0000201: Spikes on several baselines (RFI)
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ID Category Severity Reproducibility Date Submitted Last Update
0000201 [submission] 0.unknown/unspecified minor random 16-12-09 17:41 04-01-11 14:33
Reporter Gyula Jozsa View Status public  
Assigned To Hans van der Marel
Priority normal Resolution open  
Status information  
Summary 0000201: Spikes on several baselines (RFI)
Description sss submission by Oosterloo:
Many baselines have strong RFI in a band that should be (and used to be) clean. Most of the baselines with RFI are on the western end of the array, but also, e.g. 9-C has RFI.

Although this has happened in the past, the problem appears to become more severe.
Additional Information Attached a plot of spectra from a recent WSRT observation (7 dec 2009) and a plot of crosscorrelations in the affected spectral window (12 jan 2010).
Attached Files  spikes.pdf [^] (355,819 bytes) 16-12-09 17:41
 FM_interference.pdf [^] (73,236 bytes) 20-01-10 10:52

- Relationships
duplicate of 0000126information Hans van der Marel RFI in 7X7X in line obs with 1 band, 1024ch, 2polars,BW=10MHz 

- Notes
(0000212)
Gyula Jozsa
17-12-09 10:36

 The feature may be seen more frequently. I was contained in an observation from 15-12-09. The feature is not easy to spot because of a random phase. Thus, inspecting vector-averaged visibilities, it is invisible.
 
(0000215)
Harm Munk
18-12-09 13:18

 Comment from Hans van de Marel:

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.

To be continued.
 
(0000232)
Gyula Jozsa
20-01-10 11:00

 The problem occurs e.g. in recent data sets 10904910, 10905532, 11000196, 11000211. Spikes occurring can be identified with FM radio emitters (police, Smilde) entering in the IF chain in all data sets.

Fig. 2 shows the amplitude of the RFI in dependence of hour angle. The feature is roughly symmetric about HA=0h, but it is not clear whether this has any meaning.
 
(0000325)
Hans van der Marel
04-01-11 14:33

 The RFI will continue to exist as long as the Apertif/Digestif experiments are going on.
 

- Issue History
Date Modified Username Field Change
16-12-09 17:41 Gyula Jozsa New Issue
16-12-09 17:41 Gyula Jozsa File Added: spikes.pdf
17-12-09 10:36 Gyula Jozsa Note Added: 0000212
18-12-09 13:18 Harm Munk Note Added: 0000215
08-01-10 09:29 Harm Munk Status new => assigned
08-01-10 09:29 Harm Munk Assigned To  => Hans van der Marel
20-01-10 10:52 Gyula Jozsa File Added: FM_interference.pdf
20-01-10 11:00 Gyula Jozsa Note Added: 0000232
20-01-10 11:00 Gyula Jozsa Description Updated
20-01-10 11:00 Gyula Jozsa Additional Information Updated
10-02-10 11:15 Hans van der Marel Relationship added duplicate of 0000126
10-02-10 11:15 Hans van der Marel Status assigned => progressing
04-01-11 14:33 Hans van der Marel Note Added: 0000325
04-01-11 14:33 Hans van der Marel Status progressing => information