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README.md

Workspace

Here lies any code/digital work created during the project. This includes design files, measurement data, photographs and more. This is the main folder to be worked in.

Folder Description
nmr_probe Probe and Probe construction
marcos MaRCoS software and control of the spectrometer
32-channel_current_source Digitally controllable current source for driving the shims
tr_switch Transmit/Receive Switch for Switching between High-Power RF impulse transmission and low level FID signal reception
rx_lna_preamp Low-Noise Amplifier for the amplification of the FID signal for the RedPitaya
tx_power_amp Power Amplifier to amplify the excitation pulse of the RedPitaya

General Setup

+------------+                                            +--------------+
|            |     |\            |\                       | RX/TX Switch |
|            |     | \           | \                      |              |
|         TX |-----|  +----------|  +---------------------|---x  \       |         \ | /
|            |     | /           | /                      |       \      |          \|/
|            |     |/            |/                       |        \     |           |
|            |  ZFL-1000+      ZHL-3A+                    |         \    |           |
| Red Pitaya |                                            |          +---|-----------+
|            |                                            |              |
|            |       /|            /|           /|        |              |
|            |      / |           / |          / |        |              |
|         RX |-----+  |----------+  |---------+  |--------|---x          |
|            |      \ |           \ |          \ |        |              |
|            |       \|            \|           \|        |              |
|            |   ZFL-500LN+     PHA-13LN+   PHA-13LN+     |              |
+------------+                                            +--------------+

Previous NMR Work

  • Designing and building a low-cost portable FT-NMR spectrometer in 2019: A modern challenge (Alain Lous-Joseph, Philippe Lesot)
  • OpenCore NMR (Kazuyuki Takeda)
  • LabTools NMR spectrometer based on RedPitaya
  • Pavel Demin's NMR spectrometer
  • Simple Low-Cost Tabletop NMR for chemical-shift-resolution spectra measurements. Home-built magnet, 60Hz resolution at 2.45MHz (58.8mT), in a later paper down to 1ppm.

NMR Spectrometer Target Specifications

In addition to the thesis description/project proposal we tried to meet the following specifications. This includes design requirements for the physical setup partially repeated in the various sub-sections in the workspace directory. Hardware should be reusable. The end goal would be of type "Bring your own Magnet", i.e. a Red Pitaya SDRLab 122.88, our board and magnet should be enough to do spectroscopy, when building their own probes.

  • Larmor Frequency: 25MHz (0.6T)
  • RF-field strength: x.xx mT
  • pi/2 pulse length: x.xx us

TODO Write down NMR target specifications here

In the end, separate modules should be developed for ease of reconfiguration. A nice addition would be a single board, including RX/TX amplifiers, switch and filters on one standard Eurocard 3U (100mm x 160mm x 1.6mm) FR4 board, so it can be rack mounted, possibly with the common DIN 41612 (STEbus, VMEbus) connector at the back end. See also the ARTIQ/Sinara project.

Red Pitaya SDRlab 122-16 (STEMlab 122-16)

The ADC of the RedPitaya is specified for an RF input voltage of 0.5Vpp. Its absolute maximum rating is 30VDC and 1Vpp RF (~4dBm). This has been verified by looking at the used ADC and the RedPitaya Schematic. Thus the input has to be protected from the power of the transmit cycle.

The easiest way to do this is to use a TVS diode for clamping the input to safe levels. Unfortunately, this severely degrades the ADCs performance according to this Analog Devices Application Note. The same AN suggests using RB851Y Schottky diodes for ADC input protection

Basic Specs
Processor DUAL CORE ARM CORTEX A9
FPGA FPGA Xilinx Zynq 7020 SOC
RAM 512 MB (4 Gb)
System Memory Micro SD up to 32 GB
Console Connection Micro USB
Power Connector Micro USB
Power Consumption 5 V, 2 A max

RF Inputs
RF input channels 2
Sample rate 122.88 MS/s
ADC resolution 16 bit
Input impedance 50 Ohm
Full scale voltage range 0.5 Vpp/-2 dBm
Input coupling AC
Absolute max. Input voltage range DC max 50 V (AC-coupled) 1 Vpp for RF
Input ESD protection Yes
Overload protection DC voltage protection
Bandwidth 300 kHz - 550 MHz

RF Outputs
RF output channels 2
Sample rate 122.88 MS/s
DAC resolution 14 bit
Load impedance 50 Ohm
Voltage range 0.5 Vpp/ -2 dBm (50 Ohm load)
Short circut protection N/A, RF transformer & AC-coupled
Connector type SMA
Output slew rate N/A
Bandwidth 300 kHz - 60 MHz

Next Steps:

  1. Build 2x Protection diode blocks with 2x BYS-10 schottky diodes each for input protection (limit power to 4dBm/+-0.5V/1Vpp)