Abstract
Square Kilometer Array (SKA)-Low is the radio telescope operating in the lowest frequency band of the SKA, from 50 up to 350 MHz. It consists of 512 stations, each composed of 256 dual-polarization log-periodic antennas for a total of 262,144 independent signal paths. The low-frequency aperture array (LFAA) is the portion of the SKA-Low telescope including the antennas and the related electronics. Signal processing is hosted in a temperature controlled and shielded facility: the central processing facility (CPF), for all the core stations, or remote processing facilities (RPF), for stations in the array arms, to limit the maximum fiber length. Such a geographically distributed and interconnected radio telescope, spanning ∼65 km in diameter, requires that frequency and timing reference signals are distributed to the processing facilities with high stability and precision to ensure the required system performances. We present the realization of the clock and pulse per second distribution network inside the LFAA signal processing cabinet where subracks containing signal acquisition boards are housed. We describe the different parts of the chain, and we report on the total jitter introduced by this structure.
Highlights
Square Kilometer Array (SKA)-Low[1,2] is the low-frequency part of the SKA telescope[3] and is designed to detect radio signals in the frequency range of 50 to 350 MHz
The total jitter introduced by the clock distribution network, the analog-to-digital converter (ADC) clock generation in the TPM, and the ADC itself can be measured by analyzing the jitter in a monochromatic tone, digitized by the complete system
At very low frequency offsets, between 0.1 and 200 Hz, the dominant contribution is due to the phase-locked loop (PLL) in the two distribution boards
Summary
Square Kilometer Array (SKA)-Low[1,2] is the low-frequency part of the SKA telescope[3] and is designed to detect radio signals in the frequency range of 50 to 350 MHz. In the low-frequency aperture array (LFAA), a reference frequency signal is distributed and delivered to the digital part of the telescope, namely the signal processing subsystem (SPS), with the purpose of generating the sampling clock for digitization. An RF signal is generated from the laser using an optical frequency comb.[18] The Torun VLBI station[19] in Poland is remotely synchronized to an atomic clock through 350 km of fiber over the Polish fiber-optic network for time and frequency (T&F) distribution (OPTIME).[20] This system uses an integrated circuit to perform delay compensation to the stabilized 10 MHz and synchronizes both time and frequency.[21].
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