Instraal test

This is to outline the procedure for radiating a signal into the RF frequency band for accurate frequency and polarization tests. In practice the polarization check is only approximate and is only possible if we have a pair of helical antennae for that wavelength (6 and 18cm). This test should be done at least once per VLBI session for all setups to be used.

1. Setup the tied array with the correct frequencies, delay offsets, dipole positions, phase offsets. Check the setup for polarization production. If you want to test the frequencies of the VLBI video converters then set these up with the correct frequencies and bandwidths. Check that all total power levels are correct, and that the correct telescopes are in the adder box.
2. Start an observation which has a declination close to 90degrees (in practice 88.5deg is close enough and declinations higher than 89deg have large precession corrections) with a right ascension convenient for observing. This is to get the fringe rate as close to zero as possible, so that the fringe frequency is insignificant (<<1Hz) even at high frequencies (5GHz) far from the fringe stopping centre (1500m east of RT9).
3. Chose a telescope in the tied array close to the main building and point it at the building. Remove all telescopes except for that one from the adder.
4. Setup the step recovery diode signal injection system to connect to the appropriate radiating . The step recovery diode produces a large number of harmonics. At very long wavelengths (~92cm) this should be replaced by a simple synthesizer. Make sure that the signal is phase locked to the station 10MHz signal.
5. Set the frequency of the synthesizer so that (one of) the CW signal(s) should come out in the video output of the tied array. As the step recovery diode system works at frequencies from 99 to 101MHz the 50th harmonic will be used at 5GHz and the 16th at 1.6GHz. Check with the WSRT System Group about which frequency to use. Check the output frequency with a frequency counter and adjust the level of the signal so that there at all parts of the system the signal is 3dB below saturation.
6. Connect the RF injection system to the appropriate radiating antenna. At 6cm or 18cm this will be one of the two circularly polarized helical antennae (normally LCP) , and at 49cm or 92cm it will be a simple dipole. In practice the 92cm signal leaks sufficiently that it can be detected without the dipole in some cases. If this is so
7. Connect a spectrum ananlyzer to the video output of the tied array being tested. Using progressively finer frequency resolution check the exact output frequency and see if that agrees with the mixing scheme in operation. The agreement should be within 1Hz. If it is not then check that all LO signals are at the correct values and in lock and check that the mixing scheme in use is correct.
8. If the other sense of circular polarization is available then switch to it and check that the signal strength reduces by at least 10dB.