Group for Advanced Receiver Development
(GARD)

Radio & Space Science Department

Chalmers University of Technology
S-412 96 Gothenburg, Sweden


LSA Site-Testing 183GHz Water Absorption Line Monitoring Radiometer


The 183 GHz radiometer is a project started in 1996 by Professor Roy Booth, Director of the Onsala Space Observatory. The Project is part of the site-testing efforts within the Large Southern Array project. During the two years of development, the following people have been involved in the Project:

Onsala Space Observatory (OSO), Chalmers University: Denis Urbain, Victor Belitsky and Roy Booth [roy(a)oso.chalmers.se]
OSO team would like to acknowledge valuable contribution made by our technician Thomas Andersson.

Mullard Radio Astronomy Observatory (MRAO): Pierre Martin-Cocher, Martina Wiedner and Richard Hills [richard(a)mrao.cam.ac.uk]

SEST/OSO: Guillermo Delgado [gdelgado(a)eso.org] (Guillermo significantly contributed to make this site more "readable").

European Southern Observatory (ESO-Chile): Angel Otarola [aotarola(a)eso.org] and Daniel Hofstadt.

Ferilas Designs Ltd, Edinburgh: Colin Hall [cgh(a)ednet.co.uk].


Introduction

Two radiometers for the 183 GHz water vapour line have been built as a collaborative project between Onsala Space Observatory, through the Group for Advanced Receiver Development at Chalmers University of Technology, and the Mullard Radio Astronomy Observatory (MRAO), Cavendish Laboratory, Cambridge. These radiometers are doubleside band heterodyne receivers with three detection bands spaced 1.2, 4.5, and 7.5 GHz away from the line centre. The three channel spectrum measures the water line shape and intensity.

Figure 1. Schematic of the 183GHz water vapor line showing the three bands of observation.

Development done at MRAO, Cambridge

MRAO pioneered the use of the 183 GHz water line radiometry to predict the atmosphere absorption (the original design was developed for atmospheric phase measurements on Mauna Kea). These previous designs from MRAO were used to build the optics and the control electronic in the two water vapour monitors. A beam steering mirror was added to the original optic design, to "scan the sky and look for structures in the water on a range on angular scale" (Richard Hills). Calibration of the receiver is done using two reference loads, one is kept warm (40 ºC) and the other one hot (90 ºC), a tipping mirror switches the beam between the sky and the loads. Pierre Martin-Cocher did the interface and control electronics with the supervision of Richard Hills. An embedded PC runs software written in C (Colin Hall) to control the radiometer and communicate the data to a host computer.

Development done in Chile

The Chilean team (Guillermo Delgado, Angel Otárola) was involved in the development of the communication software between the host PC and the data acquisition PC (placed in the container situated at the middle of the base line). LabView was used to write the program that sends the commands to the radiometer, and read back the data. A. Otárola and D. Urbain (GARD OSO) carried out all the installation work at the site.

Recently, G. Delgado and A. Otárola have started the data retrieving and processing(http://puppis.ls.eso.org/lsa/lsahome.html)

Development done at GARD OSO

GARD OSO was responsible for the general design and integration of the two radiometers. The RF/IF section of the radiometers, as well as part of the electronic was built in Chalmers. The frame and enclosure of the radiometer were design and assemble. The power supply units were also part of the work done at GARD. A description in more details is below.

RF/IF chain of the radiometer

The local oscillator (LO) is a free running, 91.7 GHz Gunn oscillator. The LO is not phase locked because of the broadness of the water line and because the line is very symmetric. Hence any instability is compensated by the DSB detection. Also the frequency stability of the oscillator is better than 4 MHz/ºC, so by regulating the LO temperature to within 1 ºC, we obtain a negligible 0.004% frequency drift.

A variable attenuator follows the Gunn oscillator, it allows us to adjust the LO power applied to the mixer and provides some isolation between the Gunn and the mixer.

The mixers are sub-harmonically pumped Shottky types developed by Rutherford Appleton (RF 183.3 GHz, LO 91.655 GHz, IF bandwidth 0.1-8 GHz). This design has two main advantages: it does not require an external bias and we do not need to double the Gunn oscillator frequency to pump the mixer. The resulting RF circuit is simpler and more reliable.

The IF signal has a Bandwidth of 8 GHz, and it is amplified twice with a total gain of 80 dB before the filtering process. The first low noise amplifier has a gain of 33 dB +/-1.5 dB, and a noise figure inferior at 1.8 dB (noise temperature 150 K). A 3 dB pad is placed in front of the second amplifier to improve the impedance matching. The second amplifier has a gain of 48.5 dB +/-1.5 dB, and a noise figure below 4.5 dB.

Filtering and leveling of the IF signal

The signal at the output of the second amplifier has a level close to -10 dBm, too high for the power detectors. First we attenuate 10 dB the signal, then we enter the triplexer to split the 8GHz bandwidth into 3 bandwidths: 0.1-3 GHz, 3-6 GHz, and 6 GHz and above. The first and third channels go through a second set of band-pass filter to further reduce their bandwidth. The final bandwidths are 1-1.4 GHz, 3-6 GHz, and 7.1-8.1 GHz. The level of each channel is adjusted to obtain approximately the same DC output from the Crystal detectors.

Figure 2 Block diagram of the RF/IF chain of the radiometer.

Temperature control of the RF/IF box

The temperature of the RF/IF plate has to be kept as stable as possible to minimise the drift in frequency from the Gunn oscillator, and the variation of gain from the amplifiers. The first radiometer uses Peltier elements in a control loop to regulate the temperature. The advantage of the Peltier element is that following the voltage polarity, we can either cool or heat the plate. However, the reliability of the Peltier element is not well known. For the second radiometer, we decided to use a heat resistor, loosing the possibility of cooling the plate. The temperature of the plate has to be set higher, but we gain in reliability and simplicity.

Power consumption

The radiometer is an instrument designed to operate in a remote site, where the power supply is limited. A lot of effort was dedicated to the reduction of the power consumption of the radiometer. The use of a torroidal transformer in power supply units allowed us to save 60W, bringing the total power consumption below 100W for each radiometer.

Measurement and performance

The IF part of the receiver has been characterised using a Scalar Network Analyser. First, every component individually, then the all IF chain. The level of the signals at the input of the Power detectors has been carefully adjusted to get a linear response from the detectors.

The RF part of the receiver is very simple, the only variable parameter is the LO power applied to pump the mixer. We measured the performances of the receiver using Y-factor technique with the hot and cold load signal calibration. The Y factor we obtained with this method is related to the receiver temperature by the following equation (all temperatures are in Kelvin):

Trec = [ Tamb - Tcold * Y ] / ( Y - 1 )

When the Y factor increases, the receiver temperature decreases. Consequently, the optimum LO power corresponds to the highest Y factors. In the lab we measured the following receiver temperatures:

 

Channel 1-1.4GHz

1500K

 

Channel 3-6GHz

1600K

 

Channel 7.1-8.1GHz

1500K

An approximate value of the receiver temperature can be obtain using the simple equation below.

Trec = Tnoise (mixer) + Loss factor (mixer) * Tnoise (First amplifier)

All the temperatures are in Kelvin. From the data-sheet of the mixer, we have a noise temperature of approximately 900K, and a conversion loss of 6 dB, which gives a factor of 4. The first low noise amplifier has a noise temperature of 150 K. The calculated receiver temperature, using these values, is 1500 K. This has been confirmed during normal operation in the field.

Control software of the radiometer

The control software of the radiometer was written by Collin Hall. The first version was used in Cambridge and Hawaii for the water vapour monitors. The program was later modified by C. Hall here in Chalmers to accommodate the extra features of the radiometers. Controlling the movement of the external mirror was the main implementation to the software.

Installation of the radiometers in Chile

The radiometers have been installed last European summer at Llano de Chajnantor, selected site of the future large millimetre array (LSA/MMA). It is a 5,000 m high plateau, 60km SE of the village San Pedro de Atacama, In the Atacama Desert region of Chile. The receivers have been placed at the ends of an east-west orientated 300 m base line, with an underground serial link along with the power cable to the container in the centre. The container provides the electrical power, which is produced by solar panels, and houses the computer performing the data acquisition.


HARDWARE PICTURE GALLERY

ONSITE PICTURE GALLERY


This page was last modified November 9, 1998, by Victor Belitsky, belitsky(a)oso.chalmers.se