Digital Sideband Separation

Status: Mature

Introduction

In radio astronomy, sideband separating receivers (2SB) are the most popular type of analog receivers (over single sideband (SSB) and dual sideband (DSB) receivers), primary due to their high bandwidth. By cleverly manipulating the phase of the input signal before and after down-conversion the receiver can separate the upper sideband (USB) and the lower sideband (LSB) of the RF without the use of specially tuned filters. This types of receivers are also called 'Image-reject receivers' because the unwanted sideband is rejected by producing a counter-phase addition of two copies of it.

The main problem this types of receivers have is that small imbalances in phase or amplitude of its analog components translate in a poor performance of the rejection of the sidebands, measured as the sideband rejection ratio (SRR). For high bandwidth, high frequency receivers SRR of as little as 15dB and 10dB are common.

This project aims to implement a digital sideband separating receiver, that is a receiver that process its IF in the digital domain. This have the advantage that the analog RF portion can be digitally calibrated by loading digital constants in order to correct the imbalances of the analog components. This technique can be applied individually to small portion of the bandwidth of interest in order to achieve very high rejection ratio. In our project we have achieved rejection ratios of the order of 50dB to 60dB (the exact amount of rejection depends on the data size used of digitization and processing).

Documentation

• Introduction to sideband separating mixers (aka, image reject mixer): Pozar, D. Microwave Engineering, 4th Edition, pg 649

• Mathematical derivation of SRR given phase and gain imbalance: https://www.rug.nl/research/portal/files/14483708/ThesisJWK-21Nov2008.pdf page 224

• Math of digitally calibrated 2SB receiver: https://library.nrao.edu/public/memos/edir/EDIR_320.pdf

• First paper of digital 2SB: https://safe.nrao.edu/wiki/pub/Beamformer/DdlTesting/Digital_Sideband_Separating_Mixer_doc.pdf

• Ricardo Finger's Ph.D. thesis on digital 2SB on ROACH1: http://www.das.uchile.cl/lab_mwl/publicaciones/Tesis/cf-finger_rc.pdf

• Following paper about 2SB from the Millimeter-Wave Laboratory:

  • https://ui.adsabs.harvard.edu/abs/2013PASP..125..263F/abstract

  • https://ui.adsabs.harvard.edu/abs/2014PASP..126..380R/abstract

  • https://ui.adsabs.harvard.edu/abs/2015A%26A...584A...3F/abstract

  • https://ui.adsabs.harvard.edu/abs/2018A%26A...619A.153R/abstract

Code and Models

• ROACH1 and ROACH2 models, bofs, and scripts examples: https://github.com/FrancoCalan/simulink_models/tree/master/Digital_Sideband_Separation