Digitally generated excitation and near-baseband quadrature detection of rapid scan EPR signals

Abstract

The use of multiple synchronized outputs from an AWG provides the opportunity to perform EPR experiments differently than by conventional EPR. We report a method for reconstructing the quadrature EPR spectrum from periodic signals that are generated with sinusoidal magnetic field modulation such as continuous wave (CW), multiharmonic, or rapid scan experiments. The signal is down-converted to an intermediate frequency (IF) that is less than the field scan or field modulation frequency and then digitized in a single channel. This method permits use of a high-pass analog filter before digitization to remove the strong non-EPR signal at the IF, that might otherwise overwhelm the digitizer. The IF is the difference between two synchronized X-band outputs from a Tektronix AWG 70002A arbitrary waveform generator (AWG), one of which is for excitation and the other is the reference for down-conversion. To permit signal averaging, timing was selected to give an exact integer number of full cycles for each frequency. In the experiments reported here the IF was 5 kHz and the scan frequency was 40 kHz. To produce sinusoidal rapid scans with a scan frequency eight times IF, a third synchronized output generated a square wave that was converted to a sine wave. The timing of the data acquisition with a Bruker SpecJet II was synchronized by an external clock signal from the AWG. The baseband quadrature signal in the frequency domain was reconstructed. This approach has the advantages that (i) the non-EPR response at the carrier frequency is eliminated, (ii) both real and imaginary EPR signals are reconstructed from a single physical channel to produce an ideal quadrature signal, and (iii) signal bandwidth does not increase relative to baseband detection. Spectra were obtained by deconvolution of the reconstructed signals for solid BDPA (1,3-bisdiphenylene-2-phenylallyl) in air, 0.2 mM trityl OX63 in water, 15N perdeuterated tempone, and a nitroxide with a 0.5 G partially-resolved proton hyperfine splitting.

Rapid-scan EPR imaging

“…The incorporation of rapid scan into increasingly digital spectrometers is promising [59]. As rapid-scan technology has evolved to solve problems, it looks increasingly that rapid scan could replace the CW EPR that has been the standard for seventy years.…”

Field-stepped direct detection electron paramagnetic resonance

“…In rapid-scan electron paramagnetic resonance (EPR) the magnetic field is scanned through resonance in a time that is short relative to the electron spin relaxation times [1–3]. The directly detected quadrature signal is obtained using a double-balanced mixer with the reference at the resonance frequency.…”

EPR Everywhere

“…It would be 56 years later that the Eaton laboratory at the University of Denver would characterize the oscillations of LiPc and the Nycomed trityl in the first rapid-scan EPR experiments at 250 MHz [ 68 ]. Similar characterizations were performed in their laboratory at X-band before employing an entirely digital RS-EPR system with an AWG as the microwave source [ 132 ]. RS-EPR at DU has been expanded to imaging applications at 250 MHz, 700 MHz and 1 GHz in the Eaton laboratory, enabled by the ever-increasing availability of low-frequency microwave components.…”

“…Time-locked sub-sampling (TLSS) was developed by Hyde at the Medical College of Wisconsin (MCW) [142] but has not been widely incorporated due to the need for strict adherence to frequency values generating four samples in an odd number of cycles. The first direct digital detection of rapid-scan was performed at X-band in the Eaton laboratory at DU in 2014 [132]. The use of an AWG permitted downconversion to an intermediate detection frequency such that analog filters could be implemented to decrease non-EPR signals, as has been done successfully in other laboratories [132,[142][143][144].…”

“…The first direct digital detection of rapid-scan was performed at X-band in the Eaton laboratory at DU in 2014 [132]. The use of an AWG permitted downconversion to an intermediate detection frequency such that analog filters could be implemented to decrease non-EPR signals, as has been done successfully in other laboratories [132,[142][143][144]. Fully digital EPR for animal imaging has been implemented at 750 MHz [145] and 1 GHz [146], with automatic resonator tuning and matching, digital feedback control and digital automatic frequency control (digital AFC).…”