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Interferometer

30 GHz Interferometer Pathfinder

1. Introduction

Focusing our attention in terrestrial instruments for the observation of the CMB, the most usual frequency bands in the microwave range are given by the atmosphere opacity (see Figure 1).

In particular, for frequencies higher than 20 GHz, the most suitable frequency band, in which the atmosphere is more transparent, is located around 30 GHz. For this reason a lot of instruments –QUIJOTE TGI and the interferometer prototype proposed in the present Consolider project between them- are designed to work in the band from 26 to 36 GHz.

 

Figure 1: Atmosphere Opacity in the 1-275 GHz frequency range (extracted from "CRAF Handbook for Radio Astronomy", Edited by Jim Cohen, Titus Spoelstra, Roberto Ambrosini and Wim van Driel, Third Edition – 2005).

 

Although till the moment imaging instruments are broadly used, mainly due to their higher simplicity, systematic errors and their requirement of a telescope system, make the interferometers to be considered the future solution to obtain CMB ultra-sensitive measurements. The sensitivity of all these instruments is proportional to the number of receivers. In fact, by considering all the receivers to be equal, the overall effective bandwidth would be the individual effective bandwidth multiplied by the number of receivers. So, while in the imaging instruments the number of receivers is limited by the telescope focal plane area, telescope that on the other hand increases a lot the experiments cost, the interferometers do not present this limitation because they do not require such a telescope system.

  

Nevertheless, it can be verified in the literature that the interferometers fabricated until now do not present a high number of receivers (<20) due to the technical and technological complexity to correlate a big amount of wideband (typically >25%) microwave signals. In this sense, in the frame of the EPI Consolider project it has been done a detailed study to clarify the type of correlator more intended to be used in a large-format interferometer with hundred or even thousands of receivers.

2. Digital Correlators

As a first possibility it was considered the use of digital correlators, implemented by using FPGAs (“Field Programmable Gate Arrays”), GPUs (“Graphics Processing Units”), or similar modules, due to their great flexibility and easy to systematic and phase errors control.

 

It has been acquired one commercial FPGA from Agilent Technologies, in particular the U1071A model, with two 500 MHz bandwidth channels (see Figure 2).

 

Figure 2: U1071A FPGAs from Agilent Technologies.

 

Correlation tests were done using both sinusoidal signals at 100 and 250 MHz (Figure 3a) and filtered white noise signals with a bandwidth of 80 MHz and centered in 100 and 200 MHz (Figure 3b). The correlation function was implemented by means of the software Labview.

In spite of the achieved good results in terms of precision and phase control, digital correlation presents some problems as the high cost, power consumption, volume and weight. On the other hand, although in the last years it has increased a lot, the bandwidth is still reduced in terms of specifications for CMB measurement instrumentation.

 

In the particular case of the FPGAs of Figure 2, the real-time operation bandwidth (“throughput”) is of about 200 MHz, so the number of FPGAs needed to implement a 100 receiver interferometer would be 5.000. This makes totally nonviable the digital implementation of the correlator.

 

 

(a)                                                                   (b)

Figure 3: Two sinusoidal signals (a) and two band-filtered white noise signals (b) digital correlation.

3. Michelson Type Analogical Correlators

Another possibility is to use analogical correlators designed to operate in base-band or in the microwave frequency band. In principle they were considered Michelson type interferometer structures (signal correlation by pairs) as in VSA (“Very Small Array”). A base-band correlator was implemented following the scheme of Figure 4a. The single base-line correlator is shown in Figure 4b.

 

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(b)

Figure 4: ‘Plus-minus’ configuration correlator (a) and circuit mounted over FR4 substrate (b).

 

This correlator is based in the Ryle configuration where both input signals are phase-switched between 0 and 180 degrees. This kind of correlator was previously implemented in the VSA interferometer. The correct behaviour of the prototype was proven by using sinusoidal excitation signals at 250 and 250,001 MHz (see Figure 5a) and the video bandwidth was measured, resulting in a value of about 10 KHz (see Figure 5b).

(a)                                                                   (b)

Figure 5: Two sinusoidal signals analogical correlation (a) and correlator video bandwidth measurement result (b).

 

It was also implemented a 26 to 36 GHz bandwidth correlator using a correlation and detection module designed for the QUIJOTE TGI. Figure 6 shows the measurement test-set. It was also proven its correct operation and the video bandwidth was measured, resulting in a value of about 70 KHz.

 

Figure 6: Measurement Test-Set of the QUIJOTE TGI correlation module. In this case a modulation signal was applied to the input signals.

 

In spite of the achieved good results, this kind of correlators were finally discarded, due to the resulting complexity from the total number of base-lines (n(n-1)/2 been n the number of signals to correlate) and also from the routing of the microwave signals that, on the other hand, increase a lot the resulting correlator cost.

4. Fizeau Type Analogical Correlators

To solve the previous issues, Fizeau type interferometers, in which the signals are combined all with all, could be the solution because they simplify a lot the correlation structure. It was analysed the possibility of using Rotman Lenses (RLs) implemented over dielectric materials typically used for the fabrication of RF and microwave circuits. In particular, Figure 7 shows a simplified scheme of an interferometer with an analogical correlator based in RLs, T(I), Q and U input maps and the signal paths towards the detectors.

 

A “2in-4out” (two inputs and four outputs) RL was designed using the dedicated software RLD from Remmcom. As the analysis performed by this software is rather idealized, the design was exported to a much more accurate electromagnetic analysis software as is HFSS, to verify the RLD simulation results. Figure 8 shows the design of the HFSS simulated lens and the resulting matching of their input and output ports.

 

Figure 7: Scheme of a RLs-based interferometer.

 

 

     

(a)                                                                                                                 (b)

Figure 8: RL design with two input and four output ports (a) and matching of the Lens ports (b).

 

In Figure 8b it can be observed that the design works well in the center of the bandwidth, but in the ends the results are not good. The same can be observed in the Figure 9, where the phase shift between the input and output ports are shown. From these results, it seems that, due to the great wideband required, the phase control result very complicate with this technology. So, taking into account that this is a critical point for an interferometer, it was decided to discard also this correlation option.

 

(a)                                                                                                      (b)

Figure 9: Phase-shift between the output ports and the first input put (a) and the same related to the second input port (b).

5. Optical Correlators

At the moment, the most viable option is considered to be the use of Mach_Zehnder optical modulators (see Figure 10) to reduce remarkably the correlator and the resulting interferometer complexity.

 

 

Figure 10: Mach-Zehnder Modulator LN86P from Thorlabs with a bandwidth of 40 GHz.

 

These modulators divide the optical signal into two arms with a LiNbO3 substrate and a refraction index that can be modified by means of an electrical field (see Figure 11). The modulation appears because of the interference of the two arms. Applying opposite voltages in them it is possible to cancel any type of phase modulation allowing so the use of MZM as an amplitude modulator.

 

The MZM of Fig. 10 has been measured by modulating a 1550 nm optical signal with a 30 GHz sinusoidal signal (Fig. 11a) achieving the results of Fig. 11b from an optical spectrum analyser. It was seen that it is possible to the remove one of the lateral bands but, due to the high power level of the optical carrier, it will be required the use of optical filters to achieve attenuation levels of at least 40 dB.

 

 

Figure 11: MZMs operating scheme.

 

The basic idea to implement an interferometer with optical correlator is to modulate L band (1550 nm) laser signals with the microwave ones coming from the CMB to, being so in the optical range, route and correlate the signals by means of an optical system based on fibres, lenses and near-infrared cameras. Figure 13 shows a simplified scheme of a Fizeau type optical interferometer.

The use of the reported technology allows the implementation of both Michelson and Fizeau type interferometer. In principle we propose a Fizeau interferometer, in which the signals are combined all with all thanks to the lenses of the previously mentioned optical system, due to its higher simplicity but a Michelson interferometer is not discarded because surely it would suffer less from systematic errors.

 

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Figure 12: Measurement Test Set of the MZM (a); Measurement results from an optical spectrum analyser (b).

 

Figure 13: MZMs-based interferometer simplified scheme.

 

During the project remaining time, the objective is to implement an interferometer prototype with 4 receivers and following the scheme of Figure 13.