Impedance: What it is, and why it must be matched

All probes in NMR spectrometers must be “tuned” and “matched” to obtain optimum sensitivity. In the simplest electronic circuits, tuning and matching are accomplished by adjusting variable capacitors. These adjustments determine the impedance of the circuit; i.e., the extent to which the circuit opposes, or impedes alternating‐current flow. This impedance is the vector sum of the individual oppositions owing to the presence of resistors, inductors and capacitors in the circuit. When the impedance of the probe circuitry is equal to (matches) that of the high‐power, pulsed, radio‐frequency transmitter, power is efficiently transferred to the probe and pulse‐widths are minimized. Similarly, when the impedance of the probe circuitry matches that of the small‐signal preamplifier, the detected signal power is efficiently transferred to the amplifier and the signal‐to‐noise ratio is maximized.