Mazin Jouda, Erwin Fuhrer, Pedro Silva, Jan G. Korvink, and Neil MacKinnon
Karlsruhe Institute of Technology
For an efficient NMR acquisition, an NMR spectrometer usually offers the option of automatically adjusting the receiver gain. The adjustment in this case is driven by the maximum amplitude of the FID, and once the receiver gain is set to a certain value, then it cannot be changed during the acquisition. But since the NMR signal is decaying by nature, only the first few points of the FID will be acquired with the full resolution of the analog-to-digital converter (ADC), whereas the rest of the FID will be acquired with a constantly decreasing resolution.
In this contribution we describe an innovative closed-loop control mechanism to automatically adapt the receiver gain over the acquisition time such that all of the FID points are digitized with the full ADC resolution . A high performance digital lock-in amplifier was employed to extract the envelope of the FID, process it, and feed it back to control a variable-gain amplifier (VGA) inserted in the signal chain. As the FID decays the VGA gain and thus the overall receiver gain increases,even if the FID increases again due to beating for example, the VGA gain is adapted ensuring that no ADC overflow occurs.Additionally, the reported method includes a mechanism to precisely monitor the gain change over time. This is carried out via a pilot signal that propagates together with the NMR signal through the VGA. Knowledge of the gain modulation is of course essential to recover the NMR signal after the ADC stage.
The method, which we call AGAIN (Automatic Adaptive Gain), was verified experimentally. The results showed an SNR enhancement factor of 2.64 when AGAIN was applied to a custom spectrometer, while a maximum SNR enhancement factor of 1.45 was observed when applied to a commercial spectrometer. The AGAIN technique requires minimal additional hardware, which makes it general and thus can be readily combined with any commercial spectrometer.
 M. Jouda, E. Fuhrer, P. Silva, J. G. Korvink, and N. MacKinnon. “Automatic adaptive gain for magnetic resonancesensitivity enhancement”. In:Analytical Chemistry (2019).