The Hydrogen maser is required equipment for any VLBI station, and so is a formatter such as a DBBC(1,2 or 3) or DBE. Together, these ensure that the received spectrum gets digitized, timestamped with sufficient stability, and provided with headers to allow playback at a correlator. These necessities however don't come cheap.

White Rabbit is an open standard for time and frequency distribution over fiber. It was originally intended to synchronize and control the experiments at the LHC, but we show that it is possible to extend its reach and frequency stability in such a way that it can be used to transport a reference clock for VLBI.

GNU Radio is a "free and open-source software development toolkit that provides signal processing blocks to implement software defined radios". Using this software, I've designed a VLBI receiver and formatter using an off the shelf software-defined radio. It generates standard compliant VDIF data, which can be processed by the EVN SFXC correlator, and many others. Our first experiments were not able to be processed in real-time and only provided two subbands, but work is ongoing to develop this into a 1024 Mb/s capable VLBI formatter.

Using these two methods, and the H-maser at the WSRT, we have successfully demonstrated VLBI fringes between the historical 25m Dwingeloo telescope and many EVN stations.

These developments showcase that off-the-shelf hardware and (open source) software are starting to become viable technologies for our signal processing needs. This will help lower the cost to outfit a station for VLBI, which in turn could help with increasing the UV coverage for VLBI arrays.