Development of digital sideband separating down-conversion for Yuan-Tseh Lee Array

Abstract: This report presents a down-conversion method involving digital sideband separation for the Yuan Tseh Lee Array to double the processing bandwidth. The receiver consists of a MMIC HEMT LNA frontend operating at a wavelength of 3 mm, and sub-harmonic mixers that output signals at intermediate frequencies of 2 - 18 GHz. The sideband separation scheme involves an analog 90 degree hybrid followed by two mixers that provide down conversion of the IF signal to a pair of in phase (I) and quadrature (Q) signals in baseband. The I and Q baseband signals are digitized using 5 Giga sample per second analog to digital converters. A second hybrid is digitally implemented using field programmable gate arrays to produce two sidebands, each with a bandwidth of 1.6 GHz. The 2 x 1.6 GHz band can be tuned to cover any 3.2 GHz window within the aforementioned IF range of the array. Sideband rejection ratios (SRRs) above 20 dB can be obtained across the 3.2 GHz bandwidth by equalizing the power and delay between the I and Q baseband signals. Furthermore, SRRs above 30 dB can be achieved when calibration is applied.