IF Matrix Switch for the Yebes 40 Meter Radio Telescope Receiver Room
By David Cuadrado-Calle, José Antonio López-Pérez, Yebes Astronomy Center

T his work shows the design, construction and characterization of a dual 4:1 matrix switch in the DC – 2 GHz band for the IF signal distribution of the 40 meter radio telescope receivers at Yebes Astronomy Center (CAY) in Spain. As each receiver provides two IF signals, one for each circular polarization sense, the matrix switch consists of two chains, each one composed of a solid-state SP4T switch based on the Hittite HMC241QS16 integrated circuit (IC) and an amplifier based on the Mini-Circuits ERA-3SM+ IC. The resulting matrix switch has an input and output return loss better than 15 dB and 13 dB, respectively, and a minimum gain of 9 dB.

The purpose of the matrix switch is to enable the remote selection, via RS-232, of the IF signals which are sent down to the backend room through Andrew FSJ4-50B coaxial cables. The RS-232 control is implemented in a board with a MicroChip PIC16F84 microprocessor.

Introduction
The CAY 40 m radio telescope has 6 cryogenic receivers already installed (S, 3.3 GHz, C, X, 22 GHz and 3 mm bands) and one prime focus room temperature receiver for with-phase holography measurements of the 40 m main reflector in Ku band. In addition, a Q band (from 41 to 49 GHz) cryogenic receiver will be installed by the end of 2012 and a Ka band (from 28 to 33 GHz) one is in the design phase.

The IF signals of all the receivers are routed to a patch panel in the receiver room and then, they are sent down through eight coaxial cables to the backend room. As the number of coaxial cables (8) is lower than the number of IF signals from the receivers (6 x 2 = 12), a matrix switch is needed to avoid the tedious task of manual patching when the radio telescope operating frequency is changed.

The construction and integration of the matrix switch in the patch panel of the receiver room, enables the remote selection, via RS-232, of the IF signals which are sent down to the backend room. This way, the coaxial connectors don’t suffer from degradation associated to manual patching and the switching time to change the operating frequency band of the 40 meter radio telescope is reduced significantly, due to the automation of the patching process.

The inputs to the matrix switch must come from the receivers that can’t operate simultaneously due to the configuration of the receiver room optics. Hence, the selected ones are:

• IF 1 input: C band LCP and RCP
• IF 2 input: 22 GHz band RCP and LCP
• IF 3 input: Q band RCP and LCP
• IF 4 input: 3 mm band H and V

Matrix Switch Block Diagram
The matrix consists of two chains (one for each polarization sense), each one composed of a SP4T module that will select one IF signal out of four possible inputs and an amplifier that will compensate for the insertion loss of the solid-state SP4T PIN switches and cables.

As it will be shown later, the matrix switch control is implemented in a board developed at Yebes’ laboratories with a MicroChip PIC16F84 microprocessor. The firmware running in this microprocessor was developed in C language.

The block diagram of the matrix switch is shown in Figure 1.

SP4T Module
The SP4T module is a board containing one Hittite HMC241QS16 solid-state SP4T switch. Figure 2 shows the insertion loss and the return loss at the input ports of the SP4T module. These graphs have been measured in Yebes laboratories with the vector network analyzer (VNA).

According to Figure 2 the SP4T module can operate from DC to 3.5 GHz with insertion loss lower than 2 dB and return loss better than 20 dB. The isolation of the module has also been tested and it is better than 25 dB.

Hence, the frequency range of the evaluation board spans from DC to 3.5 GHz. This span is higher than the one needed for this application because the IF signals from current receivers range from 500 to 1000 MHz, except the Q band which will span from DC to 2 GHz.

Amplifier Module
The amplifier module was designed, integrated and characterized in Yebes laboratories and it is based on the Mini-Circuits ERA-3SM+ IC. Figure 3 shows the microwave layout of the amplifier as well as its final implementation.

The microstrip line substrate used for this design has been the Taconic TLX-8. It has a dielectric constant of 2.55 +/- .04, a dielectric thickness of 0.78 mm and a copper thicknes of 35 µm. To achieve an impedance of 50 ohms, the width of the microstrip line must be 2.18 millimeters.

According to the design, the amplifier module must be polarized with a source of 5 V and the operating current of the amplifier must be 35 mA. Hence, the variable resistor must be adjusted to achieve this polarization setup.

Figure 4 shows the main properties of the amplifier module: insertion loss, return loss and isolation. These measurements have been carried out with the VNA in Yebes laboratories. For each parameter there are two traces, one corresponds to the output and the other one corresponds to the monitoring port. Both traces are very similar, as expected, because they are provided by a resistive power splitter.

According to Figure 4, the amplifier module is good for operation from DC to 2.5 GHz, approximately, where a minimum gain of 10 dB and return loss of 15 dB are achieved. As mentioned above, this span is higher than the one needed for the application.

Finally, an aluminum enclosure box was designed and built for each amplifier.

Laboratory Measurements of the Matrix Switch
All the measurements have been performed with the help of a VNA, a signal generator and a spectrum analyzer (SA).

The results obtained for the four channels are very similar, hence only the results for channel 1 will be shown for simplicity.

Return loss
Figure 5 shows the return loss at the input ports (R1 and L1) of the first channel and the return loss at the corresponding output ports (Rout and Lout) of the matrix switch. On each graph, the channel with left circular polarization (L) is displayed in purple while the channel with right circular polarization (R) is represented in blue.

In the frequency range from DC to 2 GHz the input and the output return loss are better than 15 dB and 13 dB respectively.

Insertion loss measurements
Figure 6 shows the insertion loss of the first channel of the matrix switch.

It can be observed that a minimum gain of 9 dB is achieved in the frequency range from DC to 2 GHz. Depending on the application of the matrix switch, it may be necessary to design an equalizer to flatten the band shape.

Gain Curves and Compression
To obtain the gain curves of the four channels and determine the saturation point of the module, several measurements have been carried out for several frequency and power values. These measurements are shown in Figure 7 for channel 1, obtaining similar results for the channels 2, 3 and 4.

Acording to the previous graphs and taking the 1000 MHz curves as reference, it can be concluded that the output power at 1 dB compresion of the module is between 4 dBm and 6 dBm depending on the channel.

Control Module
The control of the matrix switch module is implemented in a board with a MicroChip PIC16F84 microprocessor that enables the radio telescope operator to perform remote monitor and control via a RS-232 serial port. This control board has been designed at CAY laboratories to provide the PIC with the necessary signals and connections. The firmware running in this microprocessor was developed in C language.

This way, the board receives the commands transmitted from the remote PC. Then these signals from the PC are converted to 5V in a MAX232 adapter and finally they are processed by the PIC in order to select the desired output in the SP4T module. Table 1 represents the truth table of the system and the commands that must be introduced in order to activate one output or another:

It is also possible for the operator to monitor which output is already selected by sending the command ‘?’.

Assembly and integration
The matrix switch has been integrated inside a 1U 19” rack. In addition, this rack has been successfully installed in the patch panel rack of the 40 meter radio telescope’s receiver room. Figure 8 shows the drawings of the matrix.

Conclusions
An IF matrix switch has been designed, integrated and characterized in Yebes laboratories. It will provide the 40 m radio telescope with the capability to automatically select which IF signals are sent down to the backend room, without the need of manual intervention of the operator in the patch panels.

The final performance of the matrix switch is summarized as follows:
For frequencies below 2 GHz, the module’s input and output return loss are always better than 15 dB and 13 dB, respectively.

In the band of interest between 0 and 2 GHz the gain of the matrix is above 9 dB.
The 1 dB compression point of the module is around 5 dBm (depending on the channel) for a frequency of 1 GHz.

About the Authors
Both authors work for Yebes Astronomy Center (CAY), in Spain.

David Cuadrado-Calle received his bachelor’s degree in telecommunication systems engineering in 2010 and his master’s degree in information and communication technologies in 2012, both in the University of Alcalá (Spain). He works at Yebes Astronomy Center in the receivers division and his areas of interest are RF and microwave circuits and radio astronomy receivers. He can be reached at d.cuadrado@oan.es.

José A. López-Pérez received his degree in telecommunication engineering from the Polytechnic University of Madrid in 1996. After finishing his degree, he joined the Institute de Radio Astronomie Millimétrique (IRAM) in Grenoble (France) for two years, and then he became part of the staff of the Yebes Astronomy Center (CAY) in Spain, where he actually works. He is the responsible for the design and construction of the radio astronomy receivers and for the holography measurements of the 40 meter radio telescope surface. He can be reached at ja.lopezperez@oan.es.

Yebes Astronomy Center

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