The Resonator

“Spikes” in the FID are asynchronous noise bursts at the observe frequency being generated somewhere in the NMR system.  There are several possible sources for this noise including arcing in the probe or sample coil.  A bad cable or poor quality cable that has recently been put into the system could cause the problem.  Devices external to the probe or sample coil can arc causing the noise.  Frequently we find that high power filters in-line before the Doty probe or coil are the source of troublesome noise spikes.  To isolate the cause it is a good idea to switch out parts to see if eliminating one part also eliminates the problem.  To check the Doty probe or coil for high level arcing an inline directional coupler, which will allow you to see the reflected pulse envelope on an oscilloscope, is very useful.  If the probe is arcing then you will see a sudden large jump in reflected power during the pulse at the point of initiation of the arc.  The probe can be the source of the noise even if there is no high level arc visible in the reflected pulse envelope.  This noise can be caused by a defective ground contact inside the probe and generate no other signs of arcing.  If you are convinced after checking the system that the probe is the source of the noise then the best thing to do is send it back to us for evaluation and repair.