Hi

Today downstream SNR falls suddenly by 4 Db for all our inroutes and Network goes down . I adjusted the Line card power and brought it back . it managed anyhow but the Downstream is still varying by 2 db . what might be the possible reason for this

Inroutes or Downstream carrier?  You mentioned inroutes so you are implying the upstreams (but speak of the downstram carrier)...please clarify.

Questions:

1. How is this being observed?  
2. Are all remotes down 2dB? Or a select few?

Possible causes:

1. Weather at the uplink earthstation
2. Someone may have put up a fairly large carrier the Xponder.
3. Possible problem in your uplink RFT equipment (associated cabling, HPA, UC).  

My guess would be weather or a large carrier on the same transponder.  I recommend calling your payload Ops center (intelsat, eutelsat, SES or whoever) and have them take a look.

Sorry I mean Downstream Career. Most of the remotes are affected . What troubleshooting can be done at HPA side in order to come to an assumption that either there is an issue or not ? I have checked the cablings but seems ok. The carrer is stable and no interuption or interferance occured . Still downstream c/n of all remotes are varying by 2 Db and sometimes lot off SCPC errors.

Now Downstream C/N of all Terminals suddenly goes down 8 Db and i increased the Hub TX power and managed . But still i am wonderring where the issue lies and not able to find an exact issue

Mike says:
1. Weather at the uplink earthstation
2. Someone may have put up a fairly large carrier the Xponder.
3. Possible problem in your uplink RFT equipment (associated cabling, HPA, UC).  

Have a look at the downlink spectrum on both polarisations.  Has a new x-pol carrier appeared ?   You may see a tiny matching hump on your wanted carrier. Turn off your own outlink for a moment and see if the new carrier is causing x-pol interference.  If this is the case get the satellite operator to confirm and call the interfering uplink site - which may need its polarisation adjusting.

Look at the entire transponder also and record the spectrum.  A possibility is new very large carrier elsewhere in the transponder causing a change in the operating point.  

Another possibilty is uplink co-pol interference from an uplink into an adjacent satellite and uplink intermodulation interference from an earth station transmitting multiple carriers and overloading its HPA.

Are there carriers either side of your carrier ?  If these carriers are being operated near saturation at their respective uplink earth stations then spectral regrowth will cause interference into your carrier.  It is important that all BUCs and HPAs are operated at least 1 dB below their maximum output power.

If your large hub dish uses tracking, is the tracking working properly ?  Are you using uplink power control at the hub ?  If so, is it working properly - if it relies on a beacon is the beacon receiver stable?.   Is there something lose on the antenna ?  Does it move in the wind ?  Is a waveguide broken ?

Look at the transmit spectrum, preferably nearest to the antenna feed point using a waveguide cross coupler.  Monitor the transmit carrier while waggling all the cables, IF combiners and connectors etc in the transmit up chain.   Check the HPA power supply also.   When you increased your transmit carrier level (presumably by adjusting the modem) did the uplink carrier at the antenna increase by the same amount.  Experiment up and down and check you are operating all aspects of your transmit chain in a linear fashion.   Is is bad practice, for example, to have the modem at near maximum output and the upconverter at its lowest gain.

If you have redundant paths in the hub TX chain try switching between all alternatives to see if one item is the cause.

Best regards, Eric.

smlt_vsat wrote on Nov 4^th, 2008 at 3:38am:

Now Downstream C/N of all Terminals suddenly goes down 8 Db and i increased the Hub TX power and managed . But still i am wonderring where the issue lies and not able to find an exact issue

1. How much power are you putting on the forward to buffer the downstream SNR drops (i.e. the SNR plummets observed by the remotes)?
2. Does the noise floor elevate when the power is added?


Do you have redundant HPAs?  Line cards?