Station Signal Processing Pipeline

1. Antennas

LOFAR station is a wide-band radio receiver, which operates at the frequency range from 10 MHz to 240 MHz. The full reception band is divided into low band (10 MHz – 90 MHz) and high band (110 MHz – 240 MHz). The frequencies in between low band and high band are not used due to RFI from FM radio transmitters.

The station signal processing has been originally designed assuming that three different antenna arrays would be used. These arrays would have been the low-band-low (LBL) antenna array, the low-band-high (LBH) antenna array, and the high-band antenna (HBA) array. However, the present standard stations do not have the LBL antennas, and the LBH antenna array is thus referred to as the low-band antenna (LBA) array.

Antennas used in LOFAR stations are briefly introduced in following two sections. Notice that the antenna array configurations are configurations of full antenna arrays. It is always possible to manually select only a subset of a full antenna array to be used e.g. in beamforming. Positions of individual antenna elements and antenna array definitions of a station are stored in station configuration files.

2. Receiver Control Units (RCUs)

The analogue signals from LBA elements and HBA tiles are transferred in coaxial cables to a LOFAR station cabinet, where each of the cables is connected to a receiver unit. A RCU performs input selection, followed by amplification and filtering of the analogue input signal. The conditioned analog signal is sampled with a 12-bit A/D converter. The A/D converter produces real signal samples at either 200 MHz or 160 MHz sampling frequency. Combinations of filter passband and sample clock frequency are selected so that the selected frequency band always aliases around zero frequency, without frequency aliasing inside the passband. With one of the available combinations the signal spectrum will be inverted, but it can be inverted back in subsequent processing steps.

The receiver units are numbered starting from zero. X-polarisation cables are connected to even-numbered RCUs and Y-polarisation cables are connected to odd-numbered ones.  The only deviation from this rule is the LBA outer field, whose X-polarisation cables are connected to odd-numbered RCUs and Y- polarisation cables to the even-numbered ones. A receiver unit has three inputs: LBL, LBH, and HBA. The LBL and LBH connectors have an 8 V bias voltage. The HBA X-polarisation connectors have the 48 V bias voltage for power supply. The HBA Y-polarisation connectors have a 3.3 V bias voltage only when communicating with the HBA tiles.

A core station or a remote station with 96 LBA elements and 48 HBA tiles has 96 RCUs. The LBA inner array is connected to the LBH inputs and the LBA outer array to the LBL inputs. Despite the naming convention in RCUs, both LBA inner and LBA outer use the same kind of antenna elements. The HBA tiles are connected to the HBA inputs.

Due to their larger number of HBA tiles, international stations have 192 RCUs. The 96 LBA elements of an international station are connected to the LBH inputs of the RCUs, and the 96 HBA tiles are connected to the HBA inputs. In a basic installation of an international station the LBL inputs are left empty, thus facilitating later installation of an additional antenna array.

RCU mode	antenna set	accessible frequencies [MHz]	sampling frequency [MHz]
3	low band	10-90	200
4	low band	30-90	200
5	high band	110-190	200
6	high band	170-230	160
7	high band	210-270	200

The subbands bandwidth is given by sampling frequency/1024 (1st polyphase filter). The maximum total bandwidth is given by the product of subbands bandwidth and the number of subbands (244 for 16 bit mode, and twice that number for 8 bit mode), resulting in :

sampling frequency [MHz]	bit mode	subbands bandwidth[kHz]	number of subbands	maximum total bandwidth [MHz]
200	16	195.3125	244	47.6
160	16	156.250	244	38.1
200	8	195.3125	2*244	95.3
160	8	156.250	2*244	76.3