Phase Space

7 March 2013

Researchers at the University of Cantabria in Spain have designed and fabricated a phase switch for use in radio astronomy and other microwave measurement systems.  Using existing techniques and an unconventional application of bandpass filters the group have created a phase switch that can be integrated into a microwave receiver, optimise imbalances between branches and reduce noise.

Cold comfort

Front-end receivers for radio astronomy operate at cryogenic temperatures (below 20 K) and can suffer from significant noise effects.  The traditional approach to overcome this is to include a rotating polar modulator, which will reduce 1/f noise, but the rotation introduces its own complications at temperatures below 20 K.  The team have instead used the more modern approach of phase switching.

Modern radio telescopes often employ an interferometric technique, and phase switching is a powerful way of improving resolution in these systems.  As noise will be present over the entire frequency band, it is useful to isolate and measure one frequency and thus to remove the noise from the rest of the band.   A phase switch applies a phase shift (of π or π/2) to one frequency, switching the phase/anti-phase nature of that signal.  This can then be measured separately from the rest of the band, and the entire process can finally be modulated over the required frequencies.

However, overcoming this intrinsic noise is not enough for the very sensitive measurements the group have in mind.  Radio receivers are usually divided into two branches and signal imbalance in amplitude or phase would cause inaccurate readings.  The phase imbalance can be solved by introducing bandpass filters to create a flat constant phase difference between the branches over a broad frequency range.  The bandpass filters also serve to delineate the band itself within the required measurement range.

In their Letter the group have combined all of these techniques and integrated the switch onto the receiver.  They also optimised the amplitude balance between the two branches to give extremely accurate measurements over the Ka band.

From L and X to K

Enrique Villa described the design of the team’s device as a “90° hybrid design based on the use of microstrip bandpass filters”.  The coupling between the branches is achieved by a broadband MMIC single pole double through (SPDT) switch based on pseudomorphic HEMT transistors.  Villa went on to say that “taking into account they are based on short-circuited stub π-networks, the phase features of each filter are calculated, and the individual phase expressions enable us to obtain the phase dependence between filters and to consider it as a design parameter.”

The final challenge the team faced was a result of the frequency range that radio telescopes operate in.  Previous methods were designed for low frequency (L to X) bands of the microwave spectrum, but astronomical measurements are typically at the higher K bands and this caused the group considerable problems, as Villa explained: “The design in the Ka-band challenges transmission line models and interconnections, issues which do not show up in low frequencies, and they are considered and overcome in the circuit described in the Letter. The interconnection with bond wires is especially critical at this frequency and higher, degrading the original performance of the design, so a very thorough and careful assembly is needed.”

Polarising the big bang

Now that their design is finished, the group’s radiometer is being installed at the El Teide Observatory in Tenerife.  The observatory is aiming to characterise the polarisation of the Cosmic Microwave Background along with other galactic and extra galactic radiation.  The main goal, after a year of observation, is to have covered daily an area of sky of 10,000 square degrees, with sensitivity between 1 and 2 μK and an angular resolution of 1° at 11, 13, 17, 19 and 30 Ghz.  These data will provide the most sensitive measurements yet obtained for the characterisation of anomalous microwave emission in the Milky Way, such as the synchrotron radiation emitted through the action of pulsars and, possibly, supermassive black holes.

Further reading

This article is based on the Letter: Broadband Ka-band 90º phase switch for radio astronomy (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, Volume 49, Issue 25

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