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SCPC Demodulator Unlock Help!!!

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Ex Member
Ex Member

Sep 3rd, 2011 at 8:40am  
Hi all,

I have a couple of site I am working on on SCPC link (point to point)

my hardware are
Antenna: 4.9m
ODU: SSPA 120w codan
ODU: Converter 5700 Codan
IDU: comtechefdata CDM625

I have done pick and poll with intelsat.
RF loop back test fails. rxcannot lock
RXlevel is -35db

my setting on my codan converter is

System Command  : on            
SSPA Activate   : On          
SSPA Inhibit    : Off          
Echo            : Off          
Ref. Override   : on          
SSPA Mode       : Basic    
SSPA Comp Type  : Off                        
Conv Comp Type  : Standard                  
Packet protocol : Codan        
Power Up        : Last State
tx Freq     : 6205   Atten : 15
Rx Freq     : 3980   Atten : 0
Cable Comp  : 0
Impedance   : 50
Faults      : LNA  - Enable
FAN  - Disable
SPacket addr : 0 (00H)
       For actual transceiver status use VSS command
I need help.
I have 3 site with the same problem
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« Last Edit: Sep 3rd, 2011 at 1:26pm by Admin1 »  
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Sep 3rd, 2011 at 1:22pm  
Get the handbook and study it carefully.  A 120W SSPA amplifer is dangerous and also easily damaged by too high input level.

I never heard of "pick and poll" with Intelsat. Have they verified that you are transmitting to the correct satellite and on the correct frequency and polarisation. Have they helped you set your transmit level to the correct level, probably in CW mode ? Does your transmit power (dBW) plus your calculated transmit antenna gain (dBi) make sense with the Intelsat measured power of your carrier?

You say you can't get receive lock on your carrier (looped back via the satellite).  Does your carrier look right on the analyser at the input to your receiver modem?.  Is the C/N reasonable ?

Is your main beam really pointed at the satellite ? It is possible to be peaked up on the first sidelobe, particularly at the 45 deg positions and think you have the peak. Always check in az and el to see a sweep with two equal low peaks either side of the main beam.  

Have you tried connecting one modem direct to another.  Check this works as expected.  This may resolve mismatched modulation and FEC settings.  Check for inverted and non-inverted carriers. C band LNBs often invert the receive carrier (depends on LNB LO freq) so maybe set the CDM625 receiver to inverted for loop via the satellite.

I don't know what transmit frequency you have set your modems to.  If the final frequency is not a 1 MHz increment then I suggest using a modem frequency above the centre frequency of the modem (e.g 70 MHz or 140 MHz) as this make the maths easier, so there is less likelyhood of error.

Be extremely careful with transmit power levels.  I would try with a tiny carrier (say 100 kbit/s) and maximum attenuation on everything to start with and then work on getting it all working perfectly.  Avoid having the upconverter or SSPA gain set very high and the modem to very low output level as it is then easy to accidentally increase the modem output by 10 or 20 dB.

Then gradually increase the bit rate and power to nominal, with Intelsat's assistance.  Monitor the SSPA output power very carefully. It only takes a brief click on the wrong attenuator setting and bang goes the expensive SSPA.

Best regards Eric.
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« Last Edit: Sep 3rd, 2011 at 5:41pm by Admin1 »  
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Ex Member
Ex Member

Reply #2 - Sep 3rd, 2011 at 8:44pm  
Dear Eric

I did work with intelsat on my power levels. they assisted to peaked the antenna (azimuth and elevation). Also worked on isolation= 30dB
My antenna is pointing to IS25, Transponder 18C.  I have  used another modem to check but the rx is still same. I will download to pdf documents and read it. thank you
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Eric Johnston
Senior Member

Posts: 2109
Reply #3 - Sep 3rd, 2011 at 10:38pm  
It sounds as though your transmit side is good.

Regarding receive..

Can you see your signal on an analyser ?  What is its C/N ? Reduce the level of your carrier by 3 dB.  Does the signal you can see go down also? - verifies you are looking at the correct signal.

Is your receive modem tuned correctly ? symbol rate ? modulation type ? FEC type ? scrambling on/off and type ? If in doubt put two modems back to back (just IF cable joining them) and make it work as expected. Or, simply loop one modem back on itself...

If you have a LNA and are using the CODAN to downconvert the receive signals then, if the CODAN 5700 is narrow band (option N 40 MHz wide IF) the centre frequency of the IF output will be 70 MHz.   If the CODAN  5700 is wide band (option W 80 MHz wide IF) the centre frequency of the IF output will be 70 or 140 MHz, software selectable.  Tune your modem accordingly.   The CDM625 modem tunes from 50 - 180 MHz and 950 - 2000 MHz. Setting the modem to 70 MHz and the CODAN downconverter to 3980 MHz might work. What can you see on the spectrum analyser at the CODAN receive IF output ?  Set centre freq to 100 MHz and span to 200 MHz.

If you have a LNB the L band cable output of the LNB will bypass the CODAN and go direct to the receive modem. You will need to turn on the DC power supply from the modem and also the 10 MHz reference if you are using an external reference PLL type LNB. The tuning of the modem will probably be 5150-3980 = 1170 MHz. Set carrier inverted in the receive modem config.  To monitor this signal on an analyser you will need a splitter with DC block. Tune the analyser to 1000 MHz centre frequency with 0 - 2000 MHz span for initial view of the spectrum. You should see all the carriers on the IS25 satellite - with the frequencies inverted !.

Here is the manual for the CDM625 modem: (8.67 Mbytes)

Best regards, Eric
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« Last Edit: Sep 4th, 2011 at 12:32pm by Admin1 »  
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Senior Member

Posts: 90
Reply #4 - Sep 25th, 2011 at 11:26am  
Dear eric.
problem solved. Modem setting information was wrong. Thank for all input.
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