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What may happen if probe tuning/matching is not perfect besides loss in the sensitivity, any more subtle details? Whould it ever make sense to have probe not most perfectly tuned/matched for any reason? Thx! |
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The 90 degree pulse lengths will also increase. The voltage standing wave ratio (VSWR) on the transmission line to the probe will increase, and may cause arcing problems in the probe, or any components between the power amplifier and the probe - mostly in high power situations at extreme mis-match. The amplifiers themselves are usually protected from high VSWR. Items such as pre-amps and filters are also designed to operate at 50 + 0i Ohms, and may have poorer performance in a mis-tuned situation. For liquids samples that can't achieve a perfect match (e.g. salty solutions, usually only 1H)) you can usually just calibrate the pulses at the best match that you can achieve and the resulting loss in performance is not that severe. One situation that you may want to de-tune a probe would be if you are battling radiation damping problems. Radiation damping can broaden the lines on very strong signals. Usually, diluting the sample is a better solution, and for radiation damping of the solvent (e.g. H2O) there are pulse sequences that overcome or work-around the radiation damping. |
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The circuit match is the transfer of power to and from the probe coil. Any mismatch results in reflected power, in both transmit and reception. The measured change in pulse length, due to mismatch, is the same as a perfectly matched probe run at the lower power. If half the power is reflected the pulse length with increase by the square root of two. If the probe is matched, and the power halved, the pulse length with increase by the square root of two. Signal-to-noise, in a given sample, is little changed by circuit match, as the noise is reflected the same amount as the signal. Now I can think of lots of stuff to put in a sample so the probe no longer matches. Most of that stuff is like soldering a big resistor in the circuit. A sample can cause a big enough change that the probe no longer matches. That will likely dissipate power, increasing pulse lengths and reduce signal getting to the coil from the sample. |
