In the last decade or two, it seems that the trend of technological advance in NMR spectroscopy cannot follow the trend of desire to measure NMR samples with gradually lower response signals. Recently, an accurate noise model, based on the concept of noise figure, of the most sensitive part of the NMR spectroscopy system from the aspect of noise, which is its probe-to-spectrometer receiving chain, was introduced. The main purpose of this model is to optimize the used NMR spectroscopy system and, ultimately, enable measuring NMR samples with even lower response signals than the ones measured today. All the parameters of the NMR spectroscopy system, used in the introduced model, can be easily measured using vector network analyzers and noise figure meters, or can be found in the datasheets of the respective elements, except the spectrometer’s receiving chain noise figure. In this contribution, the process of spectrometer’s receiving chain noise figure measurement, performed using the Twice Power Method, is described. A block diagram representation of the spectrometer’s receiving chain is presented here, as well as its approximative model. The respective noise figure measurement results, which are also presented, explain the general tendency of using the mid-range of the spectrometer’s gain control level when performing the actual NMR measurements.

Ever since noise was spotted and proven to cause problems for the transmission and detection of information through a communication channel, a standard procedure in the process of characterizing a detection system of the communication channel is to determine the level of the lowest detectable signal. In signal processing, this is usually done by determining the so-called threshold signal-to-noise ratio (SNR). This determination is especially important for the communication channels and systems that constantly operate with low-level signals. A good example of such a system is definitely the NMR spectroscopy system. However, to the authors’ knowledge, the threshold SNR value of NMR spectroscopy systems has not been determined yet. That is why the experts in the field of NMR spectroscopy were asked to assess, using an online questionnaire, which SNR level they considered to be the NMR threshold SNR level. Afterwards, the threshold value was calculated from the obtained data. Finally, it was compared to the existing rule of thumb and thus, a conclusion about its legitimacy was made. The described questionnaire is still available online (https://forms.gle/Y9hyDZ1v1iJoEbk27). This enables everyone to form their own opinion about the threshold SNR level, which the authors encourage the readers to do.

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