AC constant current source - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: May 07, 2020, 06:49:27 am
content: Hello, looking to build an AC constant current source in order to generate an alternating magnetic field in a coil of copper wire. I need a peak to peak current of a few amperes, a frequency of about 500kHz, as stable as I can get it. The copper wire will naturally represent a pretty low resistance which will likely change slightly as the temperature in the wire changes due to flowing current, and I would like to have the current source preserve stability in these conditions.I found some simple circuits mainly revolving around op amps to generate a sinusoid, but I don't know how they will behave when a low resistance load is applied. Is there a circuit I can build that would meet the above requirements?

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2020, 07:51:05 am
content: You'll be s--t out of luck trying to get a power OPAMP voltage controlled current source/sink to work properly at 500KHz. A better approach would be to synchronously rectify the amplified signal picked up across a non-inductive current shunt, possibly then putting it through a squaring stage for a RMS result, low pass filtering it, and then either servoing an input attenuator on your power amp, or if you are using a H-bridge rail to rail coil driver rather than a linear amplifier servo the supply voltage to maintain a constant averaged current level.

author: Circlotron
author_rank: Super Contributor
timestamp: May 07, 2020, 08:00:45 am
content: Make a series resonant circuit and put a current sense resistor in line like Ian.M says. You could regulate the current by changing the frequency.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 07, 2020, 08:12:10 am
content: With 500 kHz at a coil the copper resistance is the least problem. The inductance of the coil would be a big factor. This provides some kind of feedback on the actual field reached. Chances are the voltage is the more relevant part to regulate if one wants a constant amplitude for the field.A few amps sounds like quite some power. This may need care for too high RF emissions, if not done inside a well shielded environment.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 07, 2020, 09:06:29 am
content: Quote from: Circlotron on May 07, 2020, 08:00:45 amQuote from: Ian.M on May 07, 2020, 07:51:05 amIs there an existing project somewhere with a schematic I can look at perhaps?Quote from: Kleinstein on May 07, 2020, 08:12:10 amI can rig up a metal cage around the coils for starters. I think the skin depth for aluminum at 500kHz is around .1mm, so several layers of aluminum foil might be ok,This project is a low-field MRI machine, and I'm trying to figure out if I can get some sort of signal from it. The permanent magnetic field needs to be as high as reasonably possible for best results, and while the alternating field can be quite a bit lower, I don't know off the top of my head how low I can go, I'll have to research that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 07, 2020, 09:31:02 am
content: There isn't so powerful IC amplifiers which can deliver 5 Amps at 500 kHz.There are a lot middle power high frequency opamps, or ADSL drivers which can work at these frequencies but they are either weak (for example LM6172 100-150 mA) or expencive (LT1210 - 1A peak for $15).But some transistor audio amplifiers can work up to 1 MHz and deliver several amps at this frequency.

author: fcb
author_rank: Super Contributor
timestamp: May 07, 2020, 09:54:16 am
content: As others have said, your problem is going to be the inductance of the coil. You will require a large voltage swing (possibly 100's of volts) on your amplifier to drive several amps through your coil. Here's how I would do it:Build a simple push-pull power supply operating at your 500KHz, this could be built with a modern SMPSU drive IC, flip-flop and 1MHz oscillator - could also be as simple as a PIC10 and two MOSFET's - this would drive the primary (or push-pull primaries) of a transformer, something like an ETD39 with perhaps 3C90 ungapped cores), wind with either litz or several bifiliar 27AWG. Muck around with some capacitors and winding inductances to get a nice sinewave. The secondary of the transformer can be stepped up to generate the swing required to drive the current.Use an extra coil round your MRI coil to detect how much current is being generated, use this to windup/down the DC voltage feeding your transformer either manually, or automatically (true RMS converter>ADC/Micro/DAC or opamp if you feel like tuning a loop).Probably first thing is to measure/model/calculate the inductance of your MRI coil though.

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2020, 09:57:13 am
content: Sorry, I don't have a link to a project that implements the level stabilisation loop I described. I believe I gave a sufficient description to draw up a block diagram. The only thing that may be unclear is the synchronous rectifier, which would need its reference signal phase shifted for highest DC output. It may well be better to implement it as a high speed precision rectifier, with a fast comparator, a non-inverting buffer, and an inverting buffer all driven by the input signal. the comparator outputs a squarewave synchronous to the input signal, with edges aligned to its zero crossings, that then drives a SPDT analog switch to choose between the non-inverted and inverted signal paths. If the delays in the comparator and buffers are well matched the result will be a fast precision rectifier.Once you've got a viable block diagram, develop a circuit for each stage and SPICE sim it, individual stage by stage. When you've got the worst of the bugs out of all the stage designs, start building a dead bug / Manhatten island prototype (any sort of protoboard breadboarding without a ground plane will be so unstable its worthless), again stage by stage, verifying performance against the sim results, till you can finally close the loop and check its effective by doing step response, and swept response tests. Introduce the disturbance to the reference level rather than at the load as handling the power levels required to vary the load resistance without distortion at 500KHz is no joke.A step response test using a relay to short out an extra non-inductive low-ohm resistor inserted in series with the coil would be possible, but a swept response tests using a low-ohm rheostat is likely to be problematic as a good low-ohm high power rheostat that's non-inductive is likely to be rather expensive so you'd probably have to settle for switched resistors if you aren't a good enough machinist to DIY a carbon pile rheostat + machining graphite or bulk high density amorphous carbon is always one of the least popular jobs in the shop!

author: anvoice
author_rank: Frequent Contributor
timestamp: May 07, 2020, 08:26:56 pm
content: Lots of information and great advice there, thank you everyone! This will probably be the most challenging DIY circuit I've attempted, which is why I was hoping some sort of schematic was available. But I'll try to get this done.

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2020, 09:34:55 pm
content: The brute force approach is to use a class-AB amplifier, as commonly found in audio power amplifiers, with a current shunt to detect and control the current. The current shunt could be attached between the load and ground to simply circuit operation. Achieving high bandwidth can be challenging this way but I think 500 kHz is feasible.A more sophisticated approach would be to implement a power 2 quadrant push-pull transconductance amplifier which inherently has a high output impedance and produces an output current. This would have the advantage of much higher frequency response although this is probably not needed here. Transistor curve tracers often use this method to drive the base of the transistor under test with a controlled current.If you are not up to designing a power 2 quadrant push-pull transconductance amplifier, then there is a simplification; a constant current source of positive 2 amps placed in parallel with a variable 0 to 4 amp constant current sink results in a +2 to -2 amp current output. An advantage of this configuration besides simplicity is class-a operation for low distortion but I suspect this would not be needed in your application.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 08, 2020, 01:49:01 am
content: Quote from: David Hess on May 07, 2020, 09:34:55 pmAre any of those amenable to SPICE modeling before building the circuit? I have to admit a lot of this is over my head at the moment, though I'm definitely willing to put in the effort to learn.One thing I forgot to mention is the need to pulse the alternating field, for length of around 10 microseconds. I'm guessing an AC SSR wouldn't work for that, so is there a good solution to couple the circuit to be driven by a microcontroller?

author: Circlotron
author_rank: Super Contributor
timestamp: May 08, 2020, 04:59:41 am
content: 5 cycles at 500kHz. I think that pretty much rules out any resonant setup then.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 08, 2020, 05:05:50 am
content: I should have mentioned this at the start. So the non-resonant options, such as a current source and a variable current sink (not sure how I'd build those yet) and Ian.M's approach are still viable?

author: Berni
author_rank: Super Contributor
timestamp: May 08, 2020, 05:16:37 am
content: If it doesn't have to be a smooth sine wave then i would just use two transistors and drive it with a square wave. To get accurate current just measure it and adjust the supply voltage on the next pulse cycle. If it needs to be a sine wave then you will need a big AB class amplifier since i don't think any more efficient resonant approach can start and stop that quickly.But still measure theinductanceof the coil, not the resistance. This will determine how much voltage is required to push those 5A trough the coil. If the coil is only a few uH then it only takes a few volts to 10s of volts to do it. If the coil is a few mH of inductance then doing this might require voltages in the kV range. This is a huge difference because it means you go from having to put a like 50W of power into the coil into needing >10kW of power to drive it (Obviously making the electronics to produce that a lot more difficult).

author: anvoice
author_rank: Frequent Contributor
timestamp: May 08, 2020, 05:50:37 am
content: Quote from: Berni on May 08, 2020, 05:16:37 amIt does need to be a sinusoid around a special frequency called the Larmor frequency for it to work, which is where the 500kHz comes from.The idea is to experiment with some different coil sizes to see which works best, but I will measure the inductance of the ones I make. Over several hundred watts is out of the question for this application By the way, someone else mentioned 5A, I need a maximum of 2A RMS. Possibly I could go even lower, I believe the issue is that eventually the magnetic moment vector of the sample will start to relax back to equilibrium before I can start measuring it, which is why the field cannot be extremely low.

author: Berni
author_rank: Super Contributor
timestamp: May 08, 2020, 06:29:10 am
content: Well if this coil is just an air core then you can pretty easily calculate the inductance from the size and number of turns.Okay its 2A but my point is that the current alone is not that useful in designing a circuit to drive it. You can trade off the driving current versus driving voltage by changing the number of turns. More turns means less current is required to generate the same magnetic field strength, but this also causes higher inductance so more voltage is needed. So power is the same in the end, just shifting the requirement between low voltage/high current and high voltage/low current.So in principle the 2A could be used to generate the same magnetic field strength as a MRI machine if you can make a coil with enough turns. At the same time 100A could be used to barely pick up a paperclip with a single turn coil. But the power used flip that 100A coil at 500KHz might only be 10W but flipping a MRI machine strength field at 500KHz in that 2A coil might require megawatts of power (Because the voltage to do it would be so insanely high).These Watts are not power that is lost to heat on the coil. Most of it is the power is actually stored inside the energy of the magnetic field it creates. Now when you want to flip the magnetic field around you have to first destroy the existing field to zero and this involves taking all of that power back out of the coil and then shoving it back in with the reverse polarity. So this power is flowing in and out of the coil at a rate of 500KHz, its not turned to heat, just transformed between being in the form of a magnetic field or an electrical current. No matter what fancy superconducting coil you have this power has to be moved in and out.The issue is that an AB class amplifier can't reuse the power from the collapsing magnetic field, so all of this power has to come from the power source.But a resonant method can reuse the power by adding a capacitor across the coil. This capacitor captures the energy flowing back out, stores it and then puts it back in once the field collapses to zero. This means that the power source only needs to supply however much power was lost to heat during this energy transfer, the rest of the energy is recycled. But since you want to start and stop it in microseconds means that you do need to put all this power in at the start to start the osculation and then take it back out to stop it, so you still need a huge power source even if resonant was used.(Normally in resonant methods you will gradually pump energy into it over the course of thousands of oscillation cycles, to stop just stop providing it power and let it die out due to internal losses that can again take thousands of cycles)

author: anvoice
author_rank: Frequent Contributor
timestamp: May 08, 2020, 07:34:59 am
content: Quote from: Berni on May 08, 2020, 06:29:10 amI see, makes sense. The reason I wanted a large alternating magnetic field was to improve sensitivity. In general, at low-field (permanent) MRI the sensitivity is many times lower, so the 10 microsecond pulse was a means to salvage the situation somewhat. Assuming I use a weaker magnetic field (say, 50uT) I will need to apply it for something like 100 microseconds. At 10uT, a millisecond. I am guessing at that point a resonant method becomes more viable?The biggest issue is the weak field generated by the limited current of the main, permanent magnetic field coil. If I could generate say, 100mT instead of 10mT, I would use a frequency of 5MHz for the alternating field, which would probably work better with the resonant method. Plus bonus sensitivity.In actual MRI machines, there are four stages to generate the RF field: a frequency synthesizer, a pulse modulator, a high power amplifier, and a quadrature hybrid coupler whose function I don't quite understand yet. I don't know whether they have to lose all the coil's stored power between pulses, but I do know that the typical NMR machine (same principle as MRI) does not need a great deal of power to operate once the permanent magnetic field has been established in the superconducting coil.

author: Berni
author_rank: Super Contributor
timestamp: May 08, 2020, 10:47:34 am
content: Yes longer pulses do make the resonant method more attractive. Since you are running for longer means you get to use the recycled energy in the coil for longer before you have to dump it back out. It also gives you more time to pump energy in so less instantaneous power is needed to do it (If the slower rise in amplitude at the start is okay).At what point resonant becomes more attractive depends a lot on the coils energy storage capability. The larger and stronger field you wish to create the more energy will be contained in that field. Normally one could optimize this by using a ferrite core, the core provides a easier path for the field lines so you can concentrate the field down to a small volume (So provided the strength is the same this smaller volume will hold less energy in total) But since you are placing the whole thing inside a giant permanent magnet that wont work (Ferite would likely saturate in such a strong field)If you do need the permanent field for only a short moment too then you could possibly dump a large capacitor bank into another coil to create a very strong field. This lets you dump massive currents (>1000A) trough a large coil and get very intense fields, but it will not last for more than a few milliseconds before the non superconducting coil turns all of this into heat (Also limiting how often it can be fired). This is how the strongest man made magnetic fields are created.In any case whatever coil you try to make do some back of the napkin calculations for its inductance and check how much voltage is needed to put the required current trough it at 500KHz. This might make coils with a lot of turns unpractical to drive so fast, in that case less turns with more current might be one solution.

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2020, 01:34:26 pm
content: Quote from: anvoice on May 08, 2020, 01:49:01 amThey are all amendable to SPICE simulation but recognizing good results probably means not needing to use SPICE.QuoteSolid state relays will be too slow.So you want 10 microsecond bursts of 500 kHz drive? You might get away with one quadrant operation using only a constant current sink and the load placed inside of an h-bridge. The h-bridge allows applying and reversing the current through the load without the complex design of a two quadrant linear output stage.

author: fcb
author_rank: Super Contributor
timestamp: May 08, 2020, 01:52:55 pm
content: It's all about the inductance. If you need 5 cycles at 500KHz, then you'd be better off driving the coil from some sort of amplifier, you won't have time to stabilise a loop within 5 cycles - what's the repetition frequency?

author: anvoice
author_rank: Frequent Contributor
timestamp: May 08, 2020, 08:37:36 pm
content: Quote from: Berni on May 08, 2020, 10:47:34 amRight, thanks. I'll be doing some rough calculations soon, but I can't do anything too crazy in terms of total power, since the whole thing is meant to be low power draw in the first place. I do believe the permanent field must be permanently on, although I'll look into it.Quote from: David Hess on May 08, 2020, 01:34:26 pm10us is the ideal situation, if it's hard to design I could go with a weaker field and longer pulse. Would a constant current sink and h-bridge method give a sinusoid, or will I need a sinusoid signal to drive the H-bridge?Quote from: fcb on May 08, 2020, 01:52:55 pmI think the sample relaxation time after the pulse is on the order of a few hundred milliseconds to a second, so that is likely the most often I will need to fire the pulse. Less often for proof of concept.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2020, 07:05:39 am
content: Quote from: anvoice on May 08, 2020, 08:37:36 pmGating a 500 kHz sine drive for 10 microseconds is not any more difficult.QuoteForget I suggested it; it would not produce sine drive.

author: gf
author_rank: Super Contributor
timestamp: May 09, 2020, 10:46:52 am
content: I found this paperhttps://www.sciencedirect.com/science/article/pii/S1631074819301055which seems to give some insights regarding a potential architecture of a low-cost NRM machine, and requirements for the components. For instanceQuote@anvoice, I have of course no idea how similar (or different) your machine will be in comparision to the machine described in this paper. But I guess it may be a reasonable starting point.EDIT:And browsing through Google search results, I get the feeling that coil design for NRM is a scienece on its own.

author: Berni
author_rank: Super Contributor
timestamp: May 09, 2020, 12:08:23 pm
content: Actually perhaps capacitor energy storage coupled with a resonant LC circuit could possibly do this.Some of the strongest AC magnetic fields commonly in use are for induction heating. You can guess these are some very strong fields if anything highly conductive inside that field starts heating up so quickly. These things typically only use 1 to 10 turns but use massive currents. These massive currents come from a tank capacitor in parallel with the coil, this being a special capacitor designed for these high AC currents (In large high power machines even the capacitors are sometimes water cooled). The hundreds of amps of current are mostly supplied by the capacitor recycling energy back and forth while the power source is supplying a only few percent of it that ware lost to heat.If you do wish to generate as powerful of a AC field as possible (Not sure if this is your requirement, or does only the permanent field need to as strong as possible) you could use the same coil + capacitor circuit as induction heaters (Likely reusing off the shelf induction heater parts), but instead of slowly pumping small amount of power into it to get it oscillating you instead slowly charge up a large capacitor. Then to start you dump the energy from the capacitor into the tank capacitor of the induction heater LC cirucit (This likely needs something like a thyristor to switch such a high peak current). This should get it oscillating within pretty much one cycle and then 5 cycles later to stop it you use a triac to short out the LC cirucit and stop it. This way you can charge the main storage capacitor slowly at the 10W you have avalable but then quickly dump all that energy into the AC field you need (Giving you many kW of peak power during that short time).

author: gf
author_rank: Super Contributor
timestamp: May 09, 2020, 08:47:04 pm
content: I don't think that a freely oscillating LC tank will suffice for the use case. If I understand correctly, the carrier frequency must match the Larmor frequency very closely (ppm or better?), and it needs to be finely adjustble (since the frequency is not fixed but depends on the strength of the static field, which must be - btw - as stable as well). So IMO the origin of the carrier needs to be a decent frequency synthesizer. Furthermore the carrier is not only needed for the excitation, but also for down-mixing the received signal (quadrature demodulation), since the spectral lines to be measured obviously reside in a very narrow frequency band (just a dozen of ppm or so) around the carrier frequency. I also did read in several papers that the envelope of the excitation pulses is not necessarily a rectangle, but "soft pulses" with different envelope shapes (e.g. sinc, gaussian, and even more complex ones) are used as well, in order to synthesize excitation pulses having dedicated frequency spectra.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 10, 2020, 12:32:32 am
content: Quote from: gf on May 09, 2020, 10:46:52 amThat is reassuring. A 11.7T machine is out of the question (needs either very powerful permanent or high current cooled superconductor magnets), but the fact that a low-field one can get away with about 5W means what I have in mind may be feasible. I'll take a look at that paper right now.Quote from: Berni on May 09, 2020, 12:08:23 pmThe higher the alternating field the shorter the pulse must be, which eventually becomes a problem of its own. About 0.5T will lead to a 10us pulse, which is good enough, and even smaller fields are possible with the only downside of a loss of some sensitivity in the final spectrum. So no, I don't need a very high AC field. Thanks for the detailed explanation though, perhaps I can adapt some of that into my design.Quote from: gf on May 09, 2020, 08:47:04 pmYes, the field should match the Larmor frequency quite well, which itself would change if the static field was not stable. I doubt I can afford a good frequency synthesizer for this project though, as it is a rough proof of concept. If a minimal (and cheap) configuration starts looking promising, more investment could be made. But the goal right now is to get a detectable signal in the detector coils (which may be the alternating field coils or another orthogonal set). If that is not possible with basic, affordable electronics, low-field NMR becomes much less lucrative.You're right about the excitation pulse envelope. I may have to incorporate something like that eventually, but again, the first goal is to get any kind of signal out of a minimal setup.

author: gf
author_rank: Super Contributor
timestamp: May 10, 2020, 03:08:22 pm
content: Quote from: anvoice on May 10, 2020, 12:32:32 amSince your Larmor frequency is just ~500kHz, I'm wondering in fact whether a 10us pulse isn't much too short, as this leads to a pretty wide bandwidth of the pulse (-> about 20%, while 10us at say 100MHz rather leads to 0.1% bandwidth only). On the one hand this increases the chance that the spectrum of the pulse does include the Larmor frequency even in case of a bad match of the carrier frequency, but on the other hand it reduces the pulse's energy density per Hz of bandwidth.I'm a complete NRM layman - just did read some basic stuff yesterday, and there is one thing I did not understand so far: In the literature, B1 is obviously trated as a constant value (although the excitation signal is actually AC and therefore a function of time). For a pure sine wave at the Larmor frequency, it makes sense that the value of B1 is simply the amplitude of the sinve wave. But how is the value of B1 defined for a spread-spectrum signal whose magnetic flow per Hz of bandwdith is a function of frequency? Which frequency band of the signal contributes to the value of B1? Do the protons have a particular (let me call it) "resolution bandwidth" inside which they react to the excitation, while any frequencies otside this spectral band are just wasted excitation, having has no effect?To summarize: As far as I understand, the maximum pulse duration is eventually limited by the required bandwith (which depends in turn on the maximum chemical shift that needs to be captured). Reducing the pulse bandwith to 0.1% (which I guess is still sufficient?) whould lead to a 2ms pulse (instead of 10us) and would reduce 0.5T to only 250uT, right? And depending on the answer to my above question regarding the definition of B1, the smaller bandwidth may additionally help due to the lager magneric flow per Hz of bandwidth.Quote from: anvoice on May 10, 2020, 12:32:32 amEvery single-digit-$ radio tuner includes a reasonable frequency synthesizer today. I think that a $$ OXCO can be stabe enough for the use case (at least short term). For 500kHz, likely a DDS generator like AD9830 or similar (using an OXCO as reference for the clock) may well suffice, at least for generating the carrier for the excitation pulse.The receiver side may be more challenging than the transmitter. Here I'm not sure yet reagarding the carrier phase noise requirements for your use case. And the major issue is likely that the signal is very weak. How big is actually the amplitude of the (~500kHz AC) magnitic flow which needs to be detected? Somewhere in the uT range, or even less? What's the area of the coil, btw?EDIT: Btw, how many T is your B0?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2020, 05:36:26 pm
content: The AC excitation field should be relatively small compared to the DC field. This is at least the general assumption for the theory. Anyway 0.5 T would be really high for an AC field in air - remember a system made to get 0.1 T AC over a slightly larger volume: it was quite a beast with a few kW of RF power to a high Q resonator with water cooled coil. Kind of like induction heater, just higher frequency.I have once used a system that seem to be made for low field NMR. It had an RF amplifier for some 2 kW for pulses up to 1 ms or so. This gives odd effects when one starts to see/hear arcing at BNC connectors. The NMR coil may not be well matched to the amplifier / cable impedance. One may have to consider this with the amplifier, not all power amps like a poorly matched load.I don't think one would really need a pure sine wave, so square wave drive with a H bridge may be acceptable.A point could be dampening the LC resonance at the end of the pulse.

author: gf
author_rank: Super Contributor
timestamp: May 10, 2020, 07:54:03 pm
content: Quote from: gf on May 10, 2020, 03:08:22 pmI just recognized, 1H isotope has 42.55 MHz/T (γ=2.674e08), so 500kHz corresponds to only ~12mT. Is the static field really that low? Or do you have a different isotope in mind?EDIT:And solving "α = γ * B1 * tp" for B1 [where α=pi/2, tp=2ms (as I suggested earlier, for ~0.1% spread-spectrum bandwidth), and γ=2.674e08 for 1H isotope] results in a required RF field of onlyB1=2.94uT, in order to turn the spin vector to the x-y plane ("90° pulse"). (Please correct me if there is an error in the calculation - I could hardly believe the low number either.)According to the calculator athttps://www.accelinstruments.com/Magnetic/Magnetic-field-calculator.html,a magnetic field of 3uT should be achievable with a current of only 50mA, in the center of a short coil (air core) with a single turn and an assumed diameter of 20mm. The calculated inductance is 45nH then (-> impedance 0.14jOhm @500kHz, i.e. almost negligible). This would not reqire a powerful amp at all.But I could imagine noise problems in the detector if B0 is so low.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 10, 2020, 09:42:58 pm
content: Quote from: gf on May 10, 2020, 03:08:22 pmFrom my understanding, a pulse with a nonzero bandwidth (e.g. a sinc pulse ideally) is used for MRI for the excitation of particular slices of space. However, for simply detecting an NMR signal a pure Larmor frequency sine wave is used. In that case, a rotating reference frame is created within which the magnetic moment of the sample is pushed down from the z-axis into the xy-plane at a constant speed, i.e. as neatly a possible. At that point, the width of the pulse (which is independent of bandwidth) will determine whether the sample gets to start relaxing before it's fully in the xy-plane. If it does start relaxing (i.e. long pulse) the overall signal will be lower.My AC field only needs to be about 0.5mT (millitesla), not 0.5T. You're right that the pulse width will be inversely proportional to field strength, so the longer the pulse the lower the field I get to use. At the expense of sensitivity though, which may already be limited with a weak field.Will definitely look into these signal generators. You're right that the receiver will be a challenge: I was planning to start another topic a little later to figure out how to go about detecting the microamperes or even nanoamperes of signal I will get. I have an aged digital oscilloscope, but will need some sort of amplifier to be able to detect the low currents in the receiver coils. I have heard of Dave's ucurrent gold, which helps do this, but it's apparently out of stock.If you're asking for the area of the transmitter coil, it will probably be a few square centimeters. As far as the receiver goes, likely even smaller: maybe a square centimeter, maybe less.Yes, the static field (B0) will indeed be about that low. Creating a much higher field in air requires prohibitively high currents for an air-cooled setup.Thank you for taking the time to do this much research into the topic!Quote from: Kleinstein on May 10, 2020, 05:36:26 pmUnit error. It's 0.5mT. You're totally right, the field would be insane at 0.5T.That's a very interesting point. From what I understand, a square way may theoretically work (not 100% sure). It's usually done via sinusoid as that creates a rotating reference frame within which the magnetic moment goes from the z-axis to the xy-plane at constant speed. I believe at the very least, it would exhibit jerky movement in a square wave field. In a perfect world, I would use a sine wave so everything is as neat as possible, but if the square wave is many times easier to implement I could definitely consider it. I'm therefore officially welcoming suggestions for how to create an efficient square wave alternating field in the transmitter coils, it may be worth a try.

author: gf
author_rank: Super Contributor
timestamp: May 10, 2020, 10:23:45 pm
content: QuoteDo you only want to detect 1H resonance at all?Is your aim not to get a chemical shift spectrum?You get it for almost free anyway if you sample the down-mixed / quadrature demodulated signal and do a FFT.Note that a time-limited pulse with rectangular envelope has a (sinc-shaped) spread spectrum anyway in the frequency domain (-> 1000ppm bandwidth requires a pulse width of about 2ms, @500kHz, and shorter pulses would even have wider bandwidths - IMO wider than necessary).A sinc pulse (in the time domain) has the property that its spectrum is rectangular in the frequency domain, w/o any side lobes, so all energy is concentrated (and evenly distributed) inside this rectangle.Btw, I did update my previous message while your message arrived. Please check.EDIT:QuoteThe spread spectrum is used in FT-NMR in order to excite and capture the whole chemical shift spectrum at once, avoiding the need to sweep through frequencies in small steps.The location of MRI slices in space is IMO done by adding a gradient field to B0, making the Larmor frequency location-depedent (thus only the slice with the right Larmor frequency will respond to the excitation). Gradients can also be extended to 3D in order to address indivudial voxels in space. I guess that a spread spectrum can be used here as well in order to capture multiple neighbor voxels at once, but I don't known any details regarding MRI.QuoteMy thougths: A square wave is eventually a sum of sine waves (fundamental + harmonics).The harmonics would (undesirably) excite isotopes whose gamma is by a factor of n higher (where n in 3,5,7,...),if such isopes exist.Otherwise the harmonics will likely have no effect.

author: ssmirnov56
author_rank: Newbie
timestamp: May 11, 2020, 02:48:21 am
content: Quote from: gf on May 10, 2020, 07:54:03 pmGuten tag! Very interesting thread!I just have one quick point: the pi/2 pulse duration of millisecondsmightbe so long that the B0 and B1 relaxation eliminates much of the sample induced magnetization. If relaxation has enough time to progress, the intensity of the induced and detected signal will go down substantially (=low sensitivity). This is one reason why for the pi/2 B1 pulse duration better be as short as technically possible (especially if the goal is to ping only one type of the nuclei and not get the spectrum).Vielen dank!

author: anvoice
author_rank: Frequent Contributor
timestamp: May 11, 2020, 05:21:48 am
content: Quote from: gf on May 10, 2020, 07:54:03 pmNo, your calculations are correct. However, because the magnetic moment can relax during the pulse and not after it, sensitivity suffers greatly. I believe 2ms might be at a point where the trade off is too much. Could try to go for half a millisecond though: the field would then only need to be 12uT.Quote from: gf on May 10, 2020, 07:54:03 pmYes, the idea for the proof of concept is to detect the hydrogen resonance. If that succeeds, I would go for a spectrum. That is why I was not worrying about the receiver bandwidth spectrum at the moment. Basically, I don't know if I will get a signal at all: the idea of a basic prototype is to test that.Quote from: gf on May 10, 2020, 07:54:03 pmRight, and B0 is varied by using variable step coils. Not something I can manage right now, and again, this is far beyond the prototype's point. If there is no signal using a ~10mT B0 field, there is no point trying to get a spectrogram.Quote from: gf on May 10, 2020, 07:54:03 pmAs for the harmonics, you're right that they would be a parasitic in the spectrum, but actually a boon in the prototype, because they would add to the initial signal intensity.To summarize: you're making very valid points for a fairly advanced NMR instrument capable of resolving various molecules, but that is step 2. If step 1, which is proof of concept, fails, investing effort into a more sophisticated design will be for naught.

author: Berni
author_rank: Super Contributor
timestamp: May 11, 2020, 06:18:16 am
content: If weaker fields allow you to go for a longer pulse then this might be helpful in the reception later on.Demodulating a weak signal after it has been received back is a lot easier if you have a longer slice of time to work with. Signal processing in receivers (be it radio or radar or ultrasound) tend to have some sort of averaging like behavior present somewhere along the chain. So the longer you can look at a narrow slice of the spectrum the cleaner the resulting signal gets.This allows receivers to pull out signals that are well below the noise floor, so this is a great help when looking for really weak signals as the amplifier sensing the signal does have limits on how low noise it can get (Without cryocooling and all that jazz)

author: anvoice
author_rank: Frequent Contributor
timestamp: May 11, 2020, 06:34:23 am
content: Quote from: Berni on May 11, 2020, 06:18:16 amUnfortunately, the sample relaxation time (at which point the signal will be generated) after the excitation pulse does not depend on the transmitter pulse width. It will certainly change the spectrum if one is present because the excitation bandwidth will be different, but I don't think it will help with sensitivity to have a longer pulse. The opposite is true because the sample's magnetic moment will start the relaxation before it's fully adjusted to the xy-plane.I will start another topic about the receiver side of things so as not to clutter this one.

author: gf
author_rank: Super Contributor
timestamp: May 11, 2020, 03:02:04 pm
content: Quote from: anvoice on May 11, 2020, 05:21:48 amI'm just wondering, aren't relaxation time constants rather in the 100ms to seconds range? Then I would not expect more than a few percent of exponential decay happening alreadyduringa 2ms pulse (i.e. rather negligible). Or do I miss anything?The calculated coil was only a single turn, so it should be easy to increase B1 by a factor of N by using N turns instead (w/o need to increase the current).QuoteCan you actually calculate an estimate for the expected signal level (magnetic flux through the pick-up coil) for the planned setup?QuoteWhile they add to the total pulse intensity, I don't think they are helpful for the excitation, since their frequencies are likely too far off to excite 1H.

author: gf
author_rank: Super Contributor
timestamp: May 11, 2020, 08:00:02 pm
content: Yet another low-cost design:https://iopscience.iop.org/article/10.1088/1742-6596/1380/1/012012/pdf

author: anvoice
author_rank: Frequent Contributor
timestamp: May 12, 2020, 05:18:57 am
content: Quote from: gf on May 11, 2020, 03:02:04 pmLooking up the T1 relaxation time of water, turns out it's 4 seconds. So you are correct, it is probably negligible. I will be using a longer, weaker pulse then.QuoteYes, it should be possible but slightly nontrivial. Give me a little time to answer this question: I will need to go from the Boltzmann distribution of hydrogen spins within the B0 magnetic field, and figure out what sort of signal (a damped sine curve) will be traced out as the magnetic moment relaxes back to the z-axis after the B1 pulse. Water should give a pretty good signal compared to other substances, but if I had to answer right away I'd say tens to hundreds of nanoamperes is the best I can count on.QuoteRight, assuming they excite anything at all though, they will add to the output signal intensity. Not helpful in the long run for sure.Quote from: gf on May 11, 2020, 08:00:02 pmThat is extremely helpful, thank you. It may be a good idea to use a setup similar to theirs since it's known to work, although their permanent field is many times stronger than what I can get (more sensitivity).

author: gf
author_rank: Super Contributor
timestamp: May 12, 2020, 07:44:55 pm
content: @anvoice, I guess you might be interested in this one as well. In particular, it aims at a rather low frequencies, in your range.It isverybroadband, though (I'm not sure whether you want that).https://42f0c01f-a-62cb3a1a-s-sites.googlegroups.com/site/casecirc/publications/mandal_2014_jmr_nr_mr_final.pdf?attachauth=ANoY7cqXB7TX6lCRnwNJm0yjB64NFpEA5feF0JxI0GcUX6_GfuW2Hms64hOb4FwEX5iyp43mufIIXgMIDTKpyxevbhqjNJYz_8LE-BL-8_8F2glzf80F5s80uWb_gKfxm--B_FY4E__UAsolxwosjdR6zkQuar9aPJTd3nfEqRTIrbtFN29OuiemN6YLW-uPvlF2R6FXI7MGq-vI7X_FRYgeLPtot_HDKs5wREEX60uVhB2pxlUp71gz8c1AczRRBBopfncA2g-x&attredirects=0

author: anvoice
author_rank: Frequent Contributor
timestamp: May 17, 2020, 06:41:38 am
content: Quote from: gf on May 12, 2020, 07:44:55 pmYes, does seem interesting. Makes me wonder how many more such papers are floating around that are not in the open domain. Then again, those who publish in large journals usually have the funding to not build their own NMR.I was thinking of trying the setup in the first paper you found, with components that will work around my frequency. Still trying to understand everything completely though, and I have doubts about the AD9959 module as another forum member made a thread concluding they're not too well-made.