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TRAFFIC_HUB_MODEM_CRC

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Feb 8th, 2010 at 1:57pm  
4 out of 5 Rx Line Cards are displaying a warning "TRAFFIC_HUB_MODEM_CRC", I have run CRC Reports on PP and found 19 sites out of 30 with DATA CRC errors. What could be the problem?

With Thanks.
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Eric Johnston
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Reply #1 - Feb 8th, 2010 at 2:26pm  
Does the hub receive spectrum look normal in the vicinity of the 5 return links ?

I would suspect interference, co-pol, cross-pol or adjacent satellite.  Otherwise, is there some local degradation affecting your hub antenna system, heavy rain, LNB, cable, connectors, receive IF distribution etc ?

It is a good idea to have plotted references of your normal receive spectrum and to have a live spectrum analyser display next to your NOC desk.  The plotted spectrums should show your outlink and the noise floor (done with video averaging ON) and a separate plot of the outlink plus all return links "filled in" (done using peak hold ON for 10 minutes)

Peak hold and a fast sweep time will collect brief interference bursts.

If you see interference call the satellite operator immediately as they are probably on the phone to an installer right now trying to do a line up and it is far easier to fix now than wait till they have left their site and gone home for the night.

Best regards, Eric.
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Reply #2 - Feb 9th, 2010 at 5:47am  
am using PLL LNB (3.4 -4.2 GHZ), suspecting Wimax Interference operating at 3.5 GHz, what band pass filter could best solve the problem?

with thanks.
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Eric Johnston
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Reply #3 - Feb 9th, 2010 at 8:57am  
What is your wanted receive frequency ?  If it is at or very close to the interference frequency then a filter won't help.

You need to plot the full spectrum and investigate the interference.  If it is from WIMAX then take a C band feed horn and LNB and point it around horizontally in all directions to see where the interference is coming from.

If the wanted frequency is in the upper range 3.7 - 4.2 GHz then a transmit reject/ receive bandpass filter designed for the old band 3.7 - 4.2 GHz and an old band LNB 3.7 - 4.2 GHz might help. Such old band standard items should not be too expensive, but I would not do it unless a spectrum analyser plot showed that the interference was well down in the lower part of the band 3.4 - 3.7 GHz and the wanted carrier was in the higher band 3.7 - 4.2 GHz.  It might be best to borrow an old band transmit reject filter and old band LNB to see it solves the problem.

High quality filters are available from Microwave Filter Co, Inc..  These are mainly standard bandpass filters like 3.6 - 4.2 GHz or 3.7 - 4.2 GHz., but special filters, for example, for single wanted transponders or pre-tuned notch filters to block specific interfering carriers are available.

A full catalogue of all satellite filters is here.  https://www.microwavefilter.com/pdffiles/satcom_catalog.pdf (3.7 Mbytes pdf file)  Well worth reading for general information:

Here is an extract from the catalogue:

The model 3966 notch filter is used to remove undesired (in-band) carriers that disrupt C-band reception between (3.4-4.2) GHz.
A good example of undesired carriers are WIMAX signals that operate between ( 3.4-3.8 ) GHz :
Where operators are forced to sacrifice the lower portion of the super-extended C-band (3.4-4.2 GHz) by installing a narrower bandpass filter. While the bandpass filter removes the WIMAX, it also removes the remaining C-band (3.4-3.6 GHz or 3.4-3.7 GHz), limiting their operation to (3.6-4.2) GHz or (3.7-4.2) GHz.
In many instances, the model 3966 notch filter will allow the user to recover much of the lower C-band, since it will notch (remove) only the undesired signals (carriers) from their C-band feed, while allowing all other frequencies through.
This versatile notch filter can be configured to remove from (1-6) interfering carriers at notch depths from (15-70) dB, depending upon the signal strength of each interfering carrier.


Note that no filter can stop co-frequency interference.   Maybe change frequency, either you or the interferer ?  Site shielding or moving the dish to another location can also be considered.

Best regards, Eric.
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Reply #4 - Mar 1st, 2010 at 2:39pm  
Installed LNB with input 3.625-4.2 GHz, no TRAFFIC_HUB_MODEM_CRC errors. Thanks!
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