Assurance of high measuring sensitivity is one of the most challenging issues for any nuclear magnetic resonance (NMR) spectroscopy system. To this end, we propose an accurate noise model of the entire probe-to-spectrometer receiving chain for condensed matter physics, based on the concept of noise figure. The model predicts the propagation of both the signal and noise levels in every component of the NMR spectroscopy receiving chain. Furthermore, it enables identification of the "weakest" component and, therefore, the optimization of the whole system. The most important property of the proposed model is the possibility to find system parameters that reduce the measurement time by an a priori calculation, rather than an a posteriori approach. The model was tested experimentally on several different samples. It was found that the measurement time can still be significantly shortened, down to at least one half of the measurement time, starting from optimized conditions with commercially available components. Thus, the proposed model can be used as a tool for both quantitative analysis of the noise properties and a sensitivity prediction of practical NMR systems in physics and material science.

https://www.mdpi.com/1424-8220/19/14/3064/pdf

Sensitivity enhancement of NMR spectroscopy receiving chain used in condensed matter physics

An analog-to-digital converter based on the time-interleaved delta-sigma modulator is a proper method for high-speed ADCs. Time-interleaved delta-sigma modulators (TIDSM) can be successfully implemented with the development of the block digital filtering (BDF) technique. In this method, M mutually cross-connection delta-sigma modulators are used, with each one operating at a sampling rate of $${\text{f}}_{\text{s}}$$ hence, the effective sampling rate will be $${\text{M}}*{\text{f}}_{\text{s}}$$ However, SNDR is approximately equal to the single path standard structure. In this paper, a new structure based on the Noise Coupled time-interleaved delta-sigma modulator is proposed to increase the overall noise transfer function order without any additional active element. This improvement is analytically verified and then validated using simulations. Also, some practical issues regarding the implementation of the proposed structure, such as, finite op-amp’s gain and mismatching effects are discussed. Also, analyzes and some practical solutions are presented. The results of the simulation at the system level show that the SNDR of the proposed first-order two-channel structure is 18 dB better than its BDF technique counterpart, for the second-order two-channel TIDSM, the SNDR of the proposed structure is 13 dB better than that of the BDF technique.

A novel noise-coupled time-interleaved delta-sigma modulator with analysis of practical limitations

In order to meet the requirements for zero value stability of direct sequence spread spectrum (DSSS) signal processing in high dynamic scenario, digital automatic gain control (AGC) is employed to regulate power. However, conventional AGC causes degradation in the synchronization performance of DSSS receiver. Based on the theoretical analysis of the influence of digital AGC on DSSS signal synchronization, this paper proposes a new AGC algorithm, which is applicable to multi-channel digital DSSS signal receiver. By making power adjustment cycle and synchronization cycle coherent with each other adaptively, the influence of digital AGC on subsequent synchronization processing has been eliminated. Theoretical analysis, simulation results and experimental data verify the validity of the proposed algorithm. By virtue of the proposed algorithm, the influence of digital AGC on DSSS signal synchronization is eliminated. This algorithm applies to an aerospace engineering project successfully.

Digital AGC based on coherent adjustment cycle for DSSS receiver

The increasing role of digital radio receiving techniques and various adaptation techniques in modern radio systems has led to the appearance of variety of automatic gain control (AGC) systems used as systems of adaptation. AGC types classification and its development tendencies are considered. The paper investigates the influence of the value of the reference level of wideband digital radio receiver digital AGC on its behavior under multi-signal impact. The amplitude of the input signal effect on the value of the signal constant component is considered. The variant of AGC system design is suggested. Conclusions about the quantitative parameters and permissible changes of AGC reference level with an aim to increase performance speed are made.

Modeling of digital AGC with multi-signal impact and adaptation of the reference level

In this work we are demonstrating a novel fast settling coarse gain tuned Intermediate Frequency (IF) digital feedback Automatic Gain Control (AGC) for Bluetooth Low Energy(BLE). Feedback architecture of AGC is designed to control the gain of 4 stage VGA by using digital control signals, with each of 15 dB gain factor. This ensures to prohibit the deviation of Analog signal through the Analog to Digital converter (ADC) from its efficient dynamic range. The proposed AGC has dynamic of 60 dB with an attack time of 52.94 ns. The AGC algorithm is validated by considering different attenuation factor on incoming signal. The Feedback AGC algorithm is simulated on MATLAB-Simulink Platform by incorporating channel impairments.

Design and Evaluation of coarse tuned Feedback DAGC for Bluetooth Low Energy Receivers

A new digital Automatic gain control (AGC) circuit with two-stage gain adjustment is proposed in this paper. This AGC circuit is designed as part of an intermediate frequency (IF) processing circuit in the spectrum analyzer. The first stage controls the gain in 20 dB Steps and improves the measurement range of the RMS detector, while the second stage controls the gain in 3 dB Steps and gets a precise output signal. The gain of the two stages of amplifier is coordinated by a gain control logical module. Simulations for the control algorithm are implemented; the results show this control algorithm has some advantages in aspects of rapidity and astringency. Due to the large available gain provided by two-stage gain adjustment, a large controllable input range can be obtained. Experimental results show that the detectable range of this AGC circuit is − 105 dBm to 18 dBm at a 60 MHz IF frequency.

Implementation of a two-stage digital AGC for spectrum analyzer

This paper presents a novel Delta Sigma Modulator (DSM) in form of MASH 2-1-2, which can suppress the spurious tones in spectrum for all constant inputs with error feedback as dither. As a finite-state machine (FSM), DSM always outputs a periodic sequence (or cycle) with a constant input. However, the novel DSM was designed to break up the periodic cycles and suppress quantization noise occurred in the fractional-N frequency synthesizer. Starting from the linear model of DSM, the principle of noise shaping was analyzed and logic implementation is introduced in detail. The performance is confirmed mathematically, by simulation, and experimental results.

Zhi Gang Wang

Papers

Implementation of a two-stage digital AGC for spectrum analyzer

Design of a Delta-Sigma Modulator structured in MASH 2-1-2 with dither of error feedback