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 About cross-polarisation and 1-dB compression tests
Solution All Talia services require two radio performance tests:
  • cross-polarisation (isolation) test - maximises signal on the correct pole and minimises interference on the opposite pole of a cross-polarised service
  • 1-dB compression test - identifies the compression (saturation) point of a transmission system, including the modem, BUC, and IFL cable

Both tests are performed by transmissing an unmodulated carrier wave (CW) on a test frequency, while our NOC staff examine the carrier.  During the cross-polarisation test, we monitor both the co-pole (correct transmit polarisation) and cross-pole (opposite transmit polarisation).  Due to the physics of some satellites' beams, Talia NOC staff may not be able to monitor the opposite pole.  In such a case we will work with the satellite operator to perform this test.

During the 1-dB compression test, we only need to monitor the co-pole.  We can always perform this test ourselves.  For this reason we may request that these two tests be performed separately, to minimise the delay with the satellite operator.

Why should I care about these tests?

Both tests exist to maximise the radio signal, to ensure stability, and to minimise noise.  Failure to perform the test correctly impacts more than just that remote; it can cause interference to other remotes and even other services on the same satellite.  We are required by the satellite operator to perform them, but even if the operator permitted us to operate without these tests we would still require them.

Even if your remote is operating, it may have poor cross-pole alignment, or we may not have the correct compression point. If so, the remote will perform poorly; it will be less reliable in poor weather, have lower IP throughput, and will cause interference with other remotes. This is particularly true with DVB-S2 networks, where alignment is directly related to IP effeciency.

Note: we always ensure that customers who meet our minimum alignment requirements will acheive the expected IP service levels on DVB-S2 networks. Customers who have been warned of poor C/N may experience degraded IP performance until they resolve the issue.

I am having difficulty with the cross-polarisation test.  What can I do?

Most Talia services require a cross-polarisation isolation of 30 dB.  This means that your test carrier must show at least 30 dB more C/N on the co-pole than on the cross-pole.

Poor alignment is the most common cause of a low isolation.  Always peak the antenna before a cross-pole test.

Can I perform the cross-polarisation test with a short cable near the antenna?

Yes, and it may help your isolation.

Some customers have reported poor isolation when using a longer transmit IFL cable.  Nearly all Talia services use L-band (1-2 GHz) for IFL, which limits the length of cable that can be used because of signal loss over the cable.  Our recommended maximum for a running system is 20 m.  However, even if your cable is under 20 m, it may not acheive 30 dB of isolation.  If so, perform the cross-pole check with a short cable near the antenna.  We recommend a cable of no less than 2m in length.

Using a short cable will not affect actual cross-pole isolation when the modem is in operation.  But it will allow the modem to operate with more power for the purposes of the test.  Once the cross-pole test is complete, return the system to the longer, final install cable before the 1-dB compression test.

Can I perform the 1-dB compression test with a short cable near the antenna?

No.  You must return the system to the longer, final install cable before the 1-dB compression test.

The 1-dB compression test measures the amount of power the modem can transmit to the BUC before the BUC begins to introduce noise to the carrier.  A modem must never transmit above the compression point!

The compression point changes whenever the modem, BUC, or IFL cables change.  You must perform a new 1-dB compression test whenever you change this equipment or cable, even if the new cable is shorter.  Performing this test with a short cable near the antenna will give incorrect results when you move to the final cable.

If you need to perform a cross-pole test on a shorter cable, be ready to switch to the final, longer cable immediately after the test.  Inform the Talia NOC engineer and we will wait for you and complete the 1-dB compression test on the final cable.  The Talia NOC can normally perform a 1-dB compression test quickly, and we can even do it remotely without an engineer on site.

Article Details
Article ID: 154
Created On: 2010-03-18 17:15 +0000

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