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October 3, 2016 By Steve

A Cryogenic, Wide Band Receiver for the 500 m Aperture Spherical Telescope (FAST)

 

A Cryogenic, Wide Band Receiver for the 500 m Aperture Spherical Telescope (FAST)

 

Stephen Smith

Cosmic Microwave Technology, Inc, Lawndale, CA
steve@cosmicmicrotech.com

Sander Weinreb

California Institute of Technology, Pasadena, CA
sweinreb@caltech.edu

 

Abstract— A unique wide band receiver for radio astronomy has been designed, constructed and will be installed on the 500 m Aperture Spherical Telescope (FAST) in Guizhou, China. The receiver has a frequency range of 270 to 1620 MHz. Astronomical use of the receiver includes observing pulsars, continuum source surveys, galactic hydrogen line and extragalactic hydrogen with high red shift.

Keywords—Cryogenic; Wide band receiver; FAST

I. INTRODUCTION
A wide band receiver has been designed,

constructed and will be installed on the Five

hundred meter Aperture Spherical Telescope

(FAST) in Guizhou, China [1]. The front end of

the receiver consists of an uncooled Quad

Ridge Flared Horn feed (QRFH), and a cryostat

operating at 10 K. The cryostat includes SiGe

LNAs and cryogenic noise sources. The noise

sources are used for receiver gain calibration.

The warm electronic box consists of amplifiers,

filters, 5 bits digital attenuators, 90 degree hybrid used for polarization conversion, a

spectrum analyzer and fiber optic transmitters and receivers. Block diagram is shown in

Fig. 1.

II. DETAILED DESCRIPTION OF THE RECEIVER
The receiver consists of 4 subassemblies; the Quad Ridge Flared horn, the front end

cryostat and associated electronics, the back end warm electronics and the power supply.

The mechanical assembly is shown in Fig. 2. Design, construction and testing was

completed at Caltech.

Receiver specifications are described
• Frequency range 270 to 1620 MHz
• Receiver noise temperature < 15 K (measured

at the input to the cryogenics dewar)
• System noise temperature < 39 K
– Receiver = 15 K
– Feed = 6 K
– Spillover = 10 K
– Sky noise @ 1420 MHz = 8 K
• Receiver gain 65 dB typical
• P 1dB gain compression > -35 dBm referred to

the input
• Noise injection signal = 20 K
• System mass = 189 Kg
• Feed size = 1.45 m square x 1.2 m long
• Feed aperture efficiency > 60 % for F/D of 0.46
• Polarization-selectable as dual-linear or dual-circular
• Integrated spectrum analyzer- Rigol DSA832
• Total power detectors-Minicircuits ZX47-60+
• Monitor and control via ethernet through digital fiber link
• 2 GHz fiber optic transmitters for the RF link.

A. Antenna Feed

 

The QRFH feed is a dual polarized feed with

vertical and horizontal polarization. The feed

was designed at Caltech by Ahmed Akgiray as

part of his PhD thesis [2]. The antenna feed size

measures 1.45 m square at the face and is 1.2

m high. The design was optimized for constant

beam width over a 6:1 frequency band with low

phase center variation. The half angle beam

width at -10 dB, optimized for an F/D of 0.46, is

60 degrees. The predicted feed efficiency

across the entire frequency band is 60 % as

shown in Fig. 3.

The outputs of the feed connect to the inputs of the cryostat. Both the feed and the interconnecting cables are at a temperature of 300 K and therefore contribute 7-10 K to the system noise temperature. Low loss interconnections are vital to lowering the system noise temperature.

B. Cryogenic Dewar

 

                                                                              The cryogenic dewar is constructed from

                                                                              aluminum. The box measures 310 mm x 310

                                                                              mm x 254 mm. A key requirement of the design

                                                                              of all the electronics for the receiver is to allow

                                                                              for easy maintainability. The dewar has

                                                                              removable access panels on 3 sides and allows

                                                                              easy access to the internal electronics while

                                                                              mounted to the dish. The cooler is a CTI 350 2

                                                                              stage cooler. The first stage of the CTI cooler

                                                                              reaches a temperature of 52 K. Noise sources,

                                                                              directional couplers and a power divider are

                                                                              mounted on this stage. The second stage

                                                                              reaches a temperature of 11 K. Directional

                                                                              couplers, LNA’s and high pass filters are

mounted on this stage.

The antenna coax cables connect to the cold stage through a pair of “N” type hermetic connectors. Beryllium copper coax cables are used to bring the signal from the input connectors at a temperature of 300 K, to the directional coupler operating at

a temperature of 11 K. Signals from noise sources, mounted on the first stage at a

temperature of 50 K, along with comb generator calibration signals are coupled into the

front end of the receiver.

The output of the directional coupler is connected to a high pass filter. The filter has a cut

off of 250 MHz and is designed to reduce Radio Frequency Interference (RFI). The output of

the filter is connected to a Caltech designed LNA [3]. The cryogenic dewar with the side

panels removed is shown in Fig. 4.

C. Warm Electronics

                                                                                      The warm electronics box is an aluminum box

                                                                                      that is both environmentally and RFI shielded.

                                                                                      The box measures 605 mm x 503 mm x 250

                                                                                      mm. The warm electronics provides additional

                                                                                      RF amplification, filtering, digital controlled

                                                                                      attenuation, linear to circular conversion, fiber

                                                                                      optic transmitters, a spectrum analyzer and full

                                                                                      receiver control and monitors. Fig. 5 shows the

                                                                                      warm electronics box.

                                                                                      The design of the warm electronics box utilizes

                                                                                      field replaceable off-the-shelf components.

Each component can be removed and replaced with simple tools. The RF components are mounted onto a temperature controlled plate. A Thermal Electric Cooler (TEC) keeps the

plate and the electronics at a temperature of 40 °C.

The signals received from the dewar are further amplified, followed by a polarization

selection circuitry. A circular mode is derived from a linear mode by combining the vertical

and horizontal linear signals through a 90 degree hybrid. The astronomer can select linear

or circular polarization, depending upon the science required. Each polarization signal is

then split 4 ways. Two outputs are used as the main receiver. A 3rd output is connected to

a detector which produces a DC voltage proportional to the total RF power received by the system. For best performance, the receiver must operate within the linear range. Low DC voltages indicate the received signal has not been contaminated by RFI. The 4th output is connected to a 3.5 GHz spectrum analyzer. The observer can study the entire receiver

spectral response to identify RFI issues.

The pair of outputs used for the main receiver are further amplified. The levels of the

signals are controlled using 6 bits digital attenuators with a resolution of 0.5 dB per step.

Each signal is filtered. One channel is the wide band channel and has a low pass-high pass combination of filters for a frequency band of 270 – 1620 MHz. The second channel is

filtered with a band pass filter that has a frequency band of 1.3 to 1.62 GHz. The signals are amplified further. The RF output of the receivers drive 2 GHz fiber optic laser transmitters, designed by Caltech.

A mother board provides monitor and control signals for the receiver. Access to the

mother board and to the spectrum analyzer is accomplished through Ethernet. Monitor

signals include the receiver voltage and currents. The information measures the health of

the receiver and can provide invaluable information of diagnostics of receiver anomalies. Python is the software language written to control and monitor the receiver.

III. PERFORMANCE

 

Each component in the receiver and the completed receiver underwent extensive testing in the lab before installation onto the telescope. The LNA and the noise source components are described in the next sections followed by the system performance.

A. LNA’s

 

The LNA in the front end of the receiver is a

CITLF3, designed and manufactured by Caltech.

The CITLF3 is a Silicon Germanium (SiGe) LNA

designed for cryogenic applications. The

amplifier utilizes resistive feedback to achieve

good match (S11) and high gain stability. The

measured performance of the 2 amplifiers

used in the wide band receiver is shown in Fig.

6. The gain of the amplifier is 37-40 dB and the

noise temperature is 4-5 K.

B. Noise Sources

Each noise source is used as a secondary calibrator of system gain and can be used to

measure the system noise temperature. It is necessary to understand the fluctuations of

the receiver to determine the flux densities of radio sources. Radio telescopes use known galactic sources for the primary calibration and transfer the calibration to a stable noise

source. The usual implantation consists of a noise diode built into a temperature controlled

box, cabled through hermetic feed thrus into the cryostat, and coupled into the front end

of the receiver through a coupler operating a physical temperature of 15 K. Fluctuations in

the calibrations occur due to instabilities in the coax cables and feed thrus.

                                                                       A noise source has been built and tested that operates

                                                                       at 50 K. The noise source is mounted inside the dewar

                                                                       on the 1st stage (50 K). The 1st stage of a cryogenic

                                                                       cooler is robust and maintains a constant temperature.

                                                                       A single cable from the noise source on the 50 K stage

                                                                       connects to the receiver input located on the 2nd stage

                                                                       (11 K). The interconnect cables at 300 K are eliminated

                                                                       and reduces the variations due to changes in the coax

                                                                       cable. The hermetic SMA connection is eliminated. Fig. 7

                                                                       shows the response of the noise sources used in the

                                                                       FAST receiver.

 

C. System

 

                                                                           The performance of the receiver was measured in the

                                                                            lab at Caltech. A noise source calibrated to liquid

                                                                            nitrogen temperature was used for Y factor

                                                                            measurements. The measured noise temperature

                                                                            of the receiver is 5 K at 270 MHz and 15 K at 1.6 GHz.

                                                                            The gain of the front end is measured to be 35-40 dB. 

                                                                           Fig. 8 shows the performance of the front end.

                                                                   

                                                                   The gain

of the receiver was measured by connecting to

the input, a 50 ohm load at a temperature of

300 K. The output of the receiver was plotted

on a spectrum analyzer. The spectrum analyzer

resolution bandwidth was set to 100 KHz and

the Video bandwidth set to 1 KHz. The

attenuators of the receiver were set to 0 dB.

The total gain of the receiver was measured to

be 90 dB. The gain is shown in Fig. 9.

Spectrum analyzer resolution bandwidth = 100 KHz Gain = 174dBm – (34 dBm +

10log(100x 103) = 90 dB

IV. CONCLUSIONS

A unique wide band cryogenic receiver for the FAST telescope has been presented. The

receiver will be used to observe significant radio astronomy discoveries. The wide

bandwidth of the receiver is unique to the FAST telescope and increases the discoveries of galactic events including pulsars, continuum source surveys, galactic hydrogen line and extragalactic hydrogen with high red shift. As of writing this paper, the receiver has not

been installed on the telescope. The commissioning of the receiver on the telescope will

take place mid-year 2016.

ACKNOWLEDGEMENT

The authors would like to acknowledge Jin Chengin, Liu Hongfei and Cao Yang from Beijing Astronomical Observatory for contributions to the design and fabrication.

REFERENCES
[1] S. Weinreb, S. Smith, “The Detailed Design of the 270-1620MHz Receiver Frontend,” FAST Document FAST/JS017-2016-JZ-008, Feb 2016

[2] A. Akigiray, “New Technologies Driving Decade-Bandwidth Radio Astronomy: Quad-Ridged Flared Horn & Compound-Semiconductor LNA’s,” PhD Thesis, California Institute of Technology, 2013

[3] S. Weinreb, J. Bardin, H. Mani, and G Jones, “Matched Wideband Low- Noise Amplifiers for Radio Astronomy,” in American Institute of Physics, April, 2009.

Fig. 1. Block diagram to the wide band receiver
Fig. 2. Mechanical layout of the wide band receiver
Fig. 3. Feed efficiency
Fig. 4. Cryogenic dewar with side panels removed
Fig. 5 Warm electronics assembly
Fig. 6 measured performance of the CITLF3 LNA
Fig. 7. Measured performance of the noise source operating at 50 K
Fig. 8. Measured performance of the front end
Fig. 9. Measured gain of the receiver