Roller Inductors, Center Taps, Air Core Transformers, and Associated Craziness - Page 1
Forum section: electronics / rf-microwave

author: ikrase
author_rank: Regular Contributor
timestamp: March 01, 2018, 08:55:46 am
content: I've been studying more on how to design a quadrupole mass spectrometer power supply (summary: drive 100+pf load with 2MHz sine wave split phase having DC component and DC offset, w/ good voltage stability).I've been solidifying my schemes around a sine wave oscillator that drives either a tuned resonant inductor or a resonant-secondary transformer (when combined with a tuning capacitance and/or just the capacitance of the electrodes and cables).In order to produce the proper split-phase DC offset signal, one needs a transformer that is not merely center-tapped, but which has a split (or dual) secondary.At the frequency I'm dealing with, an air core transformer is viable and possibly the best option -- easier to design and has higher Q (and it's easier to reduce Q than increase it).It occurred to me that one could add a few-turns primary coil to a roller inductor to produce a tunable isolated air-core transformer -- is this actually a viable idea?Also, is it practical or desirable to make the "split-tapped transformer" actually be a pair of independent transformers?

author: rhb
author_rank: Super Contributor
timestamp: March 01, 2018, 04:16:46 pm
content: I can't see any reason you can't use two transformers. You'll need to account for the series primaries.Do you have suitable roller inductors? If not, I may have some. My Dad left me a few dozen (!!!) WW II surplus tuning units. There may be some suitable inductors in some of them. As a rough guess these will take several hundred watts of RF. The stuff is all silver plated with steatite insulators etc. Cost was no object. PM me if you want me to go looking through the pile.

author: ikrase
author_rank: Regular Contributor
timestamp: March 02, 2018, 08:37:09 pm
content: Interesting...Couldn't the primaries be in parallel?

author: rhb
author_rank: Super Contributor
timestamp: March 02, 2018, 10:49:29 pm
content: They could. If you're making it up yourself it can be whichever you find more convenient. You just need to account for the effect of parallel or series connection on the circuit.Making a dual secondary is pretty easy. Wind the primary on a suitable form and then wind the secondaries over the primary with an insulating sheet (aka fish paper) between the primary and the secondary.Have you calculated inductances, turns ratios, power level and approximate dimensions for an air core transformer yet? I *know* I have fixed inductors on very high grade ceramic forms. But I need an idea of what I'm looking for. It's a midsize U-Haul truck pile of stuff.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2018, 07:47:16 am
content: I would worry about imbalance. You want that thing pretty well centered, no?Tim

author: rhb
author_rank: Super Contributor
timestamp: March 03, 2018, 03:37:45 pm
content: Working with inductors wound on 2" cores with #12 wire I'd expect that tuning two transformers would not be too difficult. Fiddly perhaps. I'd actually worry more about the field interaction between the two. They would need to be very solidly mounted mechanically or there would be all sorts of headaches.Personally I'd pick a plain air core coil with the proper inductance and power rating, wrap suitable insulation around it and wind a pair of secondaries over it. I'd only resort to using two transformers if I couldn't find a suitable primary for making a dual secondary transformer.That said one might find a large toroid from a SMPS that would be suitable. That would avoid fringing field issues. The OP needs to provide a bit more information. This is an oddball RF PA. There are lots of issues one has to get right for it to work as desired.

author: ikrase
author_rank: Regular Contributor
timestamp: March 05, 2018, 06:09:41 am
content: Yes, I need a lot more information.I'm trying to build something based on this paper:https://web.stanford.edu/group/Zarelab/publinks/798.pdfwith some changes.The important things to understand are that the output is meant to be in peak-peak Voltage, not in Power-at-some-impedance and that both the isolation from ground and of the two "ends" of the amplifier from each other are important.I attached a sketch I made.The changes I would make:- Trying to push the working frequency up to 2 MHz (though lower is probably tolerable, and it's intended to be changeable)- Using a low tuning capacitance, or none at all, to achieve resonance (other than the actual load)- No automated stepper motor tuning- Using variable inductor-transformers to tune to resonance.- Replacing the expensive and probably unneccesary Apex power amp with a much cheaper boosted op-amp- Possibly boosting the primary voltage to reduce the required transformer ratio- Crystal-controlled frequency sourceAnyway, the linked work uses a cylindrical air-core transformer consisting of 3 turns primary over a secondary that isn't fully specified but has an inductance of 300 uH and looks like it probably has a secondary of around 300 turns on a 20mm former (this gives the proper numbers and dimensions that agree with the picture, as well as providing a transformer ratio of about 10 which agrees with my simulations).Since I want to go to a higher frequency, the combination of inductance and capacitance needs to be reduced. Getting rid of the tuning capacitor (which only makes things worse) is a first step.(I was somewhat resisting using ferrites because as far as I could tell, the need to baby the flux in the ferrite leads to adding another constraint -> even more headaches.Another possible idea: Minimal-inductance air core transformer (probably very small diameter to allow the combination of high turns ratio and low inductance) with the center tap / split, and the ends leading off to roller inductors (in series) that are not used as transformers.Anyway the target inductance for roller inductors would be in the 20 to 100 uH range.

author: rhb
author_rank: Super Contributor
timestamp: March 05, 2018, 03:30:20 pm
content: I've read the paper and your comments. Your requirements are self contradictory and the paper was clearly written by people who had little understanding of RF amplifiers. Litz wire in a power amplifier is nuts.Some suggestions:For the oscillator an AD9850 DDS will provide better spectral purity than the approach in the paper. They're cheap and easy to use. EBay is awash with them. They are also referenced to a crystal oscillator so the frequency accuracy is 10-20 ppm or better. a GPSDO will give you several orders of magnitude better accuracy though at 10x the cost.Feed the DDS to a wideband amp capable of giving you a watt or two of drive. MiniCircuits probably sells something that will serve well. The more power the better, as it will reduce the gain required in the output stage.The only distinction between the mass spec PS and an ordinary radio transmitter is the output LC tank. The output is intended to drive two antennae rather than one. This is done with phased array antennae where directivity is required.The load has a characteristic impedance. The power is E**2/Z, so for a 100 Vrms output into 50 ohms you need a 200 watt amplifier. That's a good bit of power, but not difficult. If your poles are higher impedance the amount of power is reduced. But you still have to match the impedance of your poles. If you don't, the power is reflected back towards the DDS which will not work well.Getting a tuning range from 300 KHz to 2 MHz is quite easy. Use a tapped inductor for the tank circuit in combination with the variable capacitor and use a multipole relay to switch taps. Step the DDS and alternate the sweep direction when you switch taps at the end of the capacitor rotation. The AD9850 can change frequency in less than a microsecond.Because your tank circuit has two outputs, you'll have to tap both sides of the secondary. As you want to tune over nearly 4 octaves, you'll need to tune the primary side of the output transformer. Your output impedance is constantly changing with frequency.To tune the output with a roller inductor you would need to wind the two sides in the opposite direction. That is perhaps possible, but keeping both halves of the coil balanced will be very difficult. With a tapped inductor, you can insert a trimmer capacitor in each tap to correct for slight differences in inductance on either side of the center tap.I'd like to strongly urge you to read the ham radio literature on RF amplifiers. This is only difficult if you follow the paper too closely.

author: ikrase
author_rank: Regular Contributor
timestamp: March 06, 2018, 06:10:47 am
content: Well, it appears that the paper writer and myself have "not being very competent with RF" in common. And I'm not very good at explaining things.I thank you very much for your criticism and advice, but I need to ask more questions. How bad actually is the Stanford paper, and what in particular strikes you as wrong?(Didn't they just use Litz wire because it was what was on the shelf?)I was however always planning to use a crystal-controlled DDS (though not necessarily specifically the AD9850) to provide the original source for the sine wave. I doubt a GPSDO is worth it, maybe just a good TCXO.Why only a watt? Most of my (admittedly rather ridiculous) simulations (and the paper) suggested that at least ~10W would be needed, I was imagining either power-transistor-boosted opamps or (as suggested in my earlier thread that was less focused on my attempt to follow the Stanford paper) one of the many ~50W QRP ham power amps targeting the 160 meter range using IRF510s. One design for a quadruopole driver used tubes, 6146B's, but it was flatly unsuitable for a QMS.I would say that there are two further important distinctions between a QMS power supply and a radio power amp. First, the QMS needs the DC offsets, and second the QMS in the final reckoning needs to tightly control voltage, not power. Of course, that just means accurately (and stably) knowing your load impedance (frequency dependent since its purely capacitative)For your tuning scheme, am I correct that you're suggesting an output transformer with (numerous) taps in the secondary to adjust the L, and then a (small) variable capacitor for analog, fine tuning? Or would this multi-tapped output tank inductor be separate from the transformer?Finally, I would also be interested in what you think of the paper's power monitoring/feedback detector circuit (and the use of the VGA that it controls.)

author: rhb
author_rank: Super Contributor
timestamp: March 06, 2018, 03:08:57 pm
content: The way the writers wrote up their work. That is not how someone who had mastered "The Art of Electronics" would describe the build or the way they'd build it. The concept is sound, but the execution is quite inept.The watt was so you'd have something you could buy off the shelf to follow a DIY DDS to drive more stages of amplification. That will let you work out the impedance matching problem without having to deal with high voltages and power levels. The impedance matching over almost 4 octaves will be very difficult. It's doable, but you should expect to make many iterations.A TCXO certainly should do fine. I doubt that you can maintain amplitude tightly enough to even need that.Calculate the load impedance vs frequency and determine what power level you need at each frequency to provide the desired output voltage. That will give you a rough idea of what sort of gain range you need to maintain a constant voltage. If you want to vary the voltage that will require additional gain.What I'm suggesting is a tapped secondary with each tap having its own trimmer cap to adjust for variations in the coils and the tap positions. You'll probably need to do the same thing on the primary to get a match over 4 octaves. Life will be much easier if you have a vector network analyzer available. The DDS and a 4 channel DSO will work with some MATLAB and SCPI programming. But it can all be done by pencil and paper.The central concept to what I'm suggesting is that the air variable is swept from max to min position, then you jump the DDS to the maximum frequency of the next tap and sweep down, then step to the next tap and sweep up. The trimmers in both tap legs will let you match the min frequency on one tap to the maximum on the adjacent tap. The number of taps required will be determined by the VSWR of the output transformer. It's really just a weird automatic antenna tuner.The DC offset really doesn't effect the RF design other than requiring an output transformer and the opposite phase outputs require a center tap transformer. Aside from that it's a normal RF amplifier with the exception of the tight output level control.Disclaimer: I am not an RF expert. I recently read a couple of upper undergraduate or early graduate level texts on RF design. I messed around a bit with RF 30 years ago and am only now getting back into it. But I di know the difference between what I do know and I don't know and I know how to learn. In the end that's what matters. I happen to think this is a very cool project. The sort of thing I'd like to do if I am granted the time.To get this done I suggest doing this:Get a DDS you can sweep under computer control. The obvious choices are the FeelTech FY-6600 and the JDS-6600. I think the latter probably has a better output stage than the former. There are extensive threads about both. You can build one, but when all is said an done you'll have spent almost as much money and a bunch of your time. You're better off spending your time on the matching problem. IIRC the JDS will deliver a 20 Vpp signal with good fidelity. The FeelTech fades around 15 Vpp unless you modify it. Both units have ungrounded switching mode power supplies and will require correcting that issue either by grounding the chassis or replacing the PS.Design and build a prototype of the transformer and air variable but skip the automation and use slide switches and manual tuning with a dummy load capacitor. Then measure the VSWR seen by the DDS at a series of frequencies. I'd do this with just the maximum inductance taps and trimmers, manually tuning the air variable each time. That will get you started on the impedance matching problem. I think you would be well advised to calculate the impedance curves for prospective tap positions before construction. It really is just a weird automatic antenna tuner with a weird antenna. There is almost certainly ham oriented software available online for calculating the impedance of a tuned circuit. I think Elsie will do this, but am not sure as I've not played with it much.The level control that they used looks OK. But I suspect they were mostly using it to compensate for poor matching. The better the matching, the easier the gain control becomes.

author: ikrase
author_rank: Regular Contributor
timestamp: March 06, 2018, 09:25:49 pm
content: Oooooooohhhh, thank you, that makes more sense.I was never planning on frequency adjustment being that automated - would be open to having manual tuning and even plug in module with transformer for each octave.I'm imagining sine wave source -> VGA -> power amp -> primary (ferrite toroid?) -> tapped centertapped secondary -> tuning capacitance -> load.Any comments on the capacitative dividers for level measurement? The Standard paper didn't specify them, I think silicon or mica may be appropriate for low tempco.

author: rhb
author_rank: Super Contributor
timestamp: March 07, 2018, 12:20:08 am
content: The FeelTech and JDS AWGs can be controlled over RS-232 or USB, so if you use one of those for the DDS you already have the variable gain amplifier. They appear to adjust in 1 mV steps. But I don't think anyone has checked them for accuracy yet.Ferrite toroids will reduce the Q thus requiring fewer taps. The materials for both air core solenoids and ferrite core toroids are cheap. I'd try both.The capacitive dividers are just to scale the amplitude of the RF without being influenced by the DC bias. If you put an RF diode bridge and capacitor feeding the ADC of an Arduino or similar you can adjust the DDS output as needed. A good design is actually easier than what the Stanford folks did. Maybe that's why it's a Junior University ;-)I'm glad you found that useful. I spent a good bit of time on it. I think a mass spec is a really cool project and would love to build one myself. I bought a VNWA 3E, but I've not actually fired it up yet. Never having used one I'm expecting it will take a few days to get acquainted. But I'm in a position to check impedances of the coupling.My parts for the Zynq project came and the USB hubs want a microUSB for power so I ordered some hubs that take 3.5 mm barrel connectors.Have Fun!Reg

author: ikrase
author_rank: Regular Contributor
timestamp: March 07, 2018, 06:26:27 am
content: I did some research and some simulations today and I think I *finally* get it.I'm kinda getting the idea that the Stanford design is clumsily trying to do DC-style "zero source impedance, infinite load impedance" with the addition of the resonant-secondary transformer to avoid needing to waste massive amounts of power charging the capacitive load backwards and forwards. And this is not working well for them as you say.So it sounds like what you're suggesting is that the load capacitance be placed in a series loop with (tunable?) inductor(s) so as to resonate at the proper frequency. Then this resonant loop should be driven (also in series) by a much-lower-inductance impedance matching transformer with multiple taps for tuning the frequency match (as well as the caps you mentioned) and the center tap to provide the DC offset.(Falstad link)The transformer will mostly be matching the power amp output impedance to the resistance (parasitic or deliberate ballast) of the tank circuit since the capacitative reactance and the inductive reactance will be cancelling out when tuned to resonance. It will likely be a *step down* transformer.It's allll finallllly making sense. I think.What about air core toroids (technically foam or plastic core probably)? I'm considering putting interchangable transformer-blocks on an Octal base. And of course airgaps. I guess I better get some rings.(Quick questions: Are the trimmer caps in series between the tap and the switch contact?)I'm intrigued by what you're doing-- you have a manufactured mass spec that you're bringing up? Would love to see pictures and/or hear commentary on construction techniques.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2018, 11:57:28 am
content: That's a thing, but notice you have to match six components(!) to ensure balance.How important is balance, anyway? I've been assuming it's important but do you have any data on common mode AC ("VDC" term varying in step with "VAC")?Tim

author: rhb
author_rank: Super Contributor
timestamp: March 07, 2018, 02:05:20 pm
content: It's probably best to put the trimmers in parallel with the active section of the secondary and use a double pole switch. to select the desired tap and trimmer. The other end of the trimmers tie to the center tap. I had been thinking of the caps in series when I mentioned them, but that results in needing excessively large capacitors.I suggest using a DP switch but leaving out the trimmer caps until you know you need them. It might be a good idea to use a 4 pole switch so that if it is necessary to select taps on both sides of the transformer that's easy to do. Mechanical fabrication can take a depressing amount of time for very simple circuit changes. And things become a nightmare if you didn't allow enough space. I'll go see if I can find you a good switch. I know what you need now and I'm sure I have some really high grade, silver plated steatite wafer switches good for several hundred volts.Aside from a VCO rather than a DDS, the conceptual design in Fig 1 of the Stanford paper looks fine. It's the actual implementation that is duff.I think that automation is important, but only when the instrument is running. Then you want to automate it so that everything is repeatable, measure the errors and correct them in the signal processing. I'd build the PSU so that it was easy to add stepper motors of appropriate size at the very end. I don't see any reason that you can't achieve a research grade instrument result.A friend of mine built the magnet mass spec at the Geosciences department at UT Austin. Sadly he was hit from behind and killed by a careless driver while waiting to make a left hand turn. His wheels were cocked to make the turn and the impact drove him into an oncoming car pretty much ripping his Sentra in two.I can't speak to the matter of air toroids. You'll just have to try it. I suspect that there are geometric constraints.I'm working on FOSS firmware for Zynq based DSOs. I *wish* I was bringing up a mass spec. My lifelong dream retirement job was as an instrument tech and builder at a university. Unfortunately things didn't work out that way. But I would like to build my own mass spec which is why I'm so interested in what you're doing. I'd greatly appreciate if you would post some of your simulation results and other information about the project. Perhaps also a documentary thread in the Projects section?As I commented in another thread about an electrokinesis transducer, I was hopelessly corrupted at an early age by Clair Stong's collection of Amateur Scientist columns from Scientific American. I was probably OK until I got to the chapter that said, "For less than the price of an average set of golf clubs you too can smash atoms in your backyard." At 10 or 12 I was enthralled.

author: ikrase
author_rank: Regular Contributor
timestamp: March 07, 2018, 04:04:11 pm
content: There are three documented (har har) DIY mass specs that I know of.Two are made by the German experimenter Thomas Rapp,http://www.rapp-instruments.de/Radioaktivitaet/div/quadropol/quadro.htma quadrupole mass spec. Note his low-capacitance design and less-than-successful power supply... and beautifully machined poles. This machine is only really suitable as an RGA.http://www.rapp-instruments.de/Radioaktivitaet/div/sector/sector.htmfor his cheap pipe tee mass spec, which is really no better than a toy but which probably presents a good platform to experiment cheaply with basic ion optics on.The other is being designed by some people in Hong Kong called M-lab -- they're mostly working on quantum computing, not mass-spectrometry, and their docu isn't great. Apparently their device is meant to be used with Ebay-garbage-can quadrupole mechanicals that have no power supplies.My (initial) intent is to make the quadrupole very much a bench-stack design -- there will be the filament controller, the multiple HV supply for different DC electrodes, the AC quadrupole supply, and my trusty Keithley electrometer.

author: rhb
author_rank: Super Contributor
timestamp: March 08, 2018, 12:38:24 am
content: Sadly I can't read German. But the workmanship is beautiful. I'll see how google translate does later.I don't know what an "RGA" is.I was mostly asking about your power supply simulations. I'd like to see what your results.Unfortunately, building a mass spec is far down on my list. I've got a scrap water tank to turn into a cupola for melting iron, and a scrap SS pool filter to build a crucible furnace.For a completely different project, I got my linear ball bearing rails today along with my steppers. I'd been planning to put them in an old 14 gauge steel chassis, but after looking at it I think I'm going to order a 12" x 12" x 2" grade B (0.0002") surface plate to mount the rails. Unfortunately shipping is more than the cost of the surface plate.

author: ikrase
author_rank: Regular Contributor
timestamp: March 08, 2018, 01:14:26 am
content: An RGA is a Residual Gas Analyzer. It's a simple mass spectrometer that measures the bulk contents of a high vacuum system and identifies the... residual gases. Very useful for finding leaks or determining why you aren't getting a very good vacuum.Recently some extremely compact RGAs such as the MKS VQM have appeared on the market, these being advertised as an upgrade for ion gauges.I included a link to Falstad in a recent post, it should bring up my simulation.Are you building your own CMM?

author: rhb
author_rank: Super Contributor
timestamp: March 08, 2018, 02:13:15 am
content: What's a CMM?I'm building a CNC XY stage and am goofy enough to scrape the extruded rails flat and true. The initial goal is just to learn how to control the thing. But I'd like to have something that could be used for precision work. I bought continuously supported rails which can't be adjusted to as tight a tolerance as unsupported rails, but they take more load. It's an experiment. I can always fit more accurate rails if needed. I am going to need a 12" machinist's level. I've got an 8", but I bought four 12" rails rather than tow eichts and two twelves.. The ballscrews are only 10", but I should be able to position a 4" platform anywhere. I've got an HeNe laser which should be sufficiently coherent to give me interferometric positioning.There's a great anecdote about this. In 1948, Scientific American did a series of articles about John Strong's construction of a ruling engine at Johns Hopkins. The article mentions that many amateurs had come to grief and financial ruin attempting to build a ruling engine. In 1958 the Amateur Scientist column featured a ruling engine built as a hobby project in the UK. The builder was employed at a university as an instrument technician and designer. So I'm not sure if "amateur" really is appropriate. What he did was use a DIY cadmium lamp interferometer to position the carriage. Because the errors were random the gratings did not suffer form Rowland's ghosts.From what I've read, I should be able to get 8-10" coherence length from the unit I assembled. I've stripped old CD & DVD drives of splitters and mirrors. I should note that the unit is not finished. I still need to mount a switch and power connector, but was terrified of botching the mechanicals.I've wanted to do this stuff for a long time, but wouldn't write the check. But having had several close friends die, I'm more willing to sign the checks. I might not get a chance if I don't get to work on it.

author: ikrase
author_rank: Regular Contributor
timestamp: March 08, 2018, 06:27:11 am
content: CMM is an ultra-accurate motion platform (usually cartesian, only sometimes motorized) equipped with a mechanical stylus, which is used to measure the shape and dimensions of arbitrarily shaped objects.The bread and butter of modern machine shop metrology -- and oh so expensive.

author: rhb
author_rank: Super Contributor
timestamp: March 08, 2018, 02:16:58 pm
content: It would have understood immediately if you had said coordinate measuring machine. I'm quite aware of those. CMMs will do for most tasks, but there are things for which they will not meet the specs and things such as toolmakers buttons made specifically for the job are required. Then it gets *really* expensive. You're making tooling to better precision than the object being made. Things like the gravity B gyros are certainly beyond CMM capabilities.Acronyms are very context sensitive even between different groups working in the same field which is why I never use an acronym without defining it on first use.Shars.com has a 12" x 12" x 2" grade B surface plate for $33. Unfortunately UPS ground will be slightly over $33. But I'll have over $200 in the mechanicals, so another $67 for the base is not disproportionate. I think I'll be able to get 0.001" accuracy and repeatability, but probably will have to switch to full round ball slides and rail to get to anything better. So I may just build on the steel chassis as originally planned using the supported rails and do a larger, higher precision table later using a larger surface plate. The supported rails are quite stiff so shimming them to fit the steel chassis will probably suffice for 0.001" tolerance in 3 axes.

author: rhb
author_rank: Super Contributor
timestamp: March 09, 2018, 01:30:51 am
content: I have been playing with the steppers for my XY stage. Which brought to mind the last couple of sentences of the Stanford paper in the "Variable Capacitor and Frequency Tuning" section.There are no forces on the variable capacitor when the system is deenergized. So the loss of holding torque has no effect. In fact, no holding torque is ever needed with an air variable capacitor unless it is subjected to mechanical vibration. The authors are mistakenly attributing to the stepper motor the effects of heating in their ridiculous choice of a coil wound with Litz wire. Very likely a close examination would reveal other heat related issues, but that is the most likely suspect.I have a lot of friends with PhDs from Stanford. Some of whom are a bit strange, but they are very capable. The authors of this paper are appalling. Their supervisor should have never allowed such a construction and the reviewers should never have allowed its publication. However, I know someone who as a PhD candidate in geophysics did not know Snell's law. And he got his degree.

author: ikrase
author_rank: Regular Contributor
timestamp: March 10, 2018, 08:21:23 am
content: I'm sorry, I should have guessed it was the acronym, not the idea.One other question: What about RF chokes -- do you think they are needed or desirable on the connections to the DC power source?

author: rhb
author_rank: Super Contributor
timestamp: March 10, 2018, 02:26:14 pm
content: Oh, yes! Leaving them out would play havoc with the voltage regulation and make your amplifier oscillate on its own.To paraphrase a remark by Ken Thompson, the primary creator of Unix, "Never underestimate the value of brute force."I'd very much like to see your calculated impedance curves for a 300 uH inductor based transformer and the variable cap tuned across the full range.I'd offer to do them except I'm trying to build a CNC XY stage and learn FPGA programming. So I've got other hills to climb at the moment. I just had someone point out that I was overlooking thermal expansion effects in the XY stage.

author: ikrase
author_rank: Regular Contributor
timestamp: March 21, 2018, 03:18:26 pm
content: So for my power amp, I'm looking at using a variant of this circuit, a Class AB amplifier using IRF510s. :http://www.g0kla.com/scpa/SimpleCheapPA.php(This is a form of the popular WA2EBY ham radio amp, which in its original form is able to go down to 160m (1.8 MHz)How would I adapt it for the range a bit below that? Few more turns on the inductors/transformers?

author: rhb
author_rank: Super Contributor
timestamp: March 21, 2018, 03:55:19 pm
content: Looks very suitable. I'd advise building the transformers as shown and measure them over 200 KHz - 2 MHz. You should also research the core material used. A different choice of core material may be appropriate for your project. It might be that just changing the core material is all that is required. The original design will have chosen a higher frequency core material. Someone plotted the performance curves for a number of core materials. I'll see if I can remember where. It was some ARRL publication is the only thing I *think* I remember.Disclaimer: I understand the principles involved. However, I do not have detailed knowledge of all the calculations, etc. I have a 5000+ volume library for dealing with that. So I swim to some new spot and in the process forget something I once knew.I'm just now at the point where I can start serious work on my major project, FOSS FW for cheap DSOs. Though the last few days have been spent on packaging an EBay BG7YBL diode noise source. And I'm on the hook to do hookup some wash basins for my sister today.

author: ikrase
author_rank: Regular Contributor
timestamp: March 21, 2018, 07:31:27 pm
content: I guess there's no excuse for not having a function generator and oscilloscope now.

author: ikrase
author_rank: Regular Contributor
timestamp: March 24, 2018, 10:03:44 am
content: Well, I did something productive: I wrote Python code that simulates the paths of ions in a quadrupole mass spectrometer.I still need to write something to actually put this to use.Code:# -*- coding: utf-8 -*-"""Created on Wed Mar 21 23:38:33 2018@author: ikraseCode to simulate the paths and stability of ions in quadrupole massspectrometers.This is heavily based on the reference ofhttp://www.chem.uky.edu/research/Lynn/pdfs/JChemEd_quads.pdf"""import numpy as npimport scipy as spimport scipy.integratefrom math import *from numpy import arraydef fMathieu_qp(t, u, au, qu):''' RHS differential equation of Matthieu for quadrupolesu = X/Y position, combined (ODE vector)t = quasi-time -- normalized to angular frequency.au, qu = dimensionless parameters, functions of all quadrupole properties.(See referenced paper)'''return np.array([u, -(au + 2*qu * cos(2*t))*u])#return np.array([u, - (w0 * w0 + eps * math.cos(w1*t))*u])def alt_fMathieu_qp(t,u,au,qu,freq):'''Alternative non-combined equation of Matthieu, in strict time domain.Freq is the actual frequency in hertz.u is [x, x', y, y']. '''w = 2 * pi * freqxpp = - ((w/2)**2) * (au + 2*qu*cos(w*t))*u# at some point I'll figure out which is the squshed axis and which is stretched.ypp = ((w/2)**2) * (au + 2*qu*cos(w*t))*ureturn array([u, xpp, u, ypp])def quadrupol_test(r, m, Vrf, Udc, freq, duration=False, tstep = 0.1E-7):'''Test whether a given quadrupole system is stable or unstable.r: quadrupole center-to-electrode radius / inscribed radius in mm.m: ion M/Z ratio in amu to electronsVrf: RF zero-to-peak voltage in voltsUdc: DC spread voltagefreq: Actual frequency (not angular) of the quadruopole in hertz'''rspan = r * 0.125 # Radius of input particle beamaqbot = ((2 * pi * freq * r)**2 * 1.036427E-8 * m ) # bottom of A and Qa = 4 * Udc / aqbotq = 2 * Vrf / aqbottb = min(0.002, 2, sqrt(a),abs(2-sqrt(a)))if tb < 1E-3: tb = 5E-2 #divide by zero and over-value guard.tlong = 2 * pi / tb # Estimate of "natural" frequency# (if there is one.)if duration == False:dur = 10 * tlongelse:dur = duration#oderes = sp.integrate.solve_ivp(lambda tt, yy: fMathieu_qp(tt, yy, a,q),# (0,dur), np.array([rspan, 0]), max_step = 0.05)oderes = sp.integrate.solve_ivp(lambda tt, yy: alt_fMathieu_qp(tt, yy, a,q, freq),(0,dur), np.array([rspan, 0, rspan, 0]),max_step = tstep)if not oderes.success:raise RuntimeError("Failed to integrate differential equation")return oderes

author: rhb
author_rank: Super Contributor
timestamp: March 24, 2018, 12:15:00 pm
content: A useful link that showed up n the VNWA mailing list:http://www.kn5l.net/43-Material-Core-Loss/

author: ikrase
author_rank: Regular Contributor
timestamp: March 25, 2018, 07:33:11 am
content: I'm having a bit of a hard time telling what the graphs refer to.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2018, 07:49:29 am
content: I think it's reflection and transmission (s11 and s21) for a 1:1 transformer (common ground, 0 degree phase), on the given core and number of turns.Which means s11 gets you a good measure of the core's intrinsic impedance characteristic (that is, not its A_L, but more general than that, its impedance per squared turn, plotted over frequency), up until high frequencies where the transmission line equivalent of the winding dominates. Which is why the one-turn transformer has a high cutoff (off scale), whereas the four-turn transformer is just beginning to cut off at the top end.Cutoff manifests as an equivalent series inductance (LF approximation, the plots don't get very close to the actual resonance (which is a notch, followed by another pass band) so this is an acceptable way to model it), hence the coincident drop in s21 and rise in Im(s11) or VSWR(s11).It's a big disingenuous to call it "core loss" when the headline plot is insertion loss. One must understand what's being measured. Indeed, |A_L| is increasing with frequency (for a core this size, typically until maybe 100MHz), so we should expect transformer insertion loss to be decreasing (i.e., a higher parallel impedance, loading down the signal less and less). You take s11, convert it to Z_core (separating Z_winding if necessary), divide by frequency to get  inductance, then divide by N^2 to finally get  A_L.Tim

author: rhb
author_rank: Super Contributor
timestamp: March 30, 2018, 09:55:15 pm
content: I was at the warehouse today. I didn't find any high power coil sets, but I did find a big pile of these. They are the RF coils from an RU-12 aircraft receiver. This particular set is marked 850-1330 KHz. They are plugins, so you might construct the RF amplifier with a set of two or three to cover the desired range. I didn't measure the wire, but I'd guess 26-28 gauge. So you might need to wind a new coil on the form with heavier wire. The coil forms appear to be phenoilc.Yours for postage if you're interested.

author: ikrase
author_rank: Regular Contributor
timestamp: March 31, 2018, 09:28:30 am
content: At least for now I am saying "Thank you, but no thank you".

author: rhb
author_rank: Super Contributor
timestamp: March 31, 2018, 01:10:57 pm
content: None of the stuff is going anywhere any time soon. I do think you should consider the pluggable coil set concept. That would greatly simplify spanning 600-2000 KHz with high Q.Offer stands for anyone else with a good use for some of these.