Application of a statically configured FPGA in the digital control system of the NQR radio spectrometer

In the case of the development of nuclear quadrupole resonance (NQR) surveillance equipment and relaxation processes in solids, it is often necessary to provide for the flexibility of a measuring system: the introduction of new multi-pulsed research techniques, the adaptation of the parameters of synthesized signals for the investigation of new cores, the implementation of a sensitive receiving path with the possibility of digital accumulation and filtration of free induction recession signals, etc. [1,2].
The purpose of this work is to create a digital control system for a NQR spectrometer [3] with the possibility of operatively changing its configuration, using field-programmable gate array (FPGA) and the syntax of dynamic modes of logical structures. In order to provide operative control over the course of the experiment and the choice of its initial conditions, a portable NQR radio spectrometer requires the presence of a flexible control system.
For the development, a low-cost platform Altera NIOS-EVALKIT-1C12 was selected, the important advantage of which is the availability of hardware necessary for implementation of the above functional tasks. The platform comprises EP1C12F324 FPGA of the Cyclone family, a 16 MB synchronous dynamic RAM memory chip, a 8 MB Flash-ROM chip, a 24 MHz clock generator, 48 I / O ports, 3.3 V and 5.0 V power supplies.
For the implementation of the algorithm a finite state machine has been synthesized, the alphabet of output sequences of which Y = {y1, y2[4:0], y3} is determined by a plurality of machine states S = {s0, s1, s2, ..., s32}, and a plurality of input characters is given by alphabet X = {x1, x2, ..., x5M}. The process continues until the state s32 = f(s2Ús3Ús4Ús5Ús6,x10) is reached. This sequence of transitions forms the source alphabet. The character y1 = g(s0) corresponds to transition from s0 to s1. A plurality of characters y2[4:0] Î Y corresponds to transitions between s1 and s31. The character y32 = g(s2Ús3Ús4Ús5Ús6) corresponds to transition of machine to state s32. Fig.1 in the form of oriented multigraph shows a fragment of diagram of machine states (transient states are not signed).

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Figure 1. Diagram of states of the synthesized finite state machine.

Configuration structure of FPGA (Fig. 2) was developed in the environment for design of projects in Altera CAD – Quartus II Web Edition, using the method of complex graphic-syntactic programming. In this structure, the main module “core”, based on the finite state machine and additional submodules of combinational (lpm_mult, lpm_decode, lpm_compare, lpm_mux) and sequential (dff, latch, lpm_ff, lpm_counter) logic, in conformity with the output data from the keyboard initialization module “key_ini”, ensures performance of functions set by program algorithm and output of control signals: Data_State[4..0]; Init_Enable; Logo/Main; Main/Screen; Digits (Pulse1-5, DDS, RF, SYS), necessary for operation of other structure modules.

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Figure 2. Configuration structure of the FPGA in the window of the Quartus II.

Recording data in registers is done according to the code on the address bus. Thus, the system of 36 separate 4-bit LPM_DFF modules with a total memory capacity of 148 bits allows you to save the settings of a number of parameters: DDS frequency, duration of the excitation pulse and transient process, pause between pulses, sequence type, transmitter power, transmission and amplification of NQR spectrometer radio channel. The synchronization is provided by the FPGA-integrated PLL system and dividers “div”, forming the frequencies: 24 MHz - for the core; 100 MHz - for graphical subsystem; 6.25 MHz - for display.
Structurally, the radio spectrometer control system is executed in the form of a modular structure, which includes the core board, LCD TD035STEB2 and I/O ports. Control of the temperature in the radio spectrometer measuring cell and transmitter heating is provided by two digital DS18B20 temperature sensors connected to IC through the 1-Wire Interface. The working model of the developed control system was tested along with the shaper of pulsed sequences of the NQR radio spectrometer [4]. The results of the model testing showed that its functionality corresponds to all the requirements for the portable equipment of relaxation and pulse-resonance spectroscopy.

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Figure 3. Modular structure of the developed control system of the NQR radio spectrometer.

CONCLUSIONS

The digital multifunctional control system for pulsed NQR Fourier radio spectrometer of laboratory type is developed, the main hardware-software methods of which are realized using FPGA with a static configuration EP1C12F324. The basis for the algorithm of IC configuration is the synthesized finite state machine, the alphabet of the output sequences Y of which is determined by the plurality of machine states S, and the plurality of input characters is given by the alphabet X. Transition functions f: S´X®S and output functions g: S®Y of finite state machine are described in VHDL language.
Experimental tests of the working model in combination with frequency synthesizer and shaper of pulse sequences of the NQR radio spectrometer have been carried out, which confirmed the functionality of the development and its compatibility with the existing standards of NQR spectroscopy equipment.

REFERENCES
[1] Weinan, Tang, Weimin, Wang, "A single-board NMR spectrometer based on a software defined radio architecture," Measurement Science and Technology 22, 0159021-8 (2011).
[2] Kazuyuki, Takeda, "Opencore NMR: Open-source core modules for implementing an integrated FPGA-based NMR spectrometer," Journal of Magnetic Resonance 192, 218-229 (2008).
[3] Khandozhko, Alexander, Khandozhko, Victor, Samila, Andriy, "A pulse coherent NQR spectrometer with effective transient suppression," Eastern-European journal of enterprise technologies 6 (12), 21-25 (2013).
[4] Samila, Andriy, Bobalo, Yuriy, Hotra, Zenon, Hotra, Oleksandra, Politans’kyy, Leonid, "Implementation of pulsed radiospectroscopy methods of NQR based on FPGA," Metrol. Meas. Syst. 22(3), 363-370 (2015).