The excitation mode is "continuous wave", as opposed to the more conventional pulsed mode. "In pulsed mode, a few microsecond pulse of the appropriate frequency excites the protons of the sample material, then one switches the RF coil to a receiver and measures the frequency of the response", explains Pascal Sommer. "In contrast, with continuous wave, the material is irradiated permanently. When the frequency reaches the resonant value, the response of the protons leads to a very small absoption of energy which is detected, filtered and amplified more than 3300x." Advantages of this system: the probe is greatly simplified as it no longer has a switch, and searching for the field value is easier. Needless to say, there is a price to pay … In order to be able to detect the nuclear magnetic resonance, the field must be modulated by means of a second coil! But rest assured, this is of no concern to the user, who can confidently read off the value displayed on the screen of his PT2025 or PT2026 Teslameter.
With the recent development of the PT2026 NMR teslameter, which has a digital frequency generator, Metrolab has taken things a step further by producing a new generation of probes (Model 1126). "This time, the probe excitation frequency is modulated directly, making the field modulation obsolete", explains Pascal Sommer. The field modulation may perturb certain types of experiments being performed in the magnet. That disadvantage vanishes with the frequency modulation technique. Moreover, the modulation coil is removed, allowing a smaller probe head. "The disadvantage of this approach is that the tuning of the excitation coil is much more complex. This is why the new probe is equipped with a built-in tuning system, needed to improve the sensitivity of the probe." Once again, the probe size stays the same.
At the same time, Metrolab is researching flowing water technology. This allows a single probe to measure a wide range of fields, including very low fields, with the high precision that characterize NMR measurements. "We are also currently exploring NMR pulsed probes", indicates Pascal Sommer. Finally, to measure weak fields (down to around 500 µT), a technique similar to NMR can be used, with a similar degree of accuracy: ESR (Electron Spin Resonance). Here it is the electrons that resonate, at a very high frequency, on the order of 28 GHz/T. Metrolab used to build this type of probe, but had to suspend production: there were no longer any suppliers proposing ESR material! But that's another story…
1060 probe range data sheet
1062 probe range data sheet
1080 probe range data sheet
1082 probe range data sheet
PT2025 data sheet
PT2026 data sheet