Digital detection and processing of multiple quadrature harmonics for EPR spectroscopy

Abstract

A quadrature digital receiver and associated signal estimation procedure are reported for L-band electron paramagnetic resonance (EPR) spectroscopy. The approach provides simultaneous acquisition and joint processing of multiple harmonics in both in-phase and out-of-phase channels. The digital receiver, based on a high-speed dual-channel analog-to-digital converter, allows direct digital down-conversion with heterodyne processing using digital capture of the microwave reference signal. Thus, the receiver avoids noise and nonlinearity associated with analog mixers. Also, the architecture allows for low-Q anti-alias filtering and does not require the sampling frequency to be time-locked to the microwave reference. A noise model applicable for arbitrary contributions of oscillator phase noise is presented, and a corresponding maximum-likelihood estimator of unknown parameters is also reported. The signal processing is applicable for Lorentzian lineshape under nonsaturating conditions. The estimation is carried out using a convergent iterative algorithm capable of jointly processing the in-phase and out-of-phase data in the presence of phase noise and unknown microwave phase. Cramér-Rao bound analysis and simulation results demonstrate a significant reduction in linewidth estimation error using quadrature detection, for both low and high values of phase noise. EPR spectroscopic data are also reported for illustration.

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

“…The current work makes the major step to CW at X-band, which was not feasible until the latest developments in digital technology. Down-conversion and subsampling techniques, which are also extensively used in NMR [1] and MRI, have been used by the Hyde lab for recording multiharmonic X-band CW EPR [15, 28, 29] [30], by the Ohio State lab for L-band CW EPR [14], and by the NCI lab to digitize an FID [26]. An AWG was used to control pulse timing in a 26.5-40 GHz pulsed EPR spectrometer [31].…”

“…A single physical channel that carries s ( t ) is digitized and separation of m x ( t ) from m y ( t ) is carried out in a computer by means of digital phase-sensitive detection. For CW, multiharmonic, or rapid scan experiments the RF or microwave power is on continuously and a very intense reflection: $$r(t,\omega )=r_y\sin \left(\omega t\right)+r_x\cos \left(\omega t\right),\omega ={\omega }_0,$$ has to be digitized along with the EPR component [14]. Because of a very small separation in the frequency domain, ω 0 vs. ω 0 + ω s , filtering of the unwanted signal is not possible.…”

Digital EPR with an arbitrary waveform generator and direct detection at the carrier frequency

“…Exponential growth in processing power and memory capacity of modern digital electronics facilitates transition from the homodyne detection that is commonly used in EPR spectrometers to digital detection either at an intermediate frequency, IF [1–6], or directly at the carrier frequency [7–10]. Digital detection has several advantages.…”

“…(iii) For sinusoidal deconvolution of rapid scan signals both channels are used and orthogonality is required, (iv) For pulse EPR, precise orthogonality of the two quadrature channels reduces the number of required phase cycles and increases the SNR [2]. (v) Direct detection enables numerical evaluation of multiple harmonics of the continuous wave (CW) modulation frequency [4,6,10,12] which is important for saturation transfer EPR spectroscopy [13,14], and allows reconstruction of the true EPR lineshape from over-modulated EPR signals with improved SNR [10,12,15–18]. …”

“…If sub-sampling to very low frequencies is used, a bandpass filter with demanding characteristics (narrow bandwidth, sharp roll-off, and low insertion loss) is needed to reject noise outside the narrow signal bandwidth. These additional components introduce additional noise and increase the cost of the spectrometer [6,7,9,10]. …”