Pulsed NMR system

This is a work in progress...

Interesting links

Some interesting links on pulsed nuclear magnetic resonance:

• Pulsed NMR at UW

• Pulsed NMR at MSU

• The Basics of NMR by Joseph P. Hornak

Short description

The system consists of one in-phase/quadrature (I/Q) digital down-converter (DDC) and of one pulse generator.

The tunable frequency range covers from 0 Hz to 60 MHz.

The I/Q data rate is configurable and six settings are available: 25, 50, 125, 250, 500, 1250 kSPS.

Hardware

The basic blocks of the system are shown in the following diagram:

The projects/pulsed_nmr directory contains three Tcl files: block_design.tcl, rx.tcl, tx.tcl. The code in these files instantiates, configures and interconnects all the needed IP cores.

The source code of the R script used to calculate the coefficients of the FIR filter can be found in projects/pulsed_nmr/filters/fir_0.r.

RF and GPIO connections

• two digital down-converters (DDC) are connected to IN1 and IN2
• pulse generator is connected to OUT1
• continuous cosine signal of the receiver's DDS is connected to OUT2
• digital output for RX/TX switch control is connected to pin DIO0_N of the extension connector E1
• general purpose digital outputs are connected to the pins DIO0_P - DIO7_P of the extension connector E1

Software

The projects/pulsed_nmr/server directory contains the source code of the TCP server (pulsed-nmr.c) that receives control commands and transmits the I/Q data streams (up to 4 x 32 bit x 1250 kSPS = 152 Mbit/s) to the control program running on a remote PC.

The projects/pulsed_nmr/client directory contains the source code of the control program (pulsed_nmr.py).

.NET library

The .NET library provides a minimalist set of commands for controlling all parameters of the signal processing modules and programming the pulse generator. The commands can be classified into five groups:

Commands to manage the connection with the TCP server:

• Connect(address)
• Disconnect()

Commands to set the frequencies and decimation rate:

• SetFreqRX(frequency)
• SetFreqTX(frequency)
• SetRateRX(rate)

Commands to program a pulse sequence:

• ClearPulses()
• AddDelay(gate, duration)
• AddPulse(level, phase, duration)

Command to start pulse sequence and receive data:

• RecieveData(size)

Various commands to control output RF signal level, GPIO pins and an external DAC:

• SetLevelTX(level)
• SetPin(pin)
• ClearPin(pin)
• SetDAC(data)

The source code of the .NET library can be found in projects/pulsed_nmr/client/PulsedNMR.cs.

Getting started with GNU/Linux

• Download SD card image zip file (more details about the SD card image can be found at this link).
• Copy the contents of the SD card image zip file to a micro SD card.
• Optionally, to start the application automatically at boot time, copy its start.sh file from apps/pulsed_nmr to the topmost directory on the SD card.
• Install the micro SD card in the Red Pitaya board and connect the power.
• Install required Python libraries:

sudo apt-get install python3-numpy python3-matplotlib python3-pyqt6

• Clone the source code repository:

git clone https://github.com/pavel-demin/red-pitaya-notes

• Run the control program:

cd red-pitaya-notes/projects/pulsed_nmr/client
python3 pulsed_nmr.py

Building from source

The installation of the development machine is described at this link.

The structure of the source code and of the development chain is described at this link.

Setting up the Vitis and Vivado environment:

source /opt/Xilinx/2025.1/Vitis/settings64.sh

Cloning the source code repository:

git clone https://github.com/pavel-demin/red-pitaya-notes
cd red-pitaya-notes

Building pulsed_nmr.bit:

make NAME=pulsed_nmr bit

Building SD card image zip file:

source helpers/build-all.sh