0000126: RFI in 7X7X in line obs with 1 band, 1024ch, 2polars,BW=10MHz

Notes
(0000156) Yuan Tang 25-05-09 08:13	20090525: Geert had talked with Hans van der Marel last week. Hans will look at the problem.

(0000213) Hans van der Marel 17-12-09 10:50	This type of interference is visible in many spectral line observations nowadays. When comparing several of these observations, the peaks pop up at the same IF frequencies. The IF frequencies correspond with the strongest radio transmitters in the FM-band at Smilde: Radio 1: 91.8 MHz Radio 2: 88.0 MHz Radio 4: 94.8 MHz Q-music: 100.4 MHz Sky radio: 101.0 MHz All tranmitters have a power of 100 kW. The most visible transmitters in your spectra are Radio 1, Radio 4 and Sky radio. The next question is how they can be coupled into the IF? My hypothesis is that the cabling scheme has been changed in the last 2 years to accomodate several tests (Apertif, Galileo, VLBI). This is done by splitting the equalizer output and connecting the second output to several different installations, which can introduce ground loops. These ground loops can cause the signal to enter the otherwise well-shielded cables. This hypothesis has to be tested. If this is the case, then transformers or isolation amplifiers have to be installed for the additional equipment. For the Galileo set-up at RT6 we already installed a buffer amplifier and my impression is that the RFI at RT6 is smaller than before. Another question can be why the FE phase switching does not cancel this 'internal' RFI? This is not clear to me. Maybe Albert Bos can answer this.

(0000254) Hans van der Marel 10-02-10 11:02	At 7Y the RFI appears to be quite strong. The 91.8 MHz can be seen with a spectrum analyzer on the SA output in the console. Disconnecting the VLBI single dish IF cables at the splitter after the equalizer does not change the level of the RFI. Also disconnecting the IF cables at the splitter of RT6 does not show a change. Cables at the DLB equalizers that are not connected to the DZB equalizer (0x,3x,4x and 4y) show the reception of the FM transmitters. These cables are open. When terminating 4x, the level drops considerably, but terminating 4y has a much negligable impact. Focus of the investigations is moving to the monitoring outputs through the old dlb equalizers.

(0000280) Hans van der Marel 19-05-10 10:11	Splitters for Apertif (RT 1,2 and 4) have been removed, which improves the situation. The same has been done for RT 7 where there were splitters for the VLBI single dish. Only very small peaks can be seen now at some of the telescopes at the FM transmitter frequencies.

(0000323) Hans van der Marel 04-01-11 14:30	The splitters have been reinstalled in the end of 2010 for Apertif/Digestif experiments without informing the operators. The RFI is reported again. This will continue as long as the experiments are going on.

(0000324) Hans van der Marel 04-01-11 14:32	No improvement expected as long as Apertif/Digestif experiments are going on.

Issue History
Date Modified	Username	Field	Change
27-04-09 12:10	Jurjen Sluman	New Issue
27-04-09 12:12	Jurjen Sluman	Assigned To	=> Hans van der Marel
27-04-09 12:12	Jurjen Sluman	Status	new => assigned
25-05-09 08:13	Yuan Tang	Note Added: 0000156
14-09-09 09:01	Yuan Tang	Priority	normal => low
17-12-09 10:50	Hans van der Marel	Note Added: 0000213
17-12-09 10:50	Hans van der Marel	Status	assigned => progressing
10-02-10 11:02	Hans van der Marel	Note Added: 0000254
10-02-10 11:15	Hans van der Marel	Relationship added	has duplicate 0000201
19-05-10 10:11	Hans van der Marel	Note Added: 0000280
04-01-11 14:30	Hans van der Marel	Note Added: 0000323
04-01-11 14:32	Hans van der Marel	Note Added: 0000324
04-01-11 14:32	Hans van der Marel	Status	progressing => information