Development, design and realization of cryogenic radio receivers of the Sardinia radio telescope

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Università degli Studi di Cagliari

DOTTORATO DI RICERCA

Ingegneria Elettronica ed Informatica

Ciclo XXVII

TITOLO TESI

D

EVELOPMENT

,

D

ESIGN AND

R

EALIZATION OF THE

C

RYOGENIC

R

ADIO

R

ECEIVERS OF THE

S

ARDINIA

R

ADIO

T

ELESCOPE

Settore/i scientifico disciplinari di afferenza

ING-INF/02

Presentata da:

Giuseppe Valente

Coordinatore Dottorato

Prof. Fabio Roli

Tutor

Prof. Giorgio Montisci

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III

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IV

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Dedicated to my Wife and my Family

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V

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VI Acknowledgments __________________________________________________________________________________

A

CKNOWLEDGMENTS

Durante gli anni di dottorato, ho avuto la fortuna di collaborare con tantissime persone, ognuna delle quali a suo modo ha contribuito a definire questo mio lavoro di tesi, nonché alla mia crescita professionale. Un primo ringraziamento va ad Alessandro Navarrini e Tonino Pisanu che in questi anni mi hanno sempre stimolato e supportato in tutte le mie scelte, anche difficili, che un normale lavoro comporta.

Un ringraziamento particolare a Pasqualino Marongiu che è stato il “creatore materiale” di tutte le mie folli idee.

Ringrazio tutto lo Staff dell’Osservatorio Astronomico di Cagliari, in particolare il gruppo ricevitori, che sin dal tempo della tesi di laurea mi ha adottato, rendendomi parte attiva della crescita e dello sviluppo dell’Osservatorio stesso e di un progetto prestigioso come il Sardinia Radio Telescope, permettendomi di affrontare nelle migliori condizioni questo percorso di dottorato.

Un doveroso ringraziamento va a tutto lo staff della Stazione Radioastronomica di Medicina (BO), il quale con i suoi preziosi consigli mi ha insegnato l’aspetto pratico della ricerca.

Ringrazio Prof. Giorgio Montisci per la sua guida nel mondo della ricerca e per i suoi continui incoraggiamenti, sperando, ma non ho alcun dubbio del contrario, che la fine del percorso di dottorato non sia il termine ma bensì l’inizio di una fruttuosa collaborazione.

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Acknowledgments VII

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Infine ringrazio mia moglie che in questi anni mi ha sopportato ma soprattutto supportato dandomi sempre la forza di superare i momenti difficili.

Marzo 2016

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VIII Abstract __________________________________________________________________________________

A

BSTRACT

Radio astronomy sources emit a very low signal of the order of 10

. Therefore, in order to detect a celestial signal, very sensitive

instruments are required, starting from the radio receivers installed on the radio telescopes. In this case, one of the main targets is to reduce the

noise of the receivers (TRIC). The work of the thesis covers a wide field of

knowledge, from the design of a single passive microwave device such as directional coupler, a 180° hybrid or waveguide to microstrip transition, to the general performances of each component in the front-end of the receiver. In this thesis, we have designed and measured the single devices at room and cryogenic temperature with different instruments, like vector network analyser or spectrum analyzer. In the first part we focus on the LP Band receiver of the Sardinia Radio Telescope. In this regard, we describe the design of the low losses devices before the low noise amplifier, and the cryogenic architecture of the whole cryogenic coaxial receiver. In the second part, we present the design of the S band multi beam cryogenic receiver of the Sardinia Radio Telescope. Measured performance of the radio telescope is provided in this case.

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Abstract IX

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X Summary __________________________________________________________________________________

S

UMMARY

Acknowledgments ... VI Abstract ... VIII Summary ... X Radio Astronomy... 1 1.1 Introduction ... 1 1.2 Reference ... 3

Sardinia Radio Telescope ... 5

2.1 Sardinia Radio Telescope Technical description ... 5

2.2 Radio Receiver ... 11

2.2.1 Sensitivity and stability in radio receiver ... 13

2.2.2 System Noise ... 14

2.2.3 Cryogenic System ... 16

2.3 Reference ... 18

Coaxial LP Cryogenic Receiver ... 21

3.1 Introduction ... 21

3.2 Scientific Aim ... 21

3.3 Coaxial dual frequency LP receiver ... 22

3.3.1 Architecture of the Rx-LP ... 23

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Summary XI

__________________________________________________________________________________

3.3.3 P-Band Coaxial orthomode Junction ... 30

3.3.4 External Corrugations ... 39

3.3.5 P-band Feed Pattern ... 39

3.3.6 Planar P-Band 180° Hybrid Directional Coupler ... 44

3.3.7 P-Band Orthomode Transducer ... 52

3.3.8 Commercial switch and band pass filter device ... 55

3.1.1 Low Noise Amplifier ... 57

3.1.2 L band RF Path ... 59

3.1.3 L Band Feed ... 59

3.1.4 L- Band Feed Pattern ... 61

3.1.5 Orthomode Transducer and Vacuum Window. ... 65

3.1.6 L- Band Directional Coupler ... 84

3.1.7 Low Noise Amplifier ... 91

3.2 Thermal Design of the cryostat ... 93

Results ... 107

4.1 Introduction ... 107

4.2 Cryogenic result ... 107

4.3 Receiver temperature ... 111

4.4 Coupling between Coaxial receiver and Sardinia Radio telescope 117 4.5 Reference ... 125

A MultiFeed S-Band Cryogenic Receiver ... 127

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XII Summary __________________________________________________________________________________

5.2 S-Band RFI Investigation ... 128

5.3 Study of S-band FPA configuration ... 129

5.4 Architecture of the receiver ... 134

Conclusion and Future works ... 145

List of Figures ... 147 Chapter 2 ... 147 Chapter 3 ... 147 Chapter 4 ... 153 Chapter 5 ... 154 List of Tables ... 157 Chapter 2 ... 157 Chapter 3 ... 157 Chapter 4 ... 158 Chapter 5 ... 158

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Summary XIII

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Radio Astronomy 1

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CHAPTER

1

R

ADIO

A

STRONOMY

1.1 Introduction

Reflector antennas are used since the discovery of Electromagnetic wave propagation in 1888 by Hertz. Very important developments have been conducted since World War II [1-2]. Subsequent demands of reflectors for use in radio astronomy, microwave communication, and satellite tracking resulted in the development of sophisticated analytical and experimental techniques for shaping the reflector surfaces and optimizing its illumination so as to maximize the antenna gain. Although reflector antennas have many geometrical configurations, some of the most popular shapes are the plane, corner and curved reflectors (especially the paraboloid).

It is well know that a beam of parallel rays impinges upon a reflector whose geometrical shape is a paraboloid, the radiation will converge (focus) in the so-called “focal point”. In the same manner, if a point source is placed in the focal point, the rays reflected by a parabolic reflector will emerge as a parallel beam. The symmetry point on the parabolic surface is known as “vertex”.

When a radio astronomy receiver is placed in the focal point of the paraboloid, this configuration is usually known as “primary focus” position. The disadvantages of the primary focus arrangement are the following:

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2 Radio Astronomy __________________________________________________________________________________

- There is a long signal path from the receiver to the back-end equipment, which causes high losses;

- high receiver gain is required to compensate for the high losses caused by the long signal path;

- system cooling is required for a low noise system ; - an undesired blockage effect is present;

Another configuration that can be used is the Gregorian antenna. It employs a paraboloid main reflector and an ellipsoid subreflector, the latter positioned beyond the focus of the main reflector. For this

arrangement, the rays emitted by the receiver illuminate the

sub-reflector and are reflected by it in the direction of the primary sub-reflector, as if they originated in the focal point of the parabola (primary reflector). The rays are then reflected by the primary reflector and are converted to parallel rays. An example of a Gregorian Antenna configuration is the Sardinia Radio Telescope (SRT), which is a Radio Telescope for radio astronomy applications [1-2].

The aim of this thesis consists in the design and characterization of some passive components of the first generation radio receivers employed in the Sardinia Radio Telescope.

The first generation cryogenic radio receivers of SRT are three, one for each focus position. A seven beam K band, which cover the frequency range 18-26 GHz is installed in the Gregorian focus (F2). A mono feed C band, which covers the frequency range 6.7-7.7 GHz is installed in F3, one of BWG foci. Finally, the coaxial dual frequency LP which covers two different frequency range, 305-410 MHz and 1.3-1.8 GHz, is installed in the primary focus [3-4].

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Radio Astronomy 3

__________________________________________________________________________________

This first part of the thesis is focused on the latter receiver. Different components have been designed and characterized: the coaxial feed, that covers the P and L band together, the coaxial orthomode junction and the directional coupler for the P band, the orthomode junction, the 180 degree hybrid and the directional coupler for the L band. For the same receiver, all the cryogenic parts required to cool down the receiver at 15 K have been designed.

In the second part of the thesis the architecture of one of the second generation receivers of SRT has been studied. In particular, the S band receiver (operating between 3 GHz and 4.5 GHz) has been considered. The receiver is placed in the primary focus of SRT and it is realized with a multifeed configuration of seven elements [5].

1.2 Reference

[1] JOHN D.KRAUS,“RADIO ASTRONMY”,MCGRAW-HILL BOOK COMPANY,1966;

[2] THOMAS WILSON, KRISTEN ROHLFS, SUSANNE HUETTEMEISTER “TOOLS OF

RADIO ASTRONOMY”(ASTRONOMY AND ASTROPHYSICS LIBRARY) 6TH ED.2014

EDITION,SPRINGER;

[3] VERTEX, “64-METER SARDINIA RADIO TELESCOPE FINAL DESIGN REPORT

OPTICS AND DESIGN”SYSTEMS DIVISION,SANTA CLARA FACILITY;

[4] J. BRAND, K.H. MACK, I. PRANDONI, “SCIENCE WITH THE SARDINAI RADIO

TELESCOPE”,MEMORIE DELLA SOCIETÀ ASTRONOMICA ITALIANA,BOLOGNA

10-11,2005;

[5] G.VALENTE;G.SERRA;F.GAUDIOMONTE; A.LADU; T.PISANU;P.MARONGIU;

A. CORONGIU;A. MELIS; M. BUTTU; D. PERRODIN; G. MONTISCI; G.

MAZZARELLA;E.EGRON;N.IACOLINA;C.TIBURZI;V.VACCA,“A MULTIFEED

S-BAND CRYOGENIC RECEIVER FOR THE SARDINIA RADIO TELESCOPE PRIMARY

FOCUS”, SPIE 9153, MILLIMETER, SUBMILLIMETER, AND FAR-INFRARED

DETECTORS AND INSTRUMENTATION FOR ASTRONOMY VII,91530Q(23JULY

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4 Radio Astronomy __________________________________________________________________________________

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Sardinia Radio Telescope 5

__________________________________________________________________________________

C

HAPTER

2

S

ARDINIA

R

ADIO

T

ELESCOPE

2.1 Sardinia

Radio

Telescope

Technical

description

Radio telescope antennas for radio astronomy application must provide high resolution and sensitivity to detect the very small flux density of cosmic radio sources. Therefore, antennas of large aperture are generally required. In order to observe a large portion of the sky a high degree of steerability is required. An arbitrary but convenient way of classifying radio telescope antennas is to divide them into three groups, on the basis of their degree of mechanical steerability, as follows:

- Complete steerability: the reflector antenna can be oriented along two coordinates, azimuth and elevation;

- Partial steerability: the reflector antenna can be oriented along one coordinate, usually elevation;

- Fixed: the main antenna is mechanically stationary

The Sardinia Radio Telescope (SRT) [6-11] is an example of antenna with

complete steerability. SRT consists in a general purpose, symmetric

64-meter paraboloidal radio telescope, capable to operate with high-efficiency in a wide frequency (between 300 MHz and 116 GHz). In Figure 1 the sketch of the antenna with some specific parts are shown.

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6 Sardinia Radio Telescope __________________________________________________________________________________

Figure 2.1: Sketch of the Sardinia Radio Telescope, SRT;

The antenna geometry developed for the Sardinia Radio Telescope consists of a shaped reflector system pair based on the classical Gregorian configuration. The main reflector (M1) consists of a back-structure that supports – through actuators – the mirror surface, itself composed of rings of reflecting panels. A quadrupod, connected to the back-structure, supports the sub-reflector (M2) and the primary focus positioner and instrumentation. Three large rooms were built behind the primary mirror to contain the secondary focus receivers, the

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beam-Sardinia Radio Telescope 7

__________________________________________________________________________________

waveguide mirrors and several electronic instrumentation and cable distributions, respectively. A fourth room is located in the lower part of the alidade, where the power drivers for the motors, the antenna control unit and the cryogenic compressors are installed. The alidade also supports the elevation wheel, which is a conical truss anchored to the reflector back-structure through a massive pyramidal structure. A key feature of the SRT is its active surface [12]. The primary reflector is composed of 1008 panels. It is built using aluminium sheets glued, by means of a layer of epoxy resin, to both longitudinal and transversal Z-shaped aluminium stiffeners. The panels are fixed upon 1116 electromechanical actuators, able to correct the deformations induced by gravity on the primary surface (or “mirror”). Effort is underway to employ this facility to also correct for non-systematic errors, such as temperature/wind-related effects. The shaped design improves the antenna performance, conserving at the same time the desirable characteristics of a pure classical Gregorian configuration. The shaped parabola-ellipse pair provides a larger focal plane than fully shaped reflectors, designed for off-axis scanning. A reduced main reflector edge illumination design produces low side lobes and low noise temperature. The central portion of the secondary reflector is designed to reduce the Voltage Standing Wave Ratio (VSWR) and blockage, and henceforth increasing the aperture efficiency and gain of the antenna. The aperture diameter of the antenna is 64m (primary reflector, M1) and the main reflector focal length is 21.0567m. Consequently the paraboloid angle is about 74 degrees and the focal ratio is F1/D=0.33. All General information on the main reflector are reported in table 1. The secondary reflector is composed of 49 fix aluminium panels. The aperture of secondary reflector (M2) is 7.9 meters and it produces an optical system

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with a focal ratio of F2/D=2.35 at the Gregorian Focus.General information on the secondary reflector are report in table 2.

Primary Paraboloid Parameter

Aperture Diameter D 64 m

Distance from Gregorian Foci to Prime

Focus fe 17.4676 m

Main Reflector focal length fp 21.0567 m

Paraboloid half angle α 74.46 deg

Primary reflector Focal Ratio F1/d 0.32897

Table 2.1: Main Reflector parameters of SRT, [13]

Secondary Reflector Parameter

Aperture Diameter D2 7.9060 m

Distance from Main reflector Vertex to

Gregorian foci Bo 3.5560 m

Distance from Gregorian foci to Secondary

reflector vertex dfs 20.3200 m

Secondary reflector intercept half angle at

Gregorian foci Θ0 12.00 deg

Gregorian reflector Focal Ratio F2/d 2.3523

Table 2.2: Secondary Reflector parameters of SRT, [13]

SRT has other four focal position in additional to above cited primary and Gregorian focus. These new four focal positions are called "BWG" foci and can be namely F3, F4, F5 and F6. The first two focus positions, F3 and F4 are used only for radio astronomy applications, whereas the F5 and F6 focus positions are used for space applications. The focal positions F3 and F4, are obtained from the installation of other three ellipsoidal

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mirrors, M3, M4 and M5 into the BWG room, see figure 2. The focal ratios for the F3 and for F4 are: F3/D=1.37 and F4/D=2.81 respectively.

Figure 2.2: Sketch of Gregorian and BeamWave Guide room. In the picture there are signed the

Gregorian and BWG focal positions F2, F3 and F4. In the picture are indicated the three ellipsoid mirrors, M3, M4 and M5,[13].

The SRT is capable of hosting many microwave receivers, located in four different antenna focal positions – primary, secondary and two beam-waveguide foci – able to cover almost continually its the in operating frequency range, in the table 3 there are the plans for the arrangement of the receivers on the focus of the SRT. The SRT will operate in single-dish (continuum, full Stokes and spectroscopy), VLBI (Very Long Baseline Interferometry) [14] and Space Science [15] modes. Thanks to its large aperture and versatility (multi-frequency agility and wide frequency coverage) the SRT is expected to have a major impact in a wide range of scientific areas for many years to come. Here we illustrate some of the

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main areas where we think the SRT can play a major role in the next future.

Focus Position Fmin Fmax F/D ratio

Primary Focus (F1) 300 MHz 20 GHz 0.33

Gregorian Focus (F2) 7.5 GHz 116 GHz 2.35

BeamWave Guide (F3) 1.4 GHz 35 GHz 1.37

BeamWave Guide (F4) 1.4 GHz 35 GHz 2.81

Table 2.3: Different Focal Position, range frequency and focal ratio (F/D) of SRT, [13]

Operations in the framework of international VLBI and Pulsar Timing networks are of top-priority for the SRT. The SRT is one of the five telescopes of the European Pulsar Timing Array (EPTA), which, together with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA) share the goal to detect gravitational waves. Thanks to its dual-band L/P receiver, SRT will be of great importance in measuring accurate dispersion measure variations, crucial to obtain ultra-precision pulsar timing, and search for signatures of space-time perturbations in the pulsar timing residuals. The SRT is also part of the Large European Array for Pulsars (LEAP)[16], The SRT is expected to have a major impact also for single-dish observations. In particular we aim at exploiting its capability to operate with high efficiency at high radio frequency. Equipped with multi-feed receivers the SRT can play a major role in conducting wide-area surveys of the sky, which is poorly explored, yet very interesting. Due to an agreement between INAF and ASI on the use of the instrument for space applications, space debris and tracking of spacecraft.

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2.2 Radio Receiver

The goal of a radio astronomical receiver is to detect and measure the radio emission of the celestial sources [17-18-19]. The power level that the receiver must detect is very low, typically below the noise that the same receiver can be generate. The power level in the international system is indicated in “Watt” but in radio astronomy the level of power is indicated in Jansky. The Jansky unit is not an international unit but it is a typical astronomical unit. The name of this astronomical unit derived from the pioneer of radio astronomy, Karl G. Jansky. Its representation is [Jy]. The Jansky is a flux density of most radio sources is of the order of

10 . Commonly it is called a “flux unit”.

The radio receivers can be classified in two groups. The first group is called “Bolometer” they can be detected only the amplitude of the signal, whereas the second group is called “Coherent receiver”. The last one is capable to detect the amplitude and the phase of the signal. At the moment, on SRT there are only coherent receivers and, in the future, there are not in the plane other type of the receivers. A typical configuration of the “coherent receiver” is the super heterodyne configuration, shown in Figure 5.

Figure 2.3: Block diagram super heterodyne receiver

FEED RF DPS marker injector OMT LNA BPF MIXER AMP OL 1

Radio Frequency Intermediate Frequency

IF

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The sketch of Figure 5 is general, and it can be modified to realize the specific observations. The first device of the receiver, i.e. the feed or horn, detects the free RF signal coming from the antenna (for example from SRT), which propagates through a transmission line (typically a waveguide) towards the, Marker Injector, the Differential Phase Shifter, and the OrthoMode Transducer. The subsequent device of the receiver chain (Fig. 5) is the Low Noise Amplifier (LNA), which is crucial in terms of gain but mainly in terms of noise. In modern radio astronomy receivers, all these devices (except the Feed) are refrigerated at cryogenic. The RF signal coming from the LNA is then received by the mixers and amplifiers, which operate at room temperature. Inside the mixer the RF signal is convoluted with a local oscillator and the output signal is shifted in a new frequency range called “Intermediate Frequency” (IF), which is the lowest frequency range. At the output of the receiver there is the backend, which works at IF frequency. The backend are device that analyse the polarization time structure or spectral properties of the signal that arrive from the receiver. The trend has been toward commercial digital components for all type of backend. [20]. The radio receiver must provide several functions:

- high gain (almost 100 dB);

- very wide frequency range (around 40%);

- low system noise, Tsys (depends on the frequency range);

- high sensitivity - high stability;

- low Intermodulation distortion; - high compression gain;

In particular the sensitivity and the system noise are parameters that characterize the ability to process very weak signals.

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2.2.1 Sensitivity and stability in radio receiver

The simplest definition of Sensitivity is: the minimum detectable signal from the system receiver and its expression is:

∆ =

∙ [ ]

Where:

∆ = minimum detectable signal [K];

= integration time [s];

From the practical experiences the ∆ must be less 10 mK.

To arrive at a very low value of sensitivity is possible work on three principal parameter of the receiver, system noise, integration time and bandwidth. Typically, to have a good sensitivity we increase the

integration time tIF, but is necessary to take care on the another

fundamental parameter of the receiver: the stability. Indeed, if the integration time is long the gain fluctuation of the amplifier or the mechanical vibration can change the stability. Whereby, is necessary to put in the formula of the sensitivity the stability:

∆ = ∙ 1_{∙ +} ∆ + ∆

Where:

∆ and G are respectively the fluctuation gain (or loss) and the gain of the amplifier or the losses of the passive components;

∆ and T are respectively the fluctuation temperature (in the time) and the physical temperature of the devices;

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To minimize the sensitivity is necessary to maximize the integration time

and/or bandwidth and/or minimize the system noise (Tsys).

The Antenna Temperature TA is a parameter that describes how much

noise an antenna produces in a given environment, usually the value is between 10 and 100 Kelvin, depending on the frequency range from antenna and receiver [21]. The signal from the radio source can be less 1 K than is necessary that the receiver has a very good sensitivity

2.2.2 System Noise

The system noise (or system temperature) is the noise that the receiver

add to the detected signal, Tsys [22-23]. Radio astronomical receiver

detect very weak signals but the noise added by components tends to obscure those very weak signals. The designer has the directly control of the system noise of the overall system when he known the gain and the noise of the single components. The sources noise is principally derived from thermal noise and the parameter that describe the noise of a passive and active device is the Noise Figure (F). The noise figure of a network to be the ration of the signal to noise at the input to the signal to noise at the output, the equation is:

=

In summary, the noise figure F of a DUT (Device Under Test) is the degradation in the signal to noise ratio. In radio astronomy the noise

temperature Te is used to describe the noise performance of a device

rather than the noise figure. Quite often temperature units are used for device. Noise temperature is the equivalent temperature of a source

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__________________________________________________________________________________

impedance into a perfect device that would produce the same added noise.

= [ ]

The Noise Figure and the noise temperature are linked from the relationship:

= − 1 ℎ = 290

Usually the noise temperature is expressed in dB:

= 10 ∙ log [ ]

The receiver can be considered as a series connection of “n” devices:

Figure 2.4: Noise system for a cascaded network.

The temperature system Tsys of the “n” different blocks connected in

series is expressed by the allow equation:

= + + _∙ + ⋯ + _∙ _{∙ ⋯ ∙}

The noise figure of the system is expressed by the follow equation:

= + − 1+ − 1_∙ + ⋯ + _{∙ ⋯}− 1

Te1

G1

Te2

G2

Te3

G3

Ten

Gn

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2.2.3 Cryogenic System

The cryogenic system is a fundamental part of the radio astronomical receiver [24-25]. As discussed in the previous section the sensitivity of the receiver is very important to detect a weak signal. To improve the sensitivity is necessary to reduce the system noise, and the system noise depends on the losses (for passive component), or on the noise figure (for active devices), but both depend from their physical temperature. Therefore, in order to improve the sensitivity and reduce the system noise, we need to reduce the operating temperature of each component. The noise temperature of an attenuator is:

= − 1 ∙

Where TPT is the physical temperature of the attenuator.

The cryogenic system employed in a radio receiver is composed from two big parts, the vacuum system and the cooling system.

The vacuum system is composed by several components:

•_{two vacuum pump: the first one is called primary pump and it creates}

a low level of vacuum, about 10-1 _{mBar, whereas the second pump is}

called molecular pump and it creates a middle level of vacuum, about

10-4_mBar;

•_{One sensor to measure and control the level of vacuum;}

•_{One vacuum chamber, where the system creates the vacuum and}

wherein all the devices that form the RF chain are arranged. A fundamental property of the vacuum chamber (dewar) is that it has a hole where the radiation go through, and this hole is called vacuum window;

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__________________________________________________________________________________

The cooling system is composed by a cryogenic compressor and a cryocooler. These two instruments are linked by two pipelines filled with liquid helium. The cryocooler uses the "Gifford-McMahon" type [24-25] to lower the temperature inside the dewar.

The aim of the system described above is to cool the devices housed inside the dewar to decrease their noise temperature. In order to optimize the operation of the cooling, we need to minimize the heat transfer from the stage at room temperature to the stage at cryogenic temperature.

The thermal load is generated by three physical phenomena: - Convection ;

- Radiation; - Conduction;

The total heat transfer can be estimated from the follow expression:

= + +

To reduce the total heat transfer is necessary to reduce (ideally cancel) the three single phenomena.

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2.3 Reference

[6] GAVRIL GRUEFF; GIOVANNI ALVITO; ROBERTO AMBROSINI; PIETRO BOLLI;

ANDREA MACCAFERRI;GIUSEPPE MACCAFERRI;MARCO MORSIANI;LEONARDO

MUREDDU; VINCENZO NATALE; LUCA OLMI;ALESSANDRO ORFEI; CLAUDIO

PERNECHELE; ANGELO POMA; IGNAZIO PORCEDDU; LUCIO ROSSI;GIANPAOLO

ZACCHIROLI,“SARDINIA RADIO TELESCOPE: THE NEW ITALIAN PROJECT”,SPIE

5489, GROUND-BASED TELESCOPES, (28 SEPTEMBER 2004);

DOI: 10.1117/12.550332;

[7] J. BRAND, K.H. MACK, I. PRANDONI, “SCIENCE WITH THE SARDINAI RADIO

TELESCOPE”,MEMORIE DELLA SOCIETÀ ASTRONOMICA ITALIANA,BOLOGNA

10-11,2005;

[8] GIANNI TOFANI; GIANNI ALVITO; ROBERTO AMBROSINI; PIETRO BOLLI;

CLAUDIO BORTOLOTTI; LOREDANA BRUCA; FRANCO BUFFA; ALESSANDRO

CATTANI; GIANNI COMORETTO; ANDREA CREMONINI; LUCA CRESCI; NICHI

D'AMICO;GIAN LUIGI DEIANA;ANTONIETTA FARA;LUIGINA FERETTI;FRANCO

FIOCCHI; ENRICO FLAMINI; FLAVIO FUSI PECCI; GAVRIL GRUEFF; GIUSEPPE

MACCAFERRI;ANDREA MACCAFERRI;FRANCO MANTOVANI; SERGIO MARIOTTI;

CARLO MIGONI;FILIPPO MESSINA; JADER MONARI;MARCO MORSIANI;MATTEO

MURGIA; JOSÉ MUSMECI; MAURO NANNI; VINCENZO NATALE; ALESSANDRO

NAVARRINI;MONIA NEGUSINI;RENZO NESTI; LUCA OLMI; ALESSANDRO ORFEI;

ANDREA ORLATI;FRANCESCO PALLA;DARIO PANELLA;CLAUDIO PERNECHELE;

SALVATORE PILLONI; TONINO PISANU; ANTONIO PODDIGHE; MARCO POLONI;

ANGELO POMA; SERGIO POPPI;IGNAZIO PORCEDDU;ISABELLA PRANDONI;JURI

RODA;MAURO ROMA;PIERGUIDO SARTI;ALESSANDRO SCALAMBRA;FRANCESCO

SCHILLIRÒ; ANDREA TARCHI; GIAN PAOLO VARGIU;GIAMPAOLO ZACCHIROLI,

“STATUS OF THE SARDINIA RADIO TELESCOPE PROJECT”,SPIE7012,GROUND

-BASED AND AIRBORNE TELESCOPES II, 70120F (10 JULY 2008);

DOI: 10.1117/12.790503;

[9] AMBROSINI, R. ,BOCCHINU, A. ; BOLLI, P. ; BUFFA, F. ; BUTTU, M. ; CATTANI,

A. ; D'AMICO, N. ; DEIANA, G.L. ; FARA, A. ;FIOCCHI, F. ; GAUDIOMONTE,

F. ; MACCAFERRI, A. ; MARIOTTI, S. ; MARONGIU, P. ; MELIS, A. ; MELIS,

G. ;MIGONI, C. ; MORSIANI, M. ; NANNI, M. ; NASYR, F. ; NESTI, R. ; ORFEI,

A. ; ORLATI, A. ; PERINI, F. ;PERNECHELE, C. ; PILLONI, S. ; PISANU,

T. ; POLONI,M. ; POPPI,S. ; PORCEDDU,I. ; RIGHINI,S. ; RODA,J. ;SCALAMBRA,

A. ; SCHIRRU, M.R. ; SERRA, G. ; STRINGHETTI, L. ; TROIS, A. ; TUVERI,

A. ; VALENTE, G. ; VARGIU, G. ; ZACCHIROLI, G., “THE SARDINIA RADIO

TELESCOPE:OVERVIEW AND STATUS“,,"IEEEINTERNATIONAL CONFERENCE ON

ELECTROMAGNETICS IN ADVANCED APPLICATIONS (ICEAA 2013), (TORINO,

ITALY),SEPTEMBER 9-13,2013.;

[10] AMBROSINI,R.;A.;BOCCHINU,A.;BOLLI,P.;BUFFA,F.;BUTTU,M.;

CATTANI,A.;D'AMICO,N.;DEIANA,G.L.;FARA,A.;FIOCCHI,F.;

GAUDIOMONTE,F.;MACCAFERRI,A.;MARIOTTI,S.;MARONGIU,P.;MELIS,A.;

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__________________________________________________________________________________

A.;ORLATI,A.;PERINI,F.;PERNECHELE,C.;PILLONI,S.;PISANU,T.;POLONI,

M.;POPPI,S.;PORCEDDU,I.;RIGHINI,S.;RODA,J.;SCALAMBRA,A.;SCHIRRU,

M.R.;SERRA,G.;STRINGHETTI,L.;TROIS,A.;TUVERI,A.;VALENTE,G.;

VARGIU,G.;ZACCHIROLI,G."COMMISSIONING OF THE SARDINIA RADIO

TELESCOPE IN ITALY:RESULTS AND PERSPECTIVES", GENERAL ASSEMBLY AND

SCIENTIFIC SYMPOSIUM (URSIGASS),2014XXXITH URSI, ON PAGE(S):1–

1;

[11] P.BOLLI,A.ORLATI,L.STRINGHETTI,A.ORFEI,S.RIGHINI,R.AMBROSINI,

M.BARTOLINI,C.BORTOLOTTI,F.BUFFA,M.BUTTU,A.CATTANI,N.D’AMICO,

G.DEIANA,A.FARA,F.FIOCCHI,F.GAUDIOMONTE,A.MACCAFERRI,S.

MARIOTTI,P.MARONGIU,A.MELIS,C.MIGONI,M.MORSIANI,M.NANNI,F.

NASYR,A.PELLIZZONI,T.PISANU,M.POLONI,S.POPPI,I.PORCEDDU,I.

PRANDONI,J.RODA,M.ROMA,A.SCALAMBRA,G.SERRA,A.TROIS,G.

VALENTE,G.P.VARGIU,G.ZACCHIROLI,“SARDINIA RADIO TELESCOPE:

GENERAL DESCRIPTION,TECHNICAL COMMISSIONING AND FIRST LIGHT”,

JOURNAL OF ASTRONOMICAL INSTRUMENTATION,VOL.4,NOS.3&4(2015)

1550008#.C WORLD SCIENTI¯C PUBLISHING COMPANY DOI:

10.1142/S2251171715500087;

[12] ACTIVE SURFACE SYSTEM FOR THE NEW SARDINIA RADIOTELESCOPE,

ALESSANDRO ORFEI; MARCO MORSIANI; GIAMPAOLO ZACCHIROLI; GIUSEPPE

MACCAFERRI;JURI RODA;FRANCO FIOCCHI,PROC.SPIE5495,ASTRONOMICAL

STRUCTURES AND MECHANISMS TECHNOLOGY, (29 SEPTEMBER 2004);

DOI:10.1117/12.548944;

[13] VERTEX,“64-METER SARDINIA RADIO TELESCOPE FINAL DESIGN REPORT

OPTICS AND DESIGN”SYSTEMS DIVISION,SANTA CLARA FACILITY;

[14] CARLO MIGONI, ANDREA MELIS, ANTONIETTA FARA, R. AMBROSINI, M.

BARTOLINI,P.BOLLI,M.BURGAY,P.CASTANGIA,S.CASU,G.COMORETTO,R.

CONCU,A.CORONGIU,N.D'AMICO,E.EGRON,A.FARA,F. GAUDIOMONTE,F.

GOVONI,D.GUIDETTI,N.IACOLINA,F.MASSI,M.MURGIA,A.ORFEI,A.ORLATI,

A.PELLIZZONI,D.PERRODIN,T.PISANU,S.POPPI,I.PORCEDDU,I.PRANDONI,R.

RICCI, A. RIDOLFI, S. RIGHINI, C. STANGHELLINI, A. TARCHI, C. TIBURZI, A.

TROIS,V.VACCA,G.VALENTE, AND A.ZANICHELLI,“VLBI OBSERVATIONS WITH

THE SARDINIA RADIO TELESCOPE:HARDWARE &SOFTWARE IMPLEMENTATION”,

REPORT N.42, RELEASED:05/12/2014REVIEWER:MARCO BUTTU;

[15] T.PISANU,R.AMBROSINI,E.EGRON,E.FLAMINI,N.IACOLINA,S.MARIOTTI,

G.MARIOTTI,P.MARONGIU,A.ORLATI,P. ORTU,A.PELLIZZONI,M. PILI,A.

SCALAMBRA, P.TORTORA, J.RODA,E. URRU, G.VALENTE,”INSTALLATION ND

CHARACTERIZATI ON OF AN X-KA RECEIVER ON THE SARDINIA RADIO

TELESCOPE, CONFERENCE:36TH ESAANTENNA WORKSHOP ON ANTENNAS AND

RFSYSTEMS FOR SPACE SCIENCE,AT NOORDWIJK,THE NETHERLANDS,2015;

[16] D. PERRODIN, R. CONCU, A. MELIS, C. BASSA, R. KARUPPUSAMY, R.

AMBROSINI,M.BARTOLINI,M.BURGAY,P.BOLLI,M.BUTTU,P.CASTANGIA,S.,

CASU, G. COMORETTO, A. CORONGIU, N. D'AMICO, E. EGRON, A. FARA, F.,

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MURGIA,F.NASIR,A.ORFEI,A.ORLATI,A.PELLIZZONI,T.PISANU,S.POPPI,I.

PORCEDDU,I.PRANDONI,R.RICCI,A.RIDOLFI°,S.RIGHINI,C.STANGHELLINI,

A.TARCHI,C.TIBURZI,A.TROIS,V. VACCA,G.VALENTE, AND A.ZANICHELLI,

“LEAP PROJECT AT SRT: HARDWARE, SOFTWARE AND IMPLEMENTATION”,

REPORT N.39, RELEASED:17/09/2014REVIEWER: A.POSSENTI;

[17] JOHN D. KRAUS, “RADIO ASTRONMY”, MCGRAW- HILL BOOK COMPANY,

1966;

[18] THOMAS WILSON,KRISTEN ROHLFS,SUSANNE HUETTEMEISTER “TOOLS OF

RADIO ASTRONOMY”(ASTRONOMY AND ASTROPHYSICS LIBRARY) 6TH ED.2014

EDITION,SPRINGER;

[19] DAVID M. POZAR, “MICROWAVE ENGINEERING”, WILY, JOHN WILEY &

SONS,INC,2005;

[20] ANDREA MELIS;GIUSEPPE VALENTE;ANDREA TARCHI;MASSIMO BARBARO;

RAIMONDO CONCU; ALESSANDRO CORONGIU; FRANCESCO GAUDIOMONTE;

CARLO MIGONI; GIORGIO MONTISCI;SERGIO POPPI; ALESSIO TROIS, “AN

INFRASTRUCTURE FOR MULTI BACK-END OBSERVATIONS WITH THE SARDINIA

RADIO TELESCOPE”, PROC. SPIE 9153, MILLIMETER, SUBMILLIMETER, AND

FAR-INFRARED DETECTORS AND INSTRUMENTATION FOR ASTRONOMY VII,

91532M(23JULY 2014); DOI: 10.1117/12.2056576;

[21] CORTÈS, G., “ANTENNA NOISE TEMPERATURE CALCULATION” SKA

TECHNICAL MEMO SERIES,OCTOBER 2004;

[22] AGILENT, “FUNDAMENTALS OF RF AND MICROWAVE NOISE FIGURE

MEASUREMENTS “,APPLICATION NOTE 57-1;

[23] AGILENT, “FUNDAMENTALS OF RF AND MICROWAVE NOISE FIGURE

MEASUREMENTS “,APPLICATION NOTE 57-1;

[24] GEORGE BEHRENS WILLIAM CAMPBELL DAVE WILLIAMS STEVEN WHITE ,

“GUIDELINES FOR THE DESIGN OF CRYOGENIC SYSTEMS “,ELECTRONICS DIVISION

INTERNAL REPORT NO.306MARCH 1997;

[25] J.G. WEINSED II, “HANDBOOK OF CRYOGENIC ENGINEERING” TAYLOR

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Coaxial LP Cryogenic Receiver 21

__________________________________________________________________________________

C

HAPTER

3

C

OAXIAL

LP

C

RYOGENIC

R

ECEIVER

3.1 Introduction

The aim of this chapter is to present the detailed electromagnetic and cryogenic architecture of the coaxial dual frequency LP band receiver. In the first part, we present the principal scientific goal of the instrument. After, we describe the EM architecture of the receiver. The chapter goes on with the discussion of the electromagnetic simulation and the final measurement of each microwave component. In the last part, we talk about the cryogenic architecture

3.2 Scientific Aim

One of the main observational activities [26], like Pulsar Survey, requires an accurate knowledge of the ionosphere dispersion. To model this, a simultaneous observation at two different and far-between frequencies is required. When performing high precision timing it is important to account for the variable (on time scales longer than few days) effects of the interstellar medium along the line-of-sight to the source. To properly remove this contribution, one has to accurately measure the delay between the times of arrival of the same pulse at two well separated RFs. The dual frequency is tailored for this task, allowing to halve the required time to obtain a high precision timing point (i.e. a time of arrival of a pulse) for a target, which in turn, implies the possibility to double the number of useful observations for the given target (in an assigned

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22 Coaxial LP Cryogenic Receiver __________________________________________________________________________________

telescope time). Other scientific activities will require multi frequency observation in order to reduce observing time. Of course, two different, but coincident and collimated beams are required; this calls for a coaxial displacement of the illuminators. Due to the fact that the two illuminators are close and each one may disturb the other one, the study should take care of the efficiency maximization, for beam symmetrization, cross-polarization and so on. Furthermore, the receiver can be used to observe a radio source with a single band: P or L band in circular or linear polarization. The VLBI observations require L band and circular polarization, RHCP and LHCP (i.e. right hand circular polarization and left hand circular polarization). On the contrary, the PULSAR observations require linear polarization, vertical and horizontal , and P and L band concurrently.

The bandwidth of the receiver has been chosen after astronomical considerations but also primarily after Radio Frequencies Interference measurement around the Sardinia Radio Telescope site [27-28]. The RFI monitoring is constant [29]. Indeed, at the start of project the P band has to cover the frequency range 305-425 MHz, but after several RFI campaigns, we decided to narrow down the band. That is why the P band cover the frequency range 305-410 MHz.

3.3 Coaxial dual frequency LP receiver

The cryogenic coaxial dual frequency is one of three receiver of first light of Sardinia Radio Telescope [30]. It has been installed on the primary focus of the antenna. The receiver is a coherent receiver than can detect the amplitude and the phases of the RF signal. This receiver is particular for several aspects. One of this is the absence of the mixer section. Indeed the two RF band of the receiver are inside the generic intermediate

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__________________________________________________________________________________

frequency (IF) of the radio telescope. The IF of SRT covers the band between 0.1 to 2.1 GHz whereby it is not necessary to shift the RF at lower frequency. In the next section, we will describe the architecture of the coaxial receiver. In the following paragraphs, we illustrate the EM performance of each component of the front-end section, with details on the devices that we have developed.

3.3.1 Architecture of the Rx-LP

In this part, we illustrate a feasibility study of a cryogenic coaxial dual frequency radio receiver. The main characteristic of such receiver is the possibility to detect two different radio astronomical bands at the same time in order to have an accurate knowledge of the ionosphere dispersion. Another possibility of the receiver is to detect both linear and circular polarization to serve the pulsar and VLBI observations. The receiver must be installed on the primary focal position. For that reason, the receiver must meet strong constraints (for example, both a dimension limit and a weight limit).

The principal features are the following [30]:

•_{Frequency range: 305-425 (λ=90cm) MHz and 1.3-1.8 (λ =20cm) GHz}

simultaneously;

•_{Polarization: linear and circular double polarization for each bands;}

•_{Edge Taper: -13dB @74 degrees;}

•_{Dimensions of the receiver: a cube of 1.5 metre for each side;}

•_{Weight: max 700 Kg;}

•_{Cross-polarization: < -35 dB}

•_T_sys_{: P band < 20 K and L< 10 K;}

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•_{Insertion loss of each components: low as possible;}

•_{Possibility to have a selectable narrow band for VLBI observations.}

Some of these specifications must be reviewed critically from the designer, because the Radio Frequency Interferences (RFIs), dimensional and weight constrains can be changed compared to the previous targets. To achieve the performance listed above the architecture of the receiver is shown in Figure 1. This figure shows the principal components that compose the front-end path.

Figure 3.1 shows the estimated length, along the antenna axis, of the whole receiver, 'a=1.5' meter, whereas the length of the coaxial feed is about 1 meter (~1m).The width of the box will be 1.5 meter (b=1.5m). In the right of the picture (see figure 1), we show the coaxial feed (0) that converts the free electromagnetic wave (in P and L band ) into a guided wave. This part works at room temperature (300K). In the left of the picture we shown the remaining components: the vacuum window and thermal gaps (1), the Ortho mode Junction (OMJ, 2), the 180° hybrid (3), the commercial switch (4), the directional coupler (5) and the low noise amplifier (LNA, 6). The vacuum window and the thermal gaps works at 300 and 70 K, whereas the rest of the components work at 20 K. The components that work into the P band are: the coaxial orthomode junction (7): it is inside the coaxial feed and is arranged outside the dewar (300K is its physical temperature), 180° hybrid directional coupler (8), the commercial switch (9), the band pass filter (10) and, finally, the low noise amplifier (LNA, 11). These four last components work at cryogenic temperature (20 K). The output signal from the dewar goes to the hot part of the receiver, which is composed by distinct sections: noise calibration and antenna unit injection (this section is the same for both bands), L and P filter selection, and L and P linear to

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__________________________________________________________________________________

circular polarization. From the setting of the sections it is possible to choose the correct configuration, narrow band and the correct polarization for VLBI observations, or send the reference signal to calibrate the receiver. The block diagram of the whole receiver is shown in Figure 2.

Figure 3.1 Front-end architecture of the coaxial dual frequency LP band receiver. The picture put

emphasis on the generic dimensions and the temperature. The colors indicate the cryogenic temperature, orange indicates the 80 K stage, blue indicates the 20K stage, other parts work at room temperature (lilac).

At the start of the project we have investigated lot of configurations, in particular, the coaxial feed and the Ortho Mode Transducers for the L band [30]. All these studies were necessary because the dimensional constraints and the contemporarily detection of the L and P band are complicate to obtain. The dimensional constrain are tightened since the wavelengths are long: in particular, for the P band, the wavelength is about 84 cm at the middle frequency (357 MHz).

For the feed we take account three different configurations:

7/16 connector 7/16 connector 300 K RF signal P-band RF signal P-band Pol. 1 -3dB, 0° Pol. 1 -3dB, 180° RF signal L-band (a) (l) (b) 1 2 3 4 5 6 8 9 10 11 0 20 K

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The first one is based on the Logarithmic Periodic Dipole Antenna (LPDA) [31]. The choice of this configuration is suggested at the low frequency in order to reduce the dimension, that in waveguide technologies is very large. However, in this case the design specifications on the dimension and the performance are not met. Another solution is

to use a dipole [32] but also this solution isnot satisfactory, since it does

not meet the basic requirements, as the gain and the cross polarization. The last configuration investigated has been the coaxial feed [33]. It is based on two concentric waveguides. The dimensions of the structure are proportionate to wavelength. Indeed, at 300 MHz the wavelength is about 1 m. Consequently, the dimensions of the coaxial feed are border the geometrical limit of the receiver. On the other hand, the first EM studies achieve acceptable performance, for example the return loss better than 10 dB, cross polarization better than -25dB. We have chosen the last configuration.

The second component that was originally investigated is the Ortho Mode Transducer for the L band. After different study we have selected a turnstile configuration [34]. This configuration assures a very good electrical performance but it is too big and heavy. Therefore it is not possible to install it inside the stage at 20 K. Therefore, we have studied a new configuration of OMT for the L band. Instead, circular waveguide to coaxial cable transition with a very compact 180° hybrid. We have, moreover, developed a compact directional coupler for L and P band. Finally, we have selected all the devices of the hot part to have a sufficient gain and the possibility to choose the correct polarization and filter. The details of the hot part are described in the next paragraph.

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__________________________________________________________________________________

Figure 3.2: The Block diagram of the whole Coaxial Dual Frequency LP Band Receiver.

L -b a n d V -p o l fro m c o ld R x L -b a n d H -p o l fro m c o ld R x BP F 310 -3 5 0M H z BP F 305 -4 1 0M H z A ll b a n d BP F 3 10 -3 5 0M H z B P F 3 05 -4 10 M H z A ll b a n d P -b a n d F ilt e r S e le c to r P -b a n d L in e a r t o C ir c u la r P o la riz e r 9 0 H y b rid 0 F ilte re d P -b a n d V o r R H C p o l F ilte re d P -b a n d H o r L H C p o l L -B a n d O M J @ 2 0 K L -P B a n d c o a x ia l f e e d @ 3 0 0 K L -b an d 1 8 0 h y b rid w ith in teg ra ted B P F @ 2 0 K 0 L -b an d -2 6 dB D ir. C o u p l., @ 2 0 K L-ba n d -2 6 d B D ir. C o u p l., @ 2 0 K L -b a n d L N A @ 2 0 K L -b a nd L N A @ 2 0 K L -ba n d H a nd V p o l. P -b an d H a n d V p o l. P -b a n d H an d V p o l. P -b a nd 1 8 0 hy b rid w ith in te g ra te d -2 6 d B d ire c tio na l c ou p le r, @ 20 K 0 P h . ba l. co a x , co p p er P -b a n d H TS B P F , @ 2 0 K P -b a n d H T S B P F , @ 2 0 K P -ba n d L N A 2 0 K P -ba n d L N A @ 2 0 K P h . ba l. co a x , c o p pe r P h . ba l. co a x , co p pe r P h . ba l. co a x , s . s te e l P h . ba l. co a x , s. s te e l P h . b a l. co a x , co pp e r D e w a r @ 2 0 K P h . b al . lo w lo ss c oa x, c o p p er P -b a n d V po l -3 d B , 1 8 0 0 P -b a n d V p o l -3 dB , 00 P -b a n d H po l -3 dB , 0 0 P -b a n d H po l -3 d B , 1 80 0 P -ba n d , V po l, 0 dB P -b a n d , H p o l, 0 d B P -b a n d , V p o l, 0 dB P -ba n d , H p o l, 0 d B L -b an d , H po l, 0 d B L -b a nd , V po l, 0 dB P -B a n d 9 0 °H y b rid P o w e r d iv id er fo r L a n d P b a n ds 300 K L -ba n d 9 0 ° H y b rid B ro a d b an d N o ise G en . L + P b a nd s 3 00 K P h a se b a lan c e d c o a x c ab le s , c o pp e r L -b an d , H p o l, 0 d B L -ba n d , V po l, 0 dB A ttn . 1 3 0 0 K A ttn . 2 3 0 0 K A ttn . 3 3 00 K A ttn . 4 3 0 0 K P ow er d iv id e r fo r L an d P ba n d s 3 00 K A n te n na U n it 3 0 0 K S . s te e l co a x S . s te e l co a x S . s te e l coa x S . s te e l co a x P -b a n d Is o la to r P -b an d Iso la to r P -b an d Is o la to r A m p . 30 0 K P -ba n d Is o la to r P -b a n d V -p o l fro m c o ld R x P -b a n d H -p o l fr o m c o ld R x P ha s e ba la nc e d co a x ca b le s, co p p er N o is e c a lib ra tio n a n d A n te n n a U n it in je c tio n L oad P -b an d 1 80 h y b rid w ith in te g ra te d -2 6 dB d ire c tion a l co u p le r, @ 2 0 K 0 L o a d L -b a nd 1 8 0 hy b rid w ith in teg ra te d B P F @ 2 0 K 0 Load Load Lo a d K & L 5 B 3 40 -3 3 0 /T 5 0 -O /O K & L 5B 3 40 -3 5 7 .5 /T1 2 0 -O /O K & L 5 B 3 4 0 -3 30 /T 5 0 -O /O K & L 5 B 34 0 -3 57 .5 /T 1 2 0 -O /O A g ile n t 8 71 0 4 A S P 6 T C oa x ia l S W A g ilen t 8 7 1 04 A S P 6T C o a x ia l S W S P 6 T C oa x ia l S W S P 6T C o a x ia l S W A g ile n t 871 0 4 A A g ile nt 87 1 0 4 A S P D T C o ax ia l SW A g ilen t 8 7 62 A -T 2 4 S P D T C o a x ia l S W A g ile n t 87 6 2 A -T 2 4 S P D T C o ax ia l S W S P D T C o ax ia l S W A g ilen t 8 7 62 A -T 2 4 A g ilen t 8 7 6 2A -T 24 H Y B -5 3 09 -X 3 S M A -7 9 Z R L _ 7 00 Z R L _ 7 0 0 A m p . 30 0 K H Y B -5 3 0 9 -X 3 S M A -79 H Y B -53 1 1 -X 3 S M A -7 9 U T E -M ic ro w a v e C T -1 8 1 9 -N U T E -M ic ro w a v e C T -1 8 1 9 -N M C L I M ic ro w a ve P S 2 -1 0 9 coa x , cop p er co a x , co p p e r S w itch Ag ile nt @ 2 0K S w itch A g ile n t @ 2 0 K S w itch Ag ilen t @ 2 0 K S w itch Ag ile nt @ 2 0K Lo a d L o ad @ 20 K S . s te e l c o a x S . s tee l co a x S . s te e l c o a x S . s tee l c oa x L o ad @ 20 K L o a d @ 20 K Lo ad @ 20 K Load Lo ad S P D T C o a x ia l S W L -b a n d L in e a r t o C ir c u la r P o la riz e r 9 0 H y b rid 0 F ilte red L -ba n d V o r R H C po l F ilte re d L -b a n d H o r L H C p o l L -ba n d Is o la to r L -b a n d Is o la to r A m p . 3 00 K P h a se b a la n ced c o ax ca b le s , c op p e r S P D T C oa xia l S W Z R L _2 1 5 0 A m p . 30 0 K Z R L _ 2 15 0 A g ilen t 8 76 2 A -T 2 4 A g ile n t 8 7 6 2A -T 2 4 S P D T C oa xia l S W A g ilen t 8 76 2 A -T 2 4 S P D T C o a x ia l S W A g ilen t 87 6 2 A -T 2 4 H Y B -5 3 11 -X 3 S M A -7 9 U T E M ic ro w a ve C T -2 0 02 -O T U TE M ic row a ve C T -20 0 2 -O T A g ile nt 8 71 0 4 A B P F 16 25 -17 15 M H z B P F 13 50 -1 45 0 M H z B PF 1 30 0-1 80 0M H z A ll b a n d B P F 1 3 50 -1 45 0M H z B P F 16 25 -1 7 15 M H z B P F 1 3 00 -1 80 0M H z A ll ba n d L -b a n d F ilt e r S e le c to r L -b a n d Is o la to r L -b a n d Iso la tor B P F 13 00 -18 00M H z N otc h 1 7 90 -1 9 50 M H z N o tc h 12 50 -1 34 0 M H z S P 6 T C o a x ia l S W S P 6 T C o ax ia l SW A g ilen t 8 7 10 4 A A g ilen t 8 7 104 A S P 6 T C o a x ia l S W S P 6 T C o a x ia l S W A g ile n t 87 10 4 A K & L 4 B 1 2 1 -1 4 0 0 /T 1 2 0 -O /O K & L 4 B 1 2 1 -1 6 55 /T 1 2 0 -O /O K & L 5 B 1 2 0 -1 54 0 /T 5 2 0 -O /O N otc h 12 5 0 -1 3 40 M H z B P F 13 00 -1 80 0 M H z N otc h 17 90 -1 95 0 M H z K & L 6 N 4 5 -1 3 20 /E 6 2 .7 -O /O K & L 5 B 12 0 -1 5 40 /T 5 20 -O /O K & L 6N S 11 -1 8 8 0 /E 1 38 -O /O K & L 5 B 120 -1 5 40 /T 52 0 -O /O K & L 5 B 1 2 0 -1 5 4 0 /T 52 0 -O /O K & L 4 B 1 2 1 -1 4 00 /T1 2 0 -O /O K & L 4 B 1 21 -1 6 5 5 /T 120 -O /O K & L 6 N 4 5 -1 3 2 0 /E 6 2 .7 -O /O K & L 6 N S 1 1 -1 88 0 /E 13 8 -O /O

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3.3.2 P Band RF path

The Figure 3.3 depicts in detail all components that compose the P band RF front-end path. Shortly, the signal’s path is composed by a coaxial feed, which converts the free RF signal into a guided wave. The wave propagates inside the coaxial waveguide until the coaxial ortho modo junction (COMJ) where each polarization is splitted in two signals, equal in amplitude and out of phase of 180 degrees.

Therefore, the two signals arrive at the input of the dewar trough two distinct equiphase cables. The signals go into the dewar and are recombined with a 180° hybrid and, after, using a directional coupler. In order to save space we have decided to arrange into the same device both the 180° hybrid and the directional coupler. These two devices are realized on the Arlon AD1000 [35] substrate in microstrip technology. At the output of the directional coupler we have, also, inserted a commercial coaxial switch. This device is not necessary for the elaboration of the celestial source signal, but it is important for the calibration of the receiver. After that, we have inserted a microstrip band pass filter to reject the strong RFIs, and to preserve the linearity of the low noise amplifier. The filter is placed before the LNA. To reduce the noise temperature, the filter was designed in HTS technology (High Temeperature Superconducting).

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__________________________________________________________________________________

Figure 3.3: Sketch of the P band path. Figure is shown only one polarization.

P -b a n d H a n d V p o l. P -b a n d H a n d V p o l. P-band H pol -3 dB, 00 P-band H pol -3 dB, 1800 Teflon Andrew FSJ2-50 290 mm -0.03dB @300K @357 MHz R191329000

uS 180HYB + Directional Coupler 0.25 dB @20K @375 MHz R404212000 R404212000 Andrew F2TNM-PL R125771000 Switch Agilent 0.01dB @20K 357 MHz R404212000 R125703000 u S H T S B a n d Pa ss F ilte r 0 .1 dB @ 2 0 K , 3 5 7 M H z Lo w N o ise A m p lif ie r G a in 2 6 d B N F 0 .4 5 K @ 2 0 K Noise Generator e AU Signal Coaxial Cable 270 mm 0.13 dB @ 20K, 357 MHz Stainless steel Output Signal RF H-pol

Coaxial Cable Stainless steel 115 mm 0.05 dB @ 20K, 357 MHz R125055901 R125154901 SMA R125055901 R125771000 R185405270 Andrew F2PDR-C H&S34-N-50-0-3/133_N H&S 34_SMA-50-0-3/111_N Second Stage 20K R 125512000 R125771000 R125055901 R125705000 R125055000 R125512000 R125512000 R125512000 Cavo coassiale 165 mm 0.08 dB @ 20K, 357 MHz Stainless steel

Coaxial Cable Copper 410 mm 0.08 dB @ 20K, 357 MHz

±1.5

Coaxial Cable Stainless steel 200mm 0.09 dB @ 20K, 1.55 GHz

Coaxial Cable steanless steel 95 mm 0.04 dB @ 160K, @357 MHz

H&S 34_SMA-50-0-3/111_N Coaxial OrthoMode Junction

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3.3.3 P-Band Coaxial orthomode Junction

The Coaxial Ortho Mode Junction (COMJ) is a coaxial waveguide to coaxial probe transition. It is composed by: a coaxial waveguide and four probes. The fundamental mode of the coaxial waveguide is the TEM mode. As known, the higher-order modes can propagate when the frequency is higher than their cut-off (Table 3.1). However, when the coaxial waveguide is excited with two signals characterized by identical amplitude and 180° out of phase, the fundamental mode (TEM) cannot

propagate. Under those conditions the higher order mode TE11 is the only

one that can propagate and its wavelength at the central frequency (357.5 MHz) is about λg = 1100 mm.

Mode Cut-off frequency [MHz]

TEM 0

TE11 232

TE21 435.6

TE31 613.6

Table 3.1: Right: Cut-off frequencies of the coaxial waveguide. The first mode TE21 is outside of

the bandwidth of the receiver.

The two degenerus TE11 modes can propagate in the coaxial waveguide

as indipendent orthogonal linea polarization, H-pol and V-pol. In these condiction the COMJ has six electical ports with two inputs and four outputs ports. The scattering matrix that describes the functionality of this component is shown below:

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The design of the P-band COMJ is illustrated in Figure 3.4. It consists of a

coaxial waveguide with outer constant diameter equivalent to DP=650

mm (which is also the P-Band aperture), and a variable inner diameter (corresponding to the L Band waveguide). The inner diameter ranges

from the maximum value of DL1=196 mm at the back-short, to the

minimum DL2=171 mm at the feed’s aperture. In the middle, a conical

section matches those external parts. The length of each section is

respectively L1=576 mm, L2=220 mm and L3=120 mm (see Figure 3.4).

Additionally, the structure presents also four matching metallic cylindrical irises arranged along the optical axis of the feed and four metallic cylindrical probes orthogonal to the optical axis, and at 90 degree each other (see Figure 3.5, left panel and Figure 3.6). Two metallic irises are located near the aperture of feed: one is attached to the P-band waveguide, while the other is attached to the external L-band waveguide. Their outer diameters, inner diameters and distance from the aperture

are respectively: DO1=650 mm, DI1=480 mm and LI1=342.5 mm for the

first iris, and DO2=272 mm, DI2=196 mm and LI2=340.5 mm for the second

one (see Figure 3.4). Two other metallic irises with outer diameters

DO3=305 mm and DO4=250 mm are located respectively at LI3=177.5 mm

and LI4=250 mm from the planar back-short of the coaxial waveguide

(see Figure 3.4). Those irises are attached to the L-band circular coaxial

                                − − − = 2 1 0 2 1 0 2 1 0 0 2 1 0 2 1 0 2 1 2 1 0 2 1 0 2 1 0 0 2 1 0 2 1 0 2 1 2 1 0 2 1 0 0 0 0 2 1 0 2 1 0 0 S

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guide and designed to match the coaxial waveguide to coaxial line transition over a wide frequency band. All mechanical dimensions are shown in the left side of Table 3.2

Band Iris DOUTER [mm] DINNER [mm] Distance [mm]

P-Band

1 DO1= 650 DI1= 480 LI1= 342.5

2 DO2= 272 DI2= 196 LI2= 340.5

3 DO3= 305 DI3= 196 LI3= 177.5

4 DO4= 250 DI4= 196 LI4= 250

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Figure 3.4: Internal 3D views of coaxial dual-band LP receiver.

E x te rn al C o rr u g at io n s D L 1 D l2 D P L 1 L 3 L 2 D = 6 5 0 m m D = 4 8 0 m m L = 3 4 2 .5 m m o 1 i1 i1 D = 2 7 2 m m D = 1 9 6 m m L = 3 4 0 .5 m m o 2 i2 i2 D = 3 0 5 m m D = 1 9 6 m m L = 1 7 7 .5 m m o 3 i3 i3 D = 2 5 0 m m D = 1 9 6 m m L = 2 5 0 m m o 4 i4 i4 D = 1 6 5 m m D = 1 3 9 m m L = 1 3 5 .5 m m o 5 i5 i5 D = 1 6 5 m m D = 1 4 5 m m L i6 = 2 2 0 ,5 m m o 6 i6 L 4 R C 1 R c 2 R_c 3

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Figure 3.5: left) Arrangement of the probes inside the coaxial waveguide and of the two iris of

the transition; right) detail of the probe

Figure 3.6: Photo probes P band integrate inside the coaxial circular waveguide P band.

The two linear polarization signals (Pol.H and Pol.V) are extracted through four metallic coaxial probes connected to the central pin of

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commercial 50 Ω 7/16-type coaxial connector (Radiall model n. 185.406.270 [36]). The four identical probes are located at a distance of

LI3=177.5 mm (less than λg/4) from the back-short of the coaxial

waveguide. Each probe consists of four cylinders of different diameters

in axis with the coaxial connectors. The diameters are: D1=38 mm, D2=59

mm, D3=12 mm, D4=6.5 mm, and their lengths are respectively: L1=45

mm, L2=33 mm, L3=59 mm and L4=18 mm (see Figure 5, right panel).

To avoid mechanical microphonics problems due either to the radio telescope movement or to the cryogenic pump the inner part of the probes have been hollowed and fixed at one of the iris through robust

teflon supports (εr=2.01). This solution allows to reduce the probes

weight as well, see figure 3.7.

Figure 3.7: photo of the P band coaxial probe. At the left of the probe there is the Teflon

support. At the right of the probe there is the robust 7/16 connector[36].

The geometrical parameters described so far (geometry of the probes, their distance from flat back-short, position and dimension of the irises) have been changed to optimize the overall electromagnetic performance. The graph in Figure 8 shows the simulated reflection coefficient at the output coaxial ports of the COMJ. The simulation was made with the software 3D Ansys HFSS [37]. The simulated reflection is below -19.5 dB across the frequency band: 305-410 MHz. Ohmic losses were not included in the simulation. It is worthwhile mentioning that such a good

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L-band waveguide diameter. The cross polarization has been simulated with the schematic diagram, in figure 3.8. The block with four port is the COMJ whereas the other two blocks are the ideal 180° hybrid. The cross

polarization is simulated with the S21 parameter. The level is below the

limit of the accuracy of the simulator.

Figure 3.8: Schematic of the simulation of the cross polarization (S21 or S12) and the output

return loss (S11 or S22).

The COMJ has only been characterized for the output reflection loss. We estimate the insertion loss and the cross polarization after some theoretical considerations and some simulations.

In figure 3.9 is shown the measurement setup for the output return loss. The reflection from the coaxial output of the COMJ was obtained by terminating its coaxial waveguide input with an ecosorb load. The return loss has been measured with a Vector Network Analyzer (VNA

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[38-39-Coaxial LP Cryogenic Receiver 37

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40-41]), Agilent 8752C, which are connected at the output of the commercial 180° power splitter. The two inputs of this device are connected at the two outputs port of the device under test (DUT). The input of the DUT was loaded with an ecosorb before the feed. To obtain the correct measurement of the transition we need to include into the calibration of the instrument both the 180° power splitter and the cable. The 180° splitter was necessary to excite together the two output ports with a signal out of phase of 180°. The measured output reflection for one polarization is shown in figure 3.10. The reflection is below -19.5 dB across the whole P band. We note that the measured output reflection loss curve is very similar in shape and overall level to the one predict by simulator, see figure 3.9 dashed green curve.

Figure 3.9: Return Loss of Coaxial Ortho Mode Junction. Vertical dashed line delimit the P band.

The green dashed curve shows the simulation whereas the red continuum curve shows the return loss measured. The measured curve shows a match into to a very large ban

The losses of the COMJ may be compared with a losses of a simple coaxial structure where the dielectric is air. We estimate the losses of the coaxial waveguide and the losses of the connector equal to 0.05 dB.