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VSAT technology and installation >> iDirect Forum: hubs and terminals >> Modem Upgrade

Message started by nkast on Nov 26th, 2009 at 9:17am

Title: Modem Upgrade
Post by nkast on Nov 26th, 2009 at 9:17am

One of our location recently went for a bandwidth upgrade from 128k to 256k. The NOC remotely pushed the option file and it went through the reboot procedure. But after that it cannnot lock to the RX. All the light are flashing. The NOC is asking to change the ACQ range. Is that is the case? also advice how to change the RX acq range from telnet?


Title: Re: Modem Upgrade
Post by nkast on Nov 26th, 2009 at 9:18am
Oh it is 3100 modem.

Title: Re: Modem Upgrade
Post by TDMAMike on Nov 26th, 2009 at 1:26pm
Check the options file and determine if they disabled DC (by chance).  It sounds like you do not have DC bias enabled, or your modem has stopped providing DC  to the LNB. If you have a multimeter, check for the presence of that DC prior to the LNB (confirmation).  Not sure why you lost rx lock but with that bit rate change you took on a new L- band center freq.  Pull out your old opt and your new opt and compare.  You can see if dc is enabled as well (when comparing).  

Not sure why they recommended for you to adjust the acq range (that is clearly out in left field).  If you have the ability to see yourself in RF (say with a biasT) please take a look with the spectrum and see if their downstream carrier is present.

My guess is your DC bias is no longer present or there is something wrong with your Lband center freq.  Did you repoint this system as well?  What about a POL change with the new bit rate?  Any changes there?

Title: Re: Modem Upgrade
Post by Eric Johnston on Nov 26th, 2009 at 1:58pm
Agree with what Mike says.

If the NOC thinks you need a wider rx acq range they should send you a new opt file with a suitable rx acq range.  The hub needs to know the manufacturer and model number of your LNB.   DRO type LNBs have poor frequency stability and need  a wide acquisition range in the opt file.  PLL type LNBs will work well with narrow acq range.

You say that you were doing a "bandwidth upgrade from 128k to 256k".   You don't say if this is the downstream or upstream.  If downstream, you certainly need PLL type LNB to find such a narrow carrier.  If it is the site upstream bit rate that has changed then I can't understand why you have receive problems.  Maybe the opt file has, in error, different rx parameters from previous version or did the hub change the downsteam also ?

Best regards, Eric.

Title: Re: Modem Upgrade
Post by nkast on Nov 26th, 2009 at 4:27pm

The bit rate was changed for upstream only. I had two other sites which also went for the upgrade. They did not have the problem. The site was working well before the upgrade. The downstream is 1024k and was not changed. We have a DRO LNB. I will check on the DC part.


Title: Re: Modem Upgrade
Post by Eric Johnston on Nov 26th, 2009 at 6:10pm
A 1 Mbit/s (info data rate), 3/4 FEC, QPSK carrier is about 793 kHz wide.

The stability of DRO oscillators is typically +/- 900 kHz so your modem has quite a challenge finding the carrier.  Tuning errors should preferably be no more than 15% of the carrier bandwidth.  In the case of a 1 Mbit/s carrier a PLL type LNB is be preferred.

If your modem RX acquisition range is set wider then it is highly likely it will find the carrier.  Once locked see if you can determine the frequency error.   It drifts a great deal with LNB temperature.  The actual tuning can then be set to the middle of this range. The problem with this is that the hub needs to set all the sites to different receive frequencies.

Without any attention you may find your modem locks up successfully as some time of day as the carrier frequency drifts around.  The modem may drop out again or you may get a high rx BER if the frequency error pushes the tuning acquisition limits too far with temperature variations of the LNB.  

Good luck.

Best regards, Eric.

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