The probe  is designed to hold the sample, transmit radio frequency signals which excite the sample and receive the emitted response. The transmission and reception is achieved by using specially designed RF coils .

The probe is inserted into the bottom of the magnet and sits inside the room temperature shims. Coaxial cables carry the excitation signals from the console amplifiers to the probe and the NMR signal back from the sample to the receiver. The cables are routed through a set of preamplifiers (the HPPR) which are situated next to the base of the magnet or on NanoBay systems are built in to the console. The  preamplifiers are needed to boost the NMR signals which are typically very weak.

 

Probes come in different sizes and types. The size of the probe is given in terms of the sample tube sizes it can hold, with 5mm and 10mm sample tube diameters the most popular. Different types of probes are used depending upon the type of experiment.  Selective  probes are specially designed to observe specific nuclei, e.g. 13C, while multinuclear (X-BB or broadband) probes may be used to analyze a wide range of nuclei. The number and design of the internal coils are what physically distinguishes one type of probe from another. In addition, the outer diameter and length of the probe is built to the specifications of the various magnets (wide bore or standard bore; different length from the bottom to the center of the magnetic field for magnets with different field strength).

Signals enter and leave the coils of the probe via connectors which are clearly labeled and located at the base of the probe. The same cable is used to carry the signal to and from the probe. Each probe has an inner coil (the  observe  coil). This coil is located closest to the sample volume to maximize sensitivity. The color coding of the inner coil BNC follows a simple rule. It always has the same color as the rectangular strip located directly above the BNC connectors. The following figure shows the labeling of a multinuclear probe. In this case the  broadband  coil is the inner coil.