to be consumables, and should not be very custom made.

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2020, 12:12:08 pm
content: how has the lack of friendly expertise and delays worked out for them? Like, technician telling you something without trying to sell you something or further a contract?from the sounds of things you are on your own. I managed to trade, mingle and 'help out' in exchange for skilled labor. If they wanted a paper trail, it would never happen, because it would be ridiculous, being charged for every bullshit and stuffed into a bureaucracy.Sometimes local people will help because they are bored or just use it as mingling (be sure to tell the professors who helped you). Obviously don't over use them, if you have this opportunity, or people get in trouble.on paper desk engineer projects that are delayed for 3 weeks by shipping because of the most minor of errors is the typical result, IMO. Not to mention excessive stress from documentation and proofreading because of the penalties for minor errors (good place to put the minds of creative people, making sure documents line up to optical clarity.. test tube holder turns into the fucking apollo project, IMO thats what accounting is, to make sure your money comes in on time in the correct amount). Normally it would be 'walk in at lunch time with a napkin sketch' (not to mention having people happy to see you and making friends). I feel like excessive outsourcing turns into compartmentalization also, everything turns secret. Top level people think it makes some kind of super focused experts but its just choking.Of course if its understaffed people want to kill you if you do any of this (might want to take it easy because of COVID).Also, for free shop work, I would recommend just having them only do the bare minimum that you can't do with hand tools. Specifically ask not to chamfuer, deburr (your a big boy that can handle a razor), polish, or anything that can be handled with a minimum tool kit, sand paper and harbor freight. And don't bog them down with wood or paint.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 12:52:39 pm
content: It works exactly as unwell as you'd expect it to, but hey, rather cut costs there than reduce the number of research groups right?I think I'm good on the machining part, I'll just channel my inner Clickspring and spend 500 hours hand filing and polishingAnd I can get a bit of help in other aspects of my project; I've been allowed to calibrate my newly aquired gaussmeter against the uni's EPR magnet!I think I actually have most of the components needed now, over the last few months I've added to my lab:- Tunable Gunn X-band source- Hall gaussmeter (F. W. Bell 615)- Brookdeal 402 50 KHz lock-in amplifier- X-band magic tee, wavemeter, attenuator- X-Y recorder- Sweepable Hz to MHz generator (for modulation)- Slow sweep generator (up to 1000 min, between 0 and 5VAnd in the works is my cavity, and the magnet.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 15, 2020, 03:51:47 pm
content: Quote from: TheUnnamedNewbie on September 12, 2020, 10:06:17 amA few reasons, some historical and some still important today, mostly related to tuned circuits, at least from my limited experience with NMR. I think ESR is similar.It's generally easier to generate high power RF with a tuned source. Sure you can easily get broadband power amplifiers now but they are less efficient and have more distortion. Likewise the transmit and receive antennas are often tuned coils that don't work well with botadband excitation.In the olden days, real time signal processing didn't exist or was expensive. The detection was often done with bandpass filters and envelope detectors rather than an FFT. You could still make this frequency tunable with a heterodyne receiver but that makes the system substantially more complicated.The final issue is that you need to avoid direct detection of your drive signal. In pulsed NMR this is done with time gating, but in CW NMR you arrange the pickup coil at 90 degrees to the transmit coil and adjust it until you get a null. Then your receiver only sees the NMR signal not the excitation. However if you sweep frequency the field configuration will change and you will wander off null.For this reason my understanding is that today CW NMR is still done with field swept but pulsed NMR can be done either way but is more often frequency swept.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 04:30:58 pm
content: That makes a lot of sense, the entire concept of sweeping is lost once we get to modern pulsed ESR/EPR, those are fixed frequency (tuned before the actual experiment to find resonance).Maybe the field of ftnmr and epr just made CW more or less obsolete before the needed electronics was available to do freq. Sweep

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 08:15:49 pm
content: I know I just said the opposite, but the more I think about it, the more I want to try frequency sweeping first.My gunn module is simply tuned by 5 to 35V, and has a modulation input. It would be a million times easier than doing it magnetically. It may not work, but it would be a fairly small amount of time wasted.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 17, 2020, 07:51:02 pm
content: Quote" Also, for free shop work, I would recommend just having them only do the bare minimum that you can't do with hand tools. Specifically ask not to chamfer, deburr (your a big boy that can handle a razor), polish, or anything that can be handled with a minimum tool kit, sand paper and harbor freight. And don't bog them down with wood or paint. "If you asked me, a Senior Technician, to let something out of my shop without deburring, edging, sandblasting, chamfer, whatsoever....Well, the answer would be: We'd be having a little talk about integrity, and I'd be upstairs, looking at your lab safety situation for a good ten minutes.~Rule one: Unless the prof specifies "quick junk", never make junk for a professor or researcher..Rule two: Any "quick junk" you make, you will have to look at for the rest of your career as it remains in use for decades.Rule three: During a public tour, the Department Chair will notice the crappy work and say something.Rule four: Your worst piece of work ever will be transferred to an undergraduate lab full of engineering students, for critique, for decades. A parent will seek you out and ask why their son or daughter does not deserve something equal or better to the plant they work in.~Besides, I can deburr at the speed of summer lightning. Most of my clientele has no idea what "break edges" means.Care to guess how those rules came about?Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 17, 2020, 08:21:24 pm
content: sounds high end, I had a different facility/campus in mind.