Alternatives to creeping contacts for high frequencies

[Baluncore]

Your diagram in post #26 shows the capacitors and inductor that form the resonator. The capacitors Cm and Ct are continuously adjustable so the probe antenna, L, can be matched to Zo of the transmission line. You have just confirmed that “Yes, the sample is inside the coil and it moves together with the coil”. But you seem to be wanting many pre-tuned CCL resonators along the magnetic gradient axis. That would require transfer of the sample through all those resonator inductors as you move the sample along that axis, which seems somehow illogical to me. What have I misunderstood or got wrong?

 

[Tom.G]

That would require transfer of the sample through all those resonator inductors as you move the sample along that axis, which seems somehow illogical to me. What have I misunderstood or got wrong?

I think the OP is trying to vary the tuning of ONE coil (which contains the sample) depending on its physical position relative to the magnetic field. Sounds do-able depending on the required tuning bandwidth.

@_maxim_ , can you tell us the frequency range of the exciting pulses and of the received signal? And, what is the frequency correlation, if any, between the exciting and received signals.

For instance if the exciting frequency is 200Mhz and 1GHz is expected back, the architecture would be different than if the frequencies were identical.
Also do you:

• Require continuously adjustable tuning or are discrete frequencies acceptable?
• Need rejection or suppression of nearby frequencies? If so, how close and how much suppresion?

Cheers,
Tom

 

[_maxim_]

Dear Baluncore

Your diagram in post #26 shows the capacitors and inductor that form the resonator. The capacitors Cm and Ct are continuously adjustable so the probe antenna, L, can be matched to Zo of the transmission line. You have just confirmed that “Yes, the sample is inside the coil and it moves together with the coil”. But you seem to be wanting many pre-tuned CCL resonators along the magnetic gradient axis. That would require transfer of the sample through all those resonator inductors as you move the sample along that axis, which seems somehow illogical to me. What have I misunderstood or got wrong?

The diagram in post #26 is a very basic thank circuit.
If the coil L does not move in the magnetic field (with its sample inside), only one thank RF circuit is required.
If the coil L moves from one position to another experiencing a different magnetic field, as in my case, a different RF thank circuit for is required.
The sample remains inside the coil all the time.

I think the OP is trying to vary the tuning of ONE coil (which contains the sample) depending on its physical position relative to the magnetic field. Sounds do-able depending on the required tuning bandwidth.

Hi Tom,

Yes, it is exactly this.

can you tell us the frequency range of the exciting pulses and of the received signal? And, what is the frequency correlation, if any, between the exciting and received signals.

Exciting pulse and receiving frequency are exactly the same since the sample will emits a tiny wave at the same frequency of the transmitting pulse: that's why it is called Resonance.

For instance if the exciting frequency is 200Mhz and 1GHz is expected back, the architecture would be different than if the frequencies were identical.

In a magnetic field of 1.75 Tesla the frequency for 1H isotope is 75 MHz, at 3.52 Tesla is 150 MHz, ..., at 14.09 Tesla is 600.13 MHz and so on. The highest field I can handle is 19.97 Tesla for a maximum frequency of about 850 MHz, with transmitting and receiving identical frequencies for the same field.

Also do you:

• Require continuously adjustable tuning or are discrete frequencies acceptable?
• Need rejection or suppression of nearby frequencies? If so, how close and how much suppresion?

Cheers,
Tom

Discrete frequencies are fine, the final tuning of caps (maybe through small variable capacities) can be done directly when the sample is inserted in laboratory outiside the magnetic field, since the dielectric will change a bit depending of the nature of the sample observed.
About rejection, I would expect that there is no interference from one frequency to another due their relative distance, let say 800 MHz, 500 MHz, 300 MHz and 50 MHz.
The point is how to switch electrically and mechanically from one thank circuit to another.

Thank to all for your interesting investigation.

 

[Baluncore]

If the coil L moves from one position to another experiencing a different magnetic field, as in my case, a different RF thank circuit for is required.
The sample remains inside the coil all the time.

The magnetic field for NMR needs to be strong, flat and stable throughout the sample volume so as to give clean sharp spectra. The reason for having a gradient field in MRI is to identify the positional source of sample resonance along an axis by resonant frequency, which makes 3D imaging possible.

With a single sample, if you have a steep magnetic gradient, your spectra will be blurred by the size of your sample. With a gentle magnetic gradient, there can be no advantage in moving the sample as it will only change the resonant frequency slightly.

So I still cannot see why you need to measure the same small sample at different magnetic field strengths between the poles of the same magnet, as all it does is upset your matching network for no additional information.

What exactly are you trying to achieve?

 

[Tom.G]

That is going to be... CHALLENGING. I'm sure there are a few RF experts around here with better ideas, but here is a semi-informed first pass.
Overall it looks like separate transmit and receive coils are a good bet. I'll be concentrating on the receive part.

At 800MHz a coil of 22nH and a capacitor of 1.8pF will resonant. To give an idea of size, a 1 turn coil, 3/8in. dia., of 40AWG 30AWG (0.010 dia) wire yields 22nH. Keeping stray capacitance below 1.8pF requires rather small circuit construction. That would indicate putting a pre-amp directly adjacent to the coil. A Mini-Circuits LHA-13LN+ is a possibility for a pre-amp https://www.minicircuits.com/WebStore/dashboard.html?model=LHA-13LN+.

Since you are talking tens of watts, the pre-amp will need input protection. A pair of PIN RF-Microwave diodes in anti-parallel across the input may suffice. Here is a link to one source of PIN diodes. https://www.microsemi.com/document-...icrowave-diode-and-transistor-product-catalog

For higher power and isolation, another design for diode switching is a bridge circuit. As used in a fullwave bridge rectifier circuit, but the RF comes in and exits where the power transformer would be, and the control voltage is applied at what would normally be the output. When the control voltage biases the diodes ON there is an RF path thru the bridge. At higher powers, the control voltage polarity can be reversed to ensure the diodes stay OFF.

The pre-amp would be wideband with the receive tuned circuits at the RF receiver, away from the head. Tuned circuit selection could be done using a diode switching approach. For examples see:
https://www.radio-electronics.com/info/circuits/diode-rf-attenuator/pin-diode-switch.php
https://www.electroschematics.com/3002/pin-diode-rf-switch/

The Transmit is "Left as an exercise for others."
edit: corrected wire size
Hope this helps.
Tom

 

[_maxim_]

Thank you Tom,
I need some time to read the documentation and to start with some trials.

 

[Electron Spin]

That is going to be... CHALLENGING. I'm sure there are a few RF experts around here with better ideas, but here is a semi-informed first pass.
Tom

Tom, that was a good first pass, I would like to see your second pass!

Thank you Tom,
I need some time to read the documentation and to start with some trials.

Maxim,

I still am a bit confused on the operating freq. Is it 18 Mhz or 850?
I could break out my EE handbook, ARRL reference book, and other relevant materials and have a go at your project.
Without knowing the prioritized technical specifics such as the optimal Q, the bandpass, input/output Z, power gain, noise limits, architectural requirements such as passive or active components, costs, specific application, environmental requirements etc. it's kinda tough on this old man.

One idea that comes to mind is that if you have access to a nice network analyzer you can kinda' build as you go as long.

Ironically I was on a design team to build a NAMPS system for a foreign military and my job was to design, build 1st articles and then order all parts and supervise the pick and place operations for a "smart 14 input (antennas), 2 output switch for the system operating around the 0.35 meter band.

Bottom line is that I was doing pretty much what you are trying to do (I think) and the HP network analyzer/ plotter pretty much saved the project from cost overruns for obvious reasons.

Anyway, maybe you could beg real hard at a university or a company like Celwave if indeed a network analyzer would help with S11, S12, S21, and S22 plus BP parameters, Z parameters plus VSWR requirements and overall build performance.

If I was still working and someone came to my office w/ a laid out plan of action I would not hesitate to let that someone drive my HP NA w/ 3 color plotter to boot! :)

Best of luck,
Alex

 

[Tom.G]

Tom, that was a good first pass, I would like to see your second pass!

Thanks!
As my post indicated, I was trying to get some direction in this thread. Your experience, having actually built a similar device, greatly exceeds mine. The only other idea I come up with is if this is a one-off item, then using/modifying a manual tuner from a very old TV set could save much time... it might even be useful for implementatin approaches.

Looking forward to how this plays out.
Tom