A Calibrated Digital Sideband Separating Spectrometer for Radio Astronomy Applications

TL;DR

This paper presents a novel FPGA-based digital spectrometer with a calibrated 2SB receiver achieving over 40dB sideband rejection across 2GHz, improving astronomical spectral observations.

Contribution

It introduces a calibration method for a digital 2SB spectrometer that significantly enhances sideband rejection compared to traditional analog approaches.

Findings

Achieved over 40dB sideband rejection ratio.

Successfully implemented a 4GHz bandwidth digital spectrometer.

Demonstrated effective calibration compensating for analog imbalances.

Abstract

Dual sideband (2SB) receivers are well suited for the spectral observation of complex astronomical signals over a wide frequency range. They are extensively used in radio astronomy, their main advantages being to avoid spectral confusion and to diminish effective system temperature by a factor two with respect to double sideband (DSB) receivers. Using available millimeter-wave analog technology, wideband 2SB receivers generally obtain sideband rejections ratios (SRR) of 10-15dB, insufficient for a number of astronomical applications. We report here the design and implementation of an FPGA-based sideband separating FFT spectrometer. A 4GHz analog front end was built to test the design and measure sideband rejection. The setup uses a 2SB front end architecture, except that the mixer outputs are directly digitized before the IF hybrid, using two 8bits ADCs sampling at 1GSPS. The IF hybrid is implemented on the FPGA together with a set of calibration vectors that, properly chosen, compensate for the analog front end amplitude and phase imbalances. The calibrated receiver exhibits a sideband rejection ratio in excess of 40dB for the entire 2GHz RF bandwidth.