Low Frequency Front End

In January 2002 discussions started between the Radio Observatory Westerbork and the Technical Laboratory (TL) in Dwingeloo to study the feasibility of low frequency receivers (120-180 MHz) for the WSRT, expected to lead to a NWO-M proposal in the beginning of 2003. One of the goals of these receivers is to enable observations in preparation for LOFAR, leading to the requirement of simple, low-cost receivers, to be realized on a timescale of one to one and a half years from the possible granting of the proposal mid-2003.

Most important concern with respect to the receiver design was the suppression of strong interfering signals from the FM- and TV-transmitters at the edges of the proposed receiver pass-band, as well as in-band interferers, requiring a number of high quality filters. A second major design issue were the antennas for proper illumination of the telescope at these relatively low frequencies. Furthermore easy implementation in the existing system was desired.

At the Feasibility Review in October 2002, a possible realisation and properties of a LFFE were presented, with cost estimates for procurement of components and the construction of a series of receivers, as described in a Feasibility Report. This information was used for the proposal to NWO at the end of January 2003, which was granted in June 2003. In the mean time preparations for the project continued, resulting in the definition of the receiver chain and possible antenna system. Tests in the telescope with a prototype receiver system helped to define the antenna configuration and also resulted in the addition to the signal chain of a filter for suppression of a strong pager signal at 169.75 MHz. The picture below is of one of the prototype receiver chains used for telescope tests, showing the filters and amplifiers.

Picture of one of the LFFE prototype receiver chains
Picture of one of the LFFE prototype receiver chains

Special attention in the mechanical design was given to a lightweight construction for moving the antennas between observing and stowed position, with the least possible effect of stowed antennas on observations in MFFE-observing bands. The result of this work heavily relied on contributions from the TL Antenna and Mechanical Design Groups, as well as cooperation with the RSW for telescope tests and evaluation. The resulting design is presented in the figure below, showing the final antenna configuration two-element arrays for two polarizations and the structures for moving the individual antennas in and out of the LFFE observing position.

At the end of 2003 a final, dual-polarization, receiver system design was available. Production of receiver components for 14 telescopes was started after the Design Review at the end of January 2004, where test results with the prototype receivers and antennas were presented and discussed (see attached Design Review Report).

The LFFE antenna system in the observing position in front of the radome, also showing the moving arrangements for the individual antennas.

See movie (komt later)

The antenna moving system was further detailed after the Design Review and parts for a prototype system were ordered in March. After successful tests the production of a series of antennas and mounting structures was started, with the installation of the first system on a telescope at the end of June. In the mean time the production of components for the LFFE-receivers continued and assembly of complete receivers was started.

After a tight schedule for assembly and installation on the telescopes, in parallel with modifications to the existing MFFEs to enable control of the LFFE through the MFFE, the installation of all LFFE-systems on the WSRT telescopes was completed in the beginning of October 2004 and the first commissioning observations were performed. Based on the first successful results it was decided to make the system available for astronomical observations from mid November.

The successful completion of the project has been realized within the envisaged timescale of one and a half year from the granting of the project, enabling operation of the WSRT in the frequency range of the LOFAR high band system, starting in the fall of 2004. (read WSRT observes in the LOFAR (high)band )

More informations : Design review report

Design: Kuenst.    Development: Dripl.    © 2014 ASTRON