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iDirect 3000 SCPC errors

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Ex Member
Ex Member

Sep 19th, 2007 at 5:41am  

In past few days I have been struggling with very strange problem on my C-Band network. (or maybe strange just to me Smiley
Anyway problem is that connection goes off at almost same time every day. It goes off for a few minutes and comes back on for a minute or two and then same proces continues all over again for a few hours.
During this time NET led on satellite modem goes off and comes back on again but all other led's are OK.
My ISP reported that satellite modem log shows too many SCPC errors during this time. We have even tried using bandpass filter but there was no difference.
Even with spectrum analyser we could not figure out possible source of interference.
BTW all this happens in Kabul - Afghanistan and it's true that we are surrounded by military bases which I suspect could be the cause but there was no problem of this kind in past two years.
Please let me know if anyone experienced similar problem before?
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Ex Member
Ex Member

Reply #1 - Oct 25th, 2007 at 3:45am  
Hello Sir,

Please try to check your network topology. As I know, iDirect 3000 series does not support SCPC. It does support only star (TDM/TDMA with MF-TDMA) topology.
That might be the main reason why you've got a lot of SCPC errors.

Anyway, if you are interested, please send me your e-mail so that I can give you useful information (pdf file) to help you solve your problem.

Best Regards,


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Maxim Usatov
Senior Member

Posts: 116
Czech Republic
Reply #2 - Oct 25th, 2007 at 10:34am  
This looks like interference indeed. It's highly unpredictable so you're left to experiment. Things that usually help:

1) Try relocating the antenna around. Hide it behind buildings/constructions from the possible direction of interference (power lines, WiFi/GSM towers, radars, etc)

2) Try changing the height of the installation. As interference sometime spreads laterally, getting the antenna to the roof or getting it down to the ground level helps sometimes.

3) Wrap up the feed in the tin foil.

Sometimes, relocating the antenna even a few meters away helps a lot. If nothing helps, consider migrating to Ku-Band.
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Eric Johnston
Senior Member

Posts: 2109
Reply #3 - Oct 25th, 2007 at 2:37pm  
If you have a spectrum analyser, insert it into the LNB cable using a splitter and DC block to the analyser and power pass for the LNB/modem.

Put the analyser on a very wide span like 0 - 10 GHz and try using max hold, and resetting it periodically.

If you can watch during an event, look out for the following:

Big spikes anywhere - interference

In the band of interest,

Is your carrier frequency wrong
- LNB local oscillator jumping or stopping with temperature
- LNB drifting too far with temperature. This could be a problem if you are tuned to a narrow iDirect outlink.  Carriers just 1 or 2 MHz wide are not easy to lock with a DRO LNB that drifts. In your options file there may be a receive acquisition range entry like: rx_acqrange = 3000000  Maybe yours is too narrow.

Does the level of everything vary?.

Interference can cause the noise floor to go up or down.
Poor connections in the cable (affected by temperature) can cause drops in level and spikes all over the place.  There should be negligible signals between 0 Hz and 3.4 GHz.  If many signals suddenly appear between 0 Hz and 1 GHz then maybe the outer braid of the coax is coming loose at one of the connectors.

Try tuning to your carrier and doing zero span.  With the sweep set to 30 seconds or 30 minutes. This gives a chart recorder effect.

Some advice on site shielding here:
Earth station site shielding for interference reduction

The earth at your modem may be at a different potential to the earth at the antenna base.  This can cause abnormal and possibly dangerous voltages/currents in the IFL cables.

Best regards, Eric.
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