Next-generation EPR spectrometer from Spectrum Instrumentations

17th April 2024
Harry Fowle

Digitisers and AWGs by Spectrum Instrumentation are at the heart of the new spectrometer instruments.

EPR spectroscopy has evolved over the past few decades thanks to advances in radar technology and the mobile telecommunications that created equipment that could be used to build EPR hardware which uses microwave technology that needs increasingly higher frequencies for better resolution. Historically, EPR spectrometers have operated at 10 GHz (X-Band), but thanks to 5G technology, there is now new commercial equipment that can operate at much higher frequencies such as 35 GHz (Q band), a frequency that is much more desired for this type of application.

The AWG generates 10 – 100 ns long pulses in the 200 to 500 MHz range as required by the experiment that are then first up-converted to X band range using an RF I/Q mixer and then up-converted to the Q band range. The microwave pulses are then fed into 100 W solid-state amplifier before sent to the EPR resonator. The reflected signal is then down-converted to an IF frequency in the 200 to 500 MHz range and sent to the digitiser. In EPR spectroscopy, the signal is traditionally down-converted to DC, however this new approach drastically reduces noise and artifacts.

An example of the type of AWG generated pulses used in a modern EPR experiment is shown in figure 2. WURST (Wideband, Uniform Rate, Smooth Truncation) pulses are broadband microwave pulses with an excitation bandwidth and profile that exceeds that of a simple rectangular pulse by far. Pulses like this allow for broadband excitation in EPR spectroscopy and heavily rely on the performance of the AWG.

Intense magnetic fields with much lighter magnets 

The other factor is being able to generate intense magnetic fields in the order of 1 to 1.5 Tesla which traditionally requires a huge, heavy electromagnet. Maly explained, “We are using a much smaller, super-conducting magnet to produce the required magnetic field strength. For this experiment, the sample always needs to be cooled to cryogenic temperatures using helium so we found a supplier of liquid cryogen-free (dry) magnets that are compact and can generate the high magnetic field of 1.2 Tesla but at a fraction of the size and weight at around 130 kg. Rather than using liquid helium, the instrument uses a cold-head and helium compressor, which is effectively a fridge achieving cryogenic temperatures in a closed cycle. This is crucial as liquid helium is becoming increasingly hard to obtain.” 

Maly concluded, “Spectrum’s 5-year warranty gives us great piece of mind as these cards are key parts of our EPR spectrometer. The technical support was also first class in helping us set up the equipment so we know we can rely on Spectrum if any issues occur for our customers in the future.”

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