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Crosspol / Isolation question

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Posts: 3
Jan 22nd, 2009 at 3:27pm  
I'm relatively new to the VSAT NOC Tech industry, but I want to learn more.

Can anyone explain what performing crosspol and isolation checks are and how to do them?  For example, if I install a remote site (for one of my customers) on an iDirect platform, what exactly would the process be to complete the check to make sure their carrier is not interfering with another one?

What is crosspol?  What is isolation?  As a NOC Tech at an iDirect hub, I will need to perform these checks with my field technicians and satellite service providers, but don't know what anything means.  I greatly appreciate the help.
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Jan 22nd, 2009 at 8:51pm  
If your are a hub NOC operator you need to control the remote site modem and change its transmit frequency to a spot frequency which has been approved for you to use for cross-pol alignment and testing with an unmodulated (CW) carrier.  A CW carrier can cause serious interference to services on the opposite polarisation and to services on nearby satellites so the exact frequency must be coordinated by the satellite operator with adjacent satellite operators before you can be authorised to use it.

You at the NOC hub need to look at the relevent frequency using a spectrum analyser with selectable narrow resolution filter bandwidths like 100kHz, 30kHz, 10kHz, 3 kHz, 1 KHz and 100Hz.  You need a PLL dual polarisation receive system at the hub.  If you look on the co-pol path you should see a C/N around 35 dB or more; use a sufficiently narrow filter resolution bandwidth to achieve this. It make take a few minutes to get a good display by adjusting the span, resolution bandwidth and video bandwidth so that you have a stable reading.  Watch out for any frequency drift.  Now look on the opposite cross-pol PLL LNB and measure again. You will see something like C/N=15 dB, or less.  The cross pol isolation is now 35 - 15 = 20 dB, which is bad.  Ask the installer at the remote site to get under the feed arm and reach up and turn the feed assembly slowly around the calculated null position.  If you watch the cross-pol signal you will see it drop down and then come back up again as the installer turns the feed assembly.  The null is very narrow and you need to talk the installer into the exact centre.  The null is only a couple of degrees wide and you need to patiently get the installer to make very small adjustments till you are exactly in the middle.  Aim for an isolation of 29 dB or better. 35dB isolation is excellent.

If you get it wrong then the cross-pol transponder on the satellite will experience click interference every time that remote transmits and this will inject errors into whoever is using the cross pol frequency.  Note that whoever is operating cross pol to you can equally interfere into your service in the same way.

Installers should read the above.  It is important not to get any part of your body, arm etc, in the beam when making the adjustment.  Be aware that the hub operator may take several long seconds to make a measurement, so after an initial smooth movement across the calculated null, make further adjustments in microscopic steps, all in the same direction.  It may take several minutes to get you to the exact null.  Don't move the feed unless told.  If you get it wrong you will probably be called back to the site when it has had to be switched off due to it causing interference to someone else.

While you have the carrier in unmodulated CW mode find the modem output power setting that corresponds to the -1dB compression point of the remote site BUC. Adjust the modem output power in 1 dB steps and check that the signal at the hub also varies in 1 dB steps.  This means you are working on the linear portion of the BUC input/output transfer curve.  Increase the power in 1 dB steps and eventually the power you measure will go up in less than 1 dB steps.  Plot on graph paper to get the idea.  Once the measured value is 1 dB behind the input level you have reached the -1dB compression point of the BUC, which is rated output power, like 2 watts. Write down the modem output power.  This is the never-to-exceed value for operational service during heavy rain.

Once you set the modem to its operation transmit frequency with burst modulation on then its operational output power needs to be set to get the nominal signal quality at the hub burst receiver ( 9 dB, I believe). This is the normal clear sky operating value.  During heavy rain the modem can automatically increase its output (up to the never-to-exceed value) to keep the hub burst receiver quality unchanged.

High polarisation purity antennas such as the new Prodelin ones with mode-matched (505 and lump) feeds and antenna types with long focal length ( f/d=0.8 ) have excellent cross pol over the -1 dB transmit beamwidth.  If you fail slightly with other antennas, try peaking up, as the antenna will have its best x-pol performance on axis.

On a C band circular polarisation system no polarisation adjustment angle is possible.  If it fails to pass the feed is faulty.

Large hub antennas should have their cross-pol measured in 9 different directions, on-axis and in 8 other -1dB directions of the transmit beam, up, down, left, right and the four 45 deg positions also.  All should exceed 30 dB.

Maybe someone can add specifically iDirect related advice regarding x-pol acceptance into service testing.

Take care never to operate a BUC above its rated -1dB compression point with the modulation ON.  The modulation looks like noise which has a high peak to mean ratio.  The peaks cause brief high voltage spikes which can permanently damage the BUC transistors.

Best regards, Eric.
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