Meyer-Mace Isotopic NMR Generator

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Kator01

Re: Meyer-Mace Isotropic NMR Generator « Reply #125 on: 2015-12-31, 14:38:49 »

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Peterae,

I have to look into my files but I think you need a buffer-stage following your colpritts and not feeding directly into the preamp.
Second: the 100 nF coupling-condenser is too big ..Xc for 50 MHz ist 0.032 Ohm which means the output-signal is fully exposed to the input-impedance of the preamp ( base-emitter-capacitance) and I do not know the value of the input-impedance.
If your colpritts has - lets say -. an output-impedance von 600 ohm and the preamp is at 50 Ohm then you can imagine what happens.
If you observe or better compare on you scope how the output of your colpritts breaks down from idle-modus to beeing connected to the preamp...what do you see ?
Is the Vpp breaking down more tha 50 % ?
Change you preamp-input-condenser down to 68 pF. Xc fpr 68 pF and 50 MHz is 46 Ohm

may be you use an old fashioned MMIC. They are yused for antenna preamps and simple in design. C_block also 68 and increase in steps to
680 pF for best results.

http://194.75.38.69/pdfs/MAR-6SM+.pdf

Mike

Peterae

Re: Meyer-Mace Isotropic NMR Generator « Reply #126 on: 2016-01-02, 13:15:35 »

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Hi Mike
That is a nice monoamp, i will digest what you have said about the decouplers, shame there's not a single package 300mw amp

i will look to see what is around, never occurred to me.

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #127 on: 2016-01-10, 21:56:21 »
Group: Administrator Hero Member Posts: 3975	OK decided to throw a bit of money into this project. Just ordered a ~~300MHz~~ 100MHz DDs system 0.001Hz http://www.ebay.co.uk/itm/281773434424 and a 20-500MHz 1.5 Watt preamp http://www.ebay.co.uk/itm/331617730370?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT ------------------------ Meyer-Mace Isotopic NMR Generator $_1.JPG (22.78 kB, 400x324 - viewed 1087 times.) Meyer-Mace Isotopic NMR Generator $_2.JPG (57.58 kB, 359x400 - viewed 1038 times.) « Last Edit: 2016-01-30, 09:03:55 by Peterae »

verpies

Re: Meyer-Mace Isotropic NMR Generator « Reply #128 on: 2016-01-11, 19:44:07 »

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Posts: 3670

Quote from: Peterae on 2016-01-10, 21:56:21

Just ordered a 300MHz DDs system 0.001Hz
http://www.ebay.co.uk/itm/281773434424

That's 300MHz sampling rate at 12bits resolution.

Quote from: Peterae on 2016-01-10, 21:56:21

and a 20-500MHz 1.5 Watt preamp
http://www.ebay.co.uk/itm/331617730370?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

If that DDS module has 0.5W output power just like most commercial SigGens, then this preamp could be a little stronger like this one.

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #129 on: 2016-01-11, 20:32:38 »
Group: Administrator Hero Member Posts: 3975	It does say that it has a 0.001Hz resolution or more up to 100MHz. The datasheet says it can do uHz this obviously depends on the frequency, the higher the lower res. The great thing here is that it is a 2 channel DDS so i can also generate the lower Hz frequency as well on the 1 board. Dam thats a nice 5 watt amp and similar price to my 1.5watt Grrr. « Last Edit: 2016-01-11, 21:09:56 by Peterae »

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #130 on: 2016-01-24, 13:31:04 »
Group: Administrator Hero Member Posts: 3975	I have the DDS & Pre amp now. The DDS is amazing, the actual DDS is up to 100MHz with atmeg controller board and touch sensitive LCD. Instructions are lacking so need to track them down on line, the dds i can chirp & fsk and need to see if there is PC software available for it. Need to check out the pre amp next when i get some time.

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #131 on: 2016-01-29, 14:25:26 »
Group: Administrator Hero Member Posts: 3975	OK next thing i need to start thinking about is the coil design. So i have my pure iron core, i have my 45MHz DDS and preamp and a 16Watt amplifier, so for starters i need to concentrate on the Driving coils. and to be honest i an not sure how i design a coil to work at 45MHz, presumably i want it's impedance to be 50 Ohms around it's operating frequency. Meyer says he used 22 Turns but he was operating at 23MHz Of course i realise i need to get hold of or will probably make some Litz So any pointers here would be handy Using XL= 2πfL then it looks like i need an inductance of 159nH to achive an impedance of 50 Ohms Using an online inductance calculator for 160nH using an iron core k=200 then i need 9.5 turns across a 10cm length with a coil diameter of 3cm $:-\$ or 4.25 turns with a diameter of 3cm and a length of 2cm

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #132 on: 2016-01-29, 14:51:23 »
Group: Administrator Hero Member Posts: 3975	I wonder if i need to spread the coil over the length of the iron bar or keep it in the middle close turns, i also need to put a 400Hz winding and 2 other bucking coils on the core and also at some point a heater winding and insulation.

gyula

Re: Meyer-Mace Isotropic NMR Generator « Reply #133 on: 2016-01-29, 23:20:06 »

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Posts: 1250

Hi Peter,

Would like to understand why you tend to choose coil reactance to be around 50 Ohm at the operating frequency? The power amplifier which is to drive this coil will not be "happy" with such inductive load and I think you know that.

Perhaps making the 159 nH coil resonant around 45 MHz with say a 74pF capacitor first to see any possible Q the core may provide at such frequency would be a first step to attempt a reasonable matching. Then, after learning the (parallel) resonant impedance of this LC circuit on your core, you could attempt either to split the 74 pF into two series members to form a capacitive divider with its low impedance input side to be around 50 Ohm as a match to the power amplifier output or to make a tap on the coil or to make a coupling coil, also for an approximate mathcing the LC impedance to near 50 Ohm. These can only be done of course when the resonant impedance for the LC at least a few times higher than the inductive reactance of the coil, and this depends mainly on the loss of the core at 45 MHz.

On you question of whether spreading the coil turns or winding the wire turns close next to each other: I think first a normal close spacing between the turns would be preferred to spreading.

I do not think you would need Litz wire at such high frequency, just use thick enameled copper wire. say any OD of 0.9 to 1.2 mm, this would give low enough loss.

I attach an article on practical self supporting air cored coils, just as a guide to get a feel on sizes and number of turns, mainly in the some ten to some hundred nanoHenry range, it is much better than no such info at all.

Of course the presence of the iron core will greatly offset the air cored L values but perhaps with some trial and error you would be within range easier with it.

Gyula

------------------------

SelfSupportingCoils.zip (1410.28 kB - downloaded 299 times.)

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #134 on: 2016-01-30, 09:10:50 »
Group: Administrator Hero Member Posts: 3975	Hi gyula Thanks for your help here. Quote Would like to understand why you tend to choose coil reactance to be around 50 Ohm at the operating frequency? The power amplifier which is to drive this coil will not be "happy" with such inductive load and I think you know that. No i did not know that the amp will not be happy with 50 Ohm load impedance, if you look at my 16watt amp module it says into 50Ohm Load? So you are saying i will be better driving my load coil as a LC resonant system.? Although when i connect my load inductor it will be in parallel with the amplifiers last stage capacitor?? I'm confused now. Also skin depth for copper wire at 45MHz is 9.7um so anything over 20um is wasted copper, hence my thinking on Litz « Last Edit: 2016-01-30, 10:38:16 by Peterae »

verpies	Re: Meyer-Mace Isotropic NMR Generator « Reply #135 on: 2016-01-30, 11:05:33 »
Group: Professor Hero Member Posts: 3670	Quote from: gyula on 2016-01-29, 23:20:06 The power amplifier which is to drive this coil will not be "happy" with such inductive load and I think you know that. If the drive coil forms a parallel LC tank with the final capacitor of the RF amp, then the transistor of this amp will be very happy to drive it at its LC resonant frequency. The additional bonus is that the RF current circulating in the LC tank can be many orders of magnitude greater than the current provided by the RF amp, ...depending on Q, Eddy current and hysteresis losses of the core. Quote from: gyula on 2016-01-29, 23:20:06 I do not think you would need Litz wire at such high frequency, just use thick enameled copper wire. say any OD of 0.9 to 1.2 mm, this would give low enough loss. No, fine Litz wire is mandatory in this application. At 45MHz a solid 1.2mm OD wire would have the same "resistance" as a 0.1mm wire at DC.

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #136 on: 2016-01-30, 11:12:36 »
Group: Administrator Hero Member Posts: 3975	OK thanks for clarification verpies. Am i trying to design my coil at 45Mhz for 50Ohms or as low Ohm as possible to reduce power losses.? Do you think it is worth spending time building a B-H curve plotter to work out the u of my core.? Cheers Peter

verpies	Re: Meyer-Mace Isotropic NMR Generator « Reply #137 on: 2016-01-30, 11:29:29 »
Group: Professor Hero Member Posts: 3670	Quote from: Peterae on 2016-01-30, 11:12:36 Am i trying to design my coil at 45Mhz for 50Ohms or as low Ohm as possible to reduce power losses.? MPTT answers this question. Note that the impedance of an ideal parallel LC tank is infinite at resonance. Quote from: Peterae on 2016-01-30, 11:12:36 Do you think it is worth spending time building a B-H curve plotter to work out the u of my core.? No, but it is worth building two mini RF current transformers (CT) that you will be able to thread the Litz wire through, in order to sense what current is circulating in the LC tank and compare it to the current supplied by the RF amp (that's what the 2^nd CT is for). You could use a CSR for the latter, too, but you do not want to introduce any extra resistance in the parallel LC tank. « Last Edit: 2024-08-03, 22:26:56 by verpies »

gyula	Re: Meyer-Mace Isotropic NMR Generator « Reply #138 on: 2016-01-30, 11:38:23 »
Group: Tech Wizard Hero Member Posts: 1250	Okay folks, I accept the use of Litz wire, thanks for the explanation. Peter, most RF/HF power amplifier is designed to drive a 50 Ohm load but by default, this load needs to be purely resistive and not capacitive or inductive. Of course exceptions do occur when a load to be matched is a specific one, in those cases the matching network should be designed accordingly. In your particular power amplifier schematic you surely recognize the Pi filter at the output, consisting of the two preset 100-150 pF capacitors and the 3.5 turn air core coil, the purpose of this filter is to match the drain impedance of TR1 to a pure 50 Ohm resistive load. What could be the drain impedance you may wonder? It can be pretty well approached by this formula Z=50V50V/16W = 156 Ohm and this should be transformered down to 50 Ohm by the Pi filter, such filter network can transform impedance and also serve as a harmonic filter. Impedance transformation depends on the ratio of capacitors (C1 and C2) of such Pi filter, it can step impedance down or up too. (You may wonder how low efficiency this amplifier may have: 16W/25W=0.64 where the 25 W comes from 50V0.5A input DC power, i.e. 64% efficiency which is typical for such, nearly linearly biased AB class amplifiers.) Yes, the amplifier's last stage includes a capacitor what could seemingly be used to tune a 'load' inductor of say 160 nH for a 45 MHz resonance but this then would represent a 50 Ohm pure resistive resonant impedance by a big luck only. This may answer verpies's suggestion to use a parallel LC tank: matching could be obtained via the Pi filter if and when the final capacitor setting would insure a near 50 Ohm resonant impedance for the tank and this impedance would then be correctly transformed back towards the drain impedance of approximately 156 Ohm. I think a better approach would be to use an additional capacitor at the output in series with your 160 nH 'load' coil, as I drew it in the schematic attached. This way C2 and C3 would constitute a capacitive (voltage, hence impedance) divider, their series resultant equivalent C value would tune the 160 nH to the 45 MHz resonance. Under such condition the transformed (stepped down) impedance would be pretty close to a resistive-like 50 Ohm what the amplifier transistor would need to see via the Pi filter. Alternatively, the Pi filter could be omitted and the 160 nH drive coil would be matched by resonating it via C2 and C3 directly, this what I indicated in the second schematic below. Sorry for confusion I may have caused for you, it was not my intention. Gyula ------------------------ Meyer-Mace Isotopic NMR Generator 50MHZ_16W_MOSFET_PA modified.jpg (101.33 kB, 567x637 - viewed 976 times.)

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #139 on: 2016-01-30, 11:49:45 »
Group: Administrator Hero Member Posts: 3975	Thanks verpies & gyula Quote Sorry for confusion I may have caused for you, it was not my intention. That is me not you i am easily confused with this stuff, very much out of my depth really, I thank you very much for your time, expertise and explanations. Quote but it is worth building two mini RF current transformers (CT) Ok i can do that, in fact they can be added later so i can concentrate on the winding. Need to find a bobbin to wind these coils on, so they can be slipped on the core, and they may have to withstand high temperature as well, i am currently thinking of embedding them in high temp silicon good up to 300 Deg C

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #140 on: 2016-01-30, 12:01:27 »
Group: Administrator Hero Member Posts: 3975	OK i like the idea of taking the pi filter out and using the 2 caps to feed the inductor. What would be the set up procedure for this, how do i adjust the 2 capacitors to find the optimum points for my inductor EDIT Also now i am wondering if the output power of the above amplifier circuit can be increased maybe by using one of these LDMOS fets with a much larger heatsink, as this could greatly simplify and cheapen this build by quiet a lot. Found some litz. http://www.ebay.co.uk/itm/Litz-Wire-High-frequency-Transformer-Wire-High-Current-40A-1000-Strands-0-1mm-/151884136336?hash=item235cffff90:g:AwUAAOSw4UtWRhnQ « Last Edit: 2016-01-30, 12:33:43 by Peterae »

gyula	Re: Meyer-Mace Isotropic NMR Generator « Reply #141 on: 2016-01-30, 20:09:19 »
Group: Tech Wizard Hero Member Posts: 1250	Well, the setup procedure I suggest would be this: If you have an L meter capable of measuring with say +/-5 % accuracy under 1 uH, then it could be used to check the few number of turns on the iron core which may give the desired 150-180 nH inductance. Use at least a wire diameter of 0.8-1 mm for this test because with the Litz wire you would get only better Q when it arrives. BUT in fact an L meter would be needed which measures at 45 MHz in this case because the iron core I assume is unspecified at this frequency and the a low frequency inductance check may be far off at 45 MHz. So you may wish to build this simple test circuit I attached and this would prove useful to estimate the resonant Q of the 45 MHz LC tank on the iron core. Of course other test circuit setups could be used for checking the Q of your iron cored coil at 45 MHz. Suppose you find a voltage ratio of 10 (say the voltage amplitude from your DDS output i.e. across the left hand side of the 1 pF capacitor (TP1) and grnd is say 910 mVpp on one of the scope channels and the voltage across its right hand side (TP2) and grnd i.e. across the coil is say 9.2 Vpp on the other scope channel, (9.2/0.91=10), then this would indicate the resonant impedance to be roughly 10*50=500 Ohm if the DDS output has 50 Ohm (expected) output impedance. If you use 0.56 pF (or two 1 pF capacitors in series), the accuracy for the Q value received increases. The (loaded) Q comes from the 500 Ohm/X_L and if X_L is around 50 Ohm at 45 MHz when the L is around 177 uH, then Q is 10 for the coil on the iron core. This would be a lossy core at 45 MHz but then you would have to stay with that... Supposing this check indeed indicates a Q of 10 (or higher), the members of the capacitive voltage divider can be determined. I will continue with that tomorrow. Their actual value depends on the actual Q value of the coil on the iron core, unfortunately. By the way I agree to stay with a coil inductance value somewhere in the 140-180 nH range because the tuning/matching capacitors may have a reasonably high value at 45 MHz, much higher above stray capacitance values. The output power could be increased by lowering (with a set low limit of course) the transformed impedance of the load the drain circuit "sees". For the 16 W output power this impedance is about 156 Ohm. I referred to how the drain impedance can be approached by considering the supply voltage V and the desired output power P with the formula Z=V²/P. If you have an IRF610 MOSFET, you could still use it (on a heat sink of course) for some higher input power till you stay within its SOA (Safe Operational Area) which is included in the data sheet. Or if it is still not enough and you have LDMOS types available you could use them too of course. Would you give type number(s) if you have. Will return to this too. Gyula ------------------------ Meyer-Mace Isotopic NMR Generator 45MHz LC tank resonance check.jpg (105.41 kB, 620x349 - viewed 1653 times.)

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #142 on: 2016-01-30, 21:36:46 »
Group: Administrator Hero Member Posts: 3975	Hi gyula Today i made 1 meter length of 165 strands of 0.125mm Litz wire and wound 4 turns on a former and glued it. Tomorrow i will attempt to measure the inductance, my LCR meter was quiet expensive and also calculates the Q but only goes to 10Mhz and i have a feeling it may not read such a low inductance value, we will know for sure in the morning. I don't think i have any capacitors with such low pf value, i think i have some 6.8pf will check and report back in morning, i could always try putting some in series. I am not sure how i will yet drive the 45MHz, my new dds does not have a powerful output but i do have the preamp i bought 1.5 watt output and i also have the 2 transistor preamp i build for my colpitts oscillator see below diagram. I dont have any LDMOS fets yet was just mulling it over for higher power. I think in the morning i will try my 2 transistor preamp connected to the DDS and check that works ok, then hook that to your circuit above. Once i have some solid data we can decide what or how to proceed. Many thanks for your help. EDIT I also have a grid dip meter i was thinking i could sniff the coil for a Self resonant frequency. Also i am wondering if i were to put say a 100pf cap across the inductor and grid dip it for the resonant frequency we could then calculate the inductance knowing the capacitance & frequency.

gyula	Re: Meyer-Mace Isotropic NMR Generator « Reply #143 on: 2016-01-30, 22:11:49 »
Group: Tech Wizard Hero Member Posts: 1250	Well, regarding the small value coupling capacitor, you may have heard of "gimmick" capacitors which are nothing else but a twisted insulated wire pair of a few cm long, open circuit at the far end? And cutting down the length at the open end you reduce the capacitor value between the two other ends of the wires. See this video: https://www.youtube.com/watch?v=v1wWL6TGWOE For the passive coil test I have suggested you do not need power drive at 45 MHz but a few hundred mV peak to peak amplitude you can see on your scope, the DDS output would be loaded by only the 1 pF capacitor in series with an LC tank, no harm or problem. Perhaps you wish to check inductance value at higher power levels too, then the MOSFET amplifier could be used at a few Watt output level. Yes the grid dip meter is also a good way to check for resonance. I have not mentioned the position of the coil on the core, obviously start at one of the very edges of the core and once you found a dip (or voltage maximum in the test circuit I drew), then you can go towards the center of the core and make notes. Gyula

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #144 on: 2016-01-30, 22:20:30 »
Group: Administrator Hero Member Posts: 3975	Quote Well, regarding the small value coupling capacitor, you may have heard of "gimmick" capacitors which are nothing else but a twisted insulated wire pair of a few cm long, open circuit at the far end? And cutting down the length at the open end you reduce the capacitor value between the two other ends of the wires. See this video: https://www.youtube.com/watch?v=v1wWL6TGWOE yes good idea Quote For the passive coil test I have suggested you do not need power drive at 45 MHz but a few hundred mV peak to peak amplitude you can see on your scope, the DDS output would be loaded by only the 1 pF capacitor in series with an LC tank, no harm or problem. Perhaps you wish to check inductance value at higher power levels too, then the MOSFET amplifier could be used at a few Watt output level. OK i will try the DDS output Here's an image of the core as previously posted in thread

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #145 on: 2016-01-30, 22:47:14 »
Group: Administrator Hero Member Posts: 3975	For the Bias coil the patent requires 0.5T I can build this 3cm long with 70 turns and 1 Amp giving me a field of 0.58T 20 SWG

gyula	Re: Meyer-Mace Isotropic NMR Generator « Reply #146 on: 2016-01-30, 23:03:15 »
Group: Tech Wizard Hero Member Posts: 1250	I was not aware of that picture, sorry and thanks for posting it again. It seems to have a closed magnetic path from the coils 'to be placed on the rod' point of view (as per the patent describes) but you will have difficulties when using the grid dip meter. You would need to use a single turn of wire penetrating under the rod and closing also in one turn coupled to the dip meter coil, perhaps the user manual includes this method. Now the placement of the few turns coil on the rod for the inductance test will probably yield small changes in inductance as you slide the bobbin position on it, I suppose, due to the closed magnetic path. Gyula

verpies	Re: Meyer-Mace Isotropic NMR Generator « Reply #147 on: 2016-01-31, 06:45:42 »
Group: Professor Hero Member Posts: 3670	Quote from: gyula on 2016-01-30, 23:03:15 I was not aware of that picture, sorry and thanks for posting it again. It seems to have a closed magnetic path from the coils 'to be placed on the rod' point of view (as per the patent describes) The solid metal yoke will allow helluva Eddy currents to develop. They will lower the Q and inductance.

verpies	Re: Meyer-Mace Isotropic NMR Generator « Reply #148 on: 2016-01-31, 07:00:04 »
Group: Professor Hero Member Posts: 3670	Quote from: Peterae on 2016-01-30, 21:36:46 I dont have any LDMOS fets yet was just mulling it over for higher power. Getting more output power with these transistors will involve increasing the supply voltage to them. While LDMOSFETs have lower gate capacitances, they still require AMPS of RF gate drive current at 45MHz. Make sure your gate driver circuits are capable of providing it. Don't be surprised if it turns out that you need a 20W input to get a 100W output. These gate drive requirements are listed in their datasheets.

Peterae	Re: Meyer-Mace Isotropic NMR Generator « Reply #149 on: 2016-01-31, 08:19:14 »
Group: Administrator Hero Member Posts: 3975	Quote I was not aware of that picture, sorry and thanks for posting it again. It seems to have a closed magnetic path We do have an air gap of i think it was 1mm around the diameter of the rod at one end where it goes through the plate, the other end is interference and pressed in place, the gap is to break the electrical path and stop a short from one end of the rod to the other. I will place information in this post as i work it out My mistake first my LCR meter only goes to 10Khz not 10Mhz So all tests here are at 10KHz L=0.58-0.59uH in middle of core and 0.57-0.58 at ends of core (a lot highr than i thought it would be) R=0.0173R Z=0.0417R Q=2.14 -------------- OK have been having problems Firstly i dont think this DDS is terminated with a 50 Ohm resistor, it appears to be missing on the pcb, so there's a multiple stage filter on the output of the DDS chip. Next i used 3 off 3.3pf caps in series to make my 1pf cap, i soldered a 100pf trimmer cap across my coil, but when i scan on my DDS i have a really big amplitude (biggest) up the top end at 75MHz with trimmer cap at lowest cap value where the vanes do not overlap, and when i adjust the trimmer cap to it's highest value when the vanes are fully overlapping then the frequency goes up to 85Mhz where the biggest amplitude is. Does not make sense, i soldered a 120pf straight across the coil and the biggest amplitude stayed about 85MHz. So something strange going on. OK video is here https://youtu.be/nKcpOm2xNNU ------------------------ Meyer-Mace Isotopic NMR Generator 20160131_122649.jpg (2836.48 kB, 2560x1920 - viewed 797 times.) « Last Edit: 2016-01-31, 13:21:11 by Peterae »