An important reason for the use of low frequency NMR pulsed spectrometers in applications to Petrophysical studies is that

A NMR probe for low frequency and short recovery time is presented in this work. The probe contains the tuning circuit, diode expanders and quarter wavelength networks to protect the receiver from both the amplifier noise and the coil ringing following the transmitter power pulse. It also possesses a coil damper which is activated by of non active components. The probe performance shows a recovery time of about of , a sensitive Q factor reduction and an increase of the signal to noise ratio of about 68% during the reception at a work frequency of . Introduction An important reason for the use of low frequency NMR pulsed spectrometers applied to petro-physics studies is that nuclear relaxation caused by fixed paramagnetic impurities, rising from the interaction between the nuclear spin and unpaired electron spins, is better detected al low fields. 1, 2 This feature is in conflict with several characteristic design of the spectrometer blocks, this is because the signal to noise ratio (ξ), the natural ringing time () and the quality factor (Q) of the probe coil have a frequency dependence that comes into play. This work describes both the design criteria design and the advantages of a relatively simple probe with a non active damping circuit for the above mentioned NMR studies. The probe, in a conventional NMR spectrometer, is connected to the power amplifier and to the receiver, in a T shape configuration, in such a way to allow the rf power to go into the probe and the nuclear signal input only to the preamplifier. These three blocks of the spectrometer, the transmitter, the probe and the receiver, are usually interconnected in the well known Lowe ⁄ arrangement. 4 The probe must be tuned in such a way that during the rf pulse its impedance should be adjusted to the output impedance of the transmitter to ensure the maximum power transfer (a). Simultaneously, the receiver should be isolated from the probe to protect it from large rf voltages (b). During the reception, the probe should be isolated from the transmitter output to avoid noise and the weak nuclear signal should be driven to the receiver input. Also the probe should have a short ringing (c) time after the rf pulses, and the signal to noise ratio of the NMR signal (d) should be as high as possible. Additionally, the interconnections between the three blocks depend also of the frequency and its variations generate instruments artifacts that adversely affect both the signal to noise ration and the Ann. Magn. Reson. Vol. 10, Issue 3, 28-36, 2011 AUREMN 29 data collection (e). Features (c) and (d) are interrelated since  depends of Q being