Report on Irbene telescope and receiver tests August 2006

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1 Torun, 24 August Participants Report on Irbene telescope and receiver tests August 2006 Guntis Ozolins Dimitry Bezrukovs Andrzej Kus Grzegorz Hrynek Eugeniusz Pazderski 2 Radio telescope Figure 1: Irbene Radio Telescope 32m Irbene radio telescope is a horizontal mount Cassegrain system with 1.5 m sub-reflector and shaped surface for enhanced efficiency. Construction is very heavy and solid. Surface of the main dish is

2 in a very good state, as is also the sub-reflector, except of its motion mechanisms, which have to be checked carefully. 3 Telescope control system Figure 2: Telescope control system The telescope is actuated by motors driven by magnetic amplifiers. Anti-backlash system in the gears is used. The telescope control software in the form of a single process under DOS is written. After process start the operator can use one letter commands eventually with coordinates as arguments. For instance t means track, o means offset, i means info, e means error, s means stop. When telnetd program is started on the control computer, remote computers can be used for telescope control. In such a case one needs to use telnet in order to log into the control computer and then run the control process in the telnet window. 4 Receiver Two polarization channels 12 GHz receiver basic devices are satellite TV converters produced by Swedish Microwave, which include PLL LO hard-wired to 11.3 GHz and use 10 MHz external reference. The converters noise temperature is around 60 Kelvin (noise figure 0.8 db). Total gain of the converter is around 50 db. Additional gain is added using three ZFL-1000 amplifiers. Output power and total gain can be tuned using 0-70dB attenuator. IF range is from 768 to 1024 MHz. 70m long coaxial cable is connecting the receiver to the laboratory, where VLBI terminal will be located. Estimated cable loss is at least 15 db. 2

3 Feedhorn GHz λ/4 choke rings Ext.ref. PLL LO LNB 0.8 db NF, 50 db gain Depolarizer Orthomode transducer 15V Overvoltage and foolproof power supply Avalanche diode noise generator Load 50 Ω 27 db directional coupler Isolator Ext.ref. PLL LO LNB 0.8 db NF, 50 db gain RCP LNB output Cal control 10 MHz reference 1dB step ZFL 1000LM attenuator ZFL 1000LM ZFL 1000LM 24dB 0 70 db 24dB 24dB LCP LNB output Figure 3: 12 GHz receiver block diagram 3

4 Figure 4: 12 GHz receiver 5 Laboratory equipment Figure 5: Microwave equipment The new equipment for VLBI and laboratory needs is now present in the Irbene Observatory. It includes active hydrogen maser, VLBI terminal Mark5A, 40 GHz Vector Network Analyzer, 40 GHz Spectrum Analyzer and 40 GHz Synthesized CW Generator. Minor equipment includes GPS receiver, 5 MHz and 10 MHz buffer amplifiers, Counter/Timer for GPS versus Hydrogen Maser 1s pulses distance measurements. 4

5 6 Coherence test Special receiver was prepared for coherence test. It consists of MHz IF filter, 35 db IF amplifier, microwave mixer, 14 MHz low pass filter and MHz video amplifier. HP 8656B Signal Generator referenced to the hydrogen maser 10 MHz was used as LO. The idea of the test was as follows: GHz CW signal from Rhode Schwartz 40 GHz externally referenced generator is sent to the input of the LNB receiver and then converted down to the 950 MHz (12.25 GHz 11.3 GHz = 0.95 GHz). In the coherence test receiver this signal comes through the MHz IF filter very close to it s higher edge and then, after amplification is converted to 10 MHz (950 MHz 960 MHz = 10 MHz). This way single sideband operation is achieved. Both the received 10 MHz and 10 MHz from the maser is observed on the 2-channel oscilloscope for mutual phase stability. Figure 6: Coherence test receiver IF input (12.25 GHz converted to 950 MHz) IF filter IF amplifier Mixer Low Pass Filter Video Amplifier Dual channel oscilloscope MHz MHz 14 MHz MHz Ref. input 10 MHz Synthesized LO source 960 MHz Figure 7: Coherence test receiver block diagram 5

6 Coherence test was done with the front-end located in the laboratory and gave satisfactory results. Figure Figure 8: Coherence test results 8 shows stable 10 MHz signal from the maser at the bottom part of the oscilloscope screen, and jittering signal converted from GHz down to 10 MHz in the upper part of the screen. Phases of both signals keep stable with the rather high but acceptable jitter. Lissajous figures were not useful because of this high jitter and maybe because of digital oscilloscope display properties. 7 Total power observations After coherence test receiver was brought to the secondary focus cabin and prepared for astronomical observations. IF output power was set to the highest available level: -19 dbm, which was checked with the microwave power meter. In the laboratory the measured power level was -35 dbm, which gave cable loss 15 db. Telescope control system was started successfully and its remote operation was established. Microwave power meter and spectrum analyzer were used as back-ends. Virgo-A, Cygnus-A, W3OH and finally Sun were tried to be observed, but there was no influence on the back-ends. After careful checking broken cables were found (for 10 MHz ref signal), but they did not justify the lack of total power changes. One of conclusions was, that LNB s have built-in automatic gain control, which keeps the output power constant. 6

7 8 Conclusions Irbene radio telescope looks to be very good, mechanically stable, accurate instrument. Telescope control system needs radical improvement, remote control should be a part of the main process, graphical clients should be written for easy telescope operation. Telescope motion control from inside VLBI Field System should be enabled some way GHz receiver needs some investigation, but looks to be good enough for VLBI fringe test. Noise figure 0.8 db for LNB s is much higher than met in the contemporary LNB s (0.2 db is now a standard). If it is true, that AGC is present in the LNB s, it has to be removed. The total power operation is needed for proper receiver and antenna testing. Existing noise adding radiometer is useful, but it s linearity has to be carefully measured. Receiver power supply has too big AC component (30 mv, sometimes 150 mv). Coherence test has shown, that after all conversions the phase of the converted signal is stable enough for fringe test. Big influence of the telescope motors on the phase stability was observed during coherence test. It means, that 10 MHz reference signal cable and IF signal cable should be put far from high power motor supply cables and maybe additionally screened. All receiver and back-ends power supplies should use 230V provided by separate cables, having no common points with those used for supplying telescope motion motors and electrical heating systems. All high frequency cables used on the telescope have to be carefully checked and all coaxial connectors need to be mounted to cables again. For each cable type one need to use connectors, which match sizes exactly. Forget to use old soviet BNC connectors with the new equipment. The new test equipment looks very impressive, but for vector network measurements the test cables and a calibration set should be purchased as soon as possible. Thanks of that one can be able to measure all receiver components in order to be conscious of the reliability of the whole receiver. The lack of the permanent Internet connection at the Observatory is a very limiting factor and a fact, that 16 dark fibers will be soon available looks very promising. Successful radio astronomy needs a stable, clever, well paid team, where astronomers cooperate with engineers on the permanent development of the telescope equipment and software. The lack of such a team is a kind of brake for the success of the Irbene Observatory. Properties of the devices delivered to Irbene Observatory 7

8 0 Image filter MHz Transfer function db Input/output return loss GHz Mon Aug 14 23:00:

9 40 IF amplifier 30 Gain db Output Return Loss Input Return Loss GHz Mon Aug 14 23:16:

10 0 Mixer insertion loss. LO level +7 dbm. IF low pass filter included. mix.dat db LO frequency 710MHz MHz Mon Aug 14 22:36:

11 0 Low pass filter 14 MHz db MHz Mon Aug 14 22:22:

12 30 Low frequency amplifier. 25 Gain db Input Return Loss -15 Output Return Loss GHz Mon Aug 14 22:46:

13 15 10 Broadband amplifier ERA-1 Gain db -10 Input Return Loss Output Return Loss GHz Mon Aug 14 23:11:

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