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Wednesday, April 9, 2008

Electronics for Simple Single Frequency NMR Probes

For good reason, many NMR users find the electronics in an NMR probe daunting as they can be quite complicated, depending on the configuration of the probe (double resonance, triple resonance etc ...). The electronics in a simple single frequency NMR probe, on the other hand, are very simple and can be built on the bench in a matter of minutes. A single frequency probe can have the following tank circuit with only two capacitors and a coil. Of course, all of the components must be nonmagnetic.
CT is the tuning capacitor. It is used to tune the circuit to a specific frequency (i.e. from one nucleus to another). CM is the matching capacitor. It is used to alter the quality factor of the circuit. L is the coil in which the sample is placed. It may be a solenoid or Helmholtz coil. The inductance of the coil is an important property to consider in achieving the desired tuning range of the probe . The coil must be effective at both transmitting rf to the sample as well as detecting the small NMR signal from the sample.

9 comments:

Anonymous said...

Could you recommend a book that would help one calculate the correct range of tuning and matching capacitance for a given frequency and inductance?

Glenn Facey said...

Anonymous,

I have never calculated the values of capacitance myself. I used to do it by trial and error with a sweep generator and scope. The ranges will probably be 3 - 100 pF. Two books which you may find usefula are:

Fukushima and Roeder "Experimental Pulse NMR"

and

Malmstadt, Enke and Crouch " Electronics and Instrumentation for Scientists"

Glenn

Magdalena said...

I have sometimes seen a small coil connected in parallel with the coax. Do you have any idea what this is for? I have heard that it is a matching coil? How does that help circuit?

Magdalena

Glenn Facey said...

Magdalena,

I'm not sure what you mean by a coil being in parallel with the coax. I have certainly seen coils in parallel with capacitors as tuning elements.

Glenn

Magdalena said...

Hi,

I am not sure what it is called, but I have seen coils (ranging from 1 to 5 turns and various sizes) connected in parallel with the matching capacitor (I mispoke earlier). It is supposed to improve Q - factor? I am just not sure how. Any thoughts?

Magdalena

Glenn Facey said...

Magdalena,

Yes, these coils can be used to change the Q of the probe or even the tuning range. You can think of the coil (an inductor) as an element of the circuit. If you change the number of turns or shape of the coil you will affect both the matching and tuning of the circuit. The diagram in this BLOG post is only meant to represent the very simplest of probe circuits.

Glenn

Anonymous said...

I have seen a probe with capacitor attached between the ground and the coil to raise the frequency. Does it have particular disadvantages or side effects ?

Glenn Facey said...

Anonymous,

There are many different configurations of probe circuits and I am certainly not an electronics expert so cannot provide you with an answer to your question.

Glenn

Anonymous said...

There are many different resonators considered as an NMR circuits. At low frequency NMR as long as you need quit high inductance in order to optimize the resonance I prefer to use the balanced capacitive network [(Ground---CT/2---L/2---L/2---CT/2---Ground ) === CM---Signal )] where you can divide the inductance and the tuning capacitor in 2 halves. One has to calculate the values of inductance L/2 and the CT/2 before building the circuit in order to be able to set the voltage across the two half tuning capacitors equal in amplitude and opposite in phase, one of the tuning capacitors should be trimmer (tunable), but of course the values have to be calculated in advance, I am using my self-programmed software to calculate (simulate) the values for any given frequency, but one can use a simple software called mini ring core calculator, be sure you use R-L-C analyzer to watch the voltage phase and the quality factor of the circuit (phase difference less than 155 degree is reduce QF a lot). Use high quality capacitors with reasonable tolerance, avoid the dielectric capacitors, ceramic capacitors are preferable in order to overcome the poor SN ratio at low frequency NMR.
At high freq. NMR one can use single inductance circuits but double have matching capacitors in balance with the voltage amplitude and the opposition in the phase.
I am preparing publication in this work while I have developed NMR resonator at permanent NMR magnets (low frequencies).
Write me for any further questions

AHMAD