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How does a radio wave become a picture? Part II: Compact receivers

Published: Wed, 22/01/2020
compact receiver

With the help of radio telescopes, we can look into space by capturing radio signals from the universe. This can be done, for instance, with the 14 dishes in Westerbork or with LOFAR, which consists of several fields full of small antennae. But what actually happens when a radio signal, after traveling thousands, millions if not billions of lightyears, reaches a radio telescope antenna?

By Mischa Brendel

“A receiver picks up the radio signal through an antenna. The (very weak) signal is amplified and is then digitized,” prof. dr. Mark Bentum explains. Bentum is one of the researchers of the radio group at ASTRON working on the development of compact receivers.

Westerbork Synthesis Radio Telescope, one of the dishes viewed from above, credit: ASTRON

In radio telescopes, receivers consist of an antenna and several electronic components to transform the electrical signal. In a compact receiver these various components are integrated. But why make the effort to put all those separate components together in one single part? There are several reasons for that, Bentum and his colleague André Gunst, system architect of the digital & signal producing group, explain.

First and foremost is that with a compact receiver far less of the captured signal is lost and that more data can be collected. Gunst: “Currently we capture a radio signal and send this through a piece of coax cable of a few hundred meters before we digitize it, which is not without risk. For instance, a difference in temperature between several of these cables can cause a variation in how quickly it transports the signal.” The researchers want to receive the various signals of all antennae at exactly the same time to make certain that the data they receive from each individual antenna is from the exact same moment and location of looking into space. By digitizing the signal close to the antenna and putting a time stamp on it there, the signal is not sensitive for delay variations and loss anymore.

compact receiver
A compact receiver, credit: ASTRON

Gunst: “Digitizing the signal much closer to the antenna is exactly what we plan to do, but this is also our biggest challenge. This is because we need to boost the signals received by the antenna greatly before we can digitize them. That process creates noise which interferes with the measurements. Because of this issue the housing for the compact receivers needs to be designed in such a way that this interference is limited.” A second challenge to overcome is to distribute a clock signal to the antennas for the exact timing.

All in all this sounds like a rather costly affair, but that is not so. In fact, cost reduction is another important reason for the development of the compact receiver. Bentum: “When we have developed the integrated compact receiver, we plan to have a chip manufacturer to actually build the chips. The first one will be very costly, but the second one less so, and the third one will be even cheaper, and so forth. And these compact systems will function better and will be more stable than with separate components.” Another advantage: when a malfunction occurs, a compact receiver can be easily replaced, whereas a separate part in a larger receiver system needs to be disconnected, replaced and then reconnected to all the other parts.

Doing maintenance on LOFAR, credit: ASTRON

Several parts needed to construct a compact receiver are already available on the market. Gunst: “Analog to digital converters are already for sale for instance.” The compact receiver will eventually be available to the mass market as well, although both researchers suspect the system won’t cause any instantaneous huge impacts for the average consumer. Bentum: “But the technologies we develop here can have a major impact in the daily lives of people. WiFi for example was developed because we needed algorithms for the quick transportation of data.”

So far, Bentum and Gunst don’t foresee any problems with the development of the compact receivers and the two gentlemen expect that ASTRON – despite numerous challenges that still need to be overcome in its development – will be presenting the compact receiver in a couple of years.