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GCU receive problem for Linkstar HUB

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Ex Member

Apr 30th, 2008 at 7:54pm  
Hello everybody,

Please advice me for the problem below.
When i give tfpr command to GCU,the Attenuation for few RCSTs are showing very less like 2,3,4 as compare to 10,11 for some sites.

When my Linkstar Hub was installed,i had to start and shutdown the HUB everyday,i found problem in bringing up atleast one RCST.SO i changed the RxAttenuation of each GCU to 7 from 16 ,after that it worked fine with right  attenuation values(10,11,12 with Automatic Tx power of -21 set at HUB for the RCST).Now i am noticing the attenuation value has dropped for few RCSTs,by the time i check my outbound and inbound on Spectrum analyzer.The neighbouring users are almost touching our Spectrum from both sides.Is it causing the spectral interference .Or do i have to do some GCU alignment or should i increase the RxAttenuation from 7 to 16.

I will Appriciate if i will get some help,i will give more information if required.

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Eric Johnston
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Reply #1 - Apr 30th, 2008 at 11:58pm  
I have considerable experience getting TDMA burst receivers working properly, but none on the specific LinkStar hub you refer to, so here is some general advice.

To test a burst receiver, put another known good receiver on the same inbound frequency and look for abnormalities on the unit under test.  You probably need to remove the traffic cable so you don't duplicate all the received packets into your hub LAN.

To get the burst receiver input level right read the manual to find out what is the correct input level, or experiment with a wide range of levels with known good signal bursts and see how the burst BER varies from extremely low to extremely high input levels.  Set the receive IF burst (~10 dB C/N) level about 4 dB above the middle of the optimum range.  This gives low level incoming bursts (~6 dB C/N) the best chance of successful reception.

The IF distribution network must be designed so that signals never get too high or too low level.  If there are booster amplifiers amongst cables and splitters, the operating point of these amplifiers must be designed with consideration of both intermodulation and the noise floor.

To set the receive level by measurement is difficult, since all the signals observable, the noise floor and the bursts themselves are noise like.  One way is to measure the level of the noise floor with the spectrum analyser MARKER NOISE ON function.  This will tell you the power per Hz, e.g. -100 dBm/Hz.   Multiply by the bandwidth. i.e.  +10log(Bandwidth in Hz) of your incoming bursts and add 10 dB or so to give you the burst power of a carrier with C/N=10 dB.

It is most interesting to observe the incoming bursts.   Set the analyser to the burst carrier frequency and the resolution bandwidth the same as the carrier bandwidth if possible (otherwise less).  Set the span to ZERO and sweep to say 1 second, you can vary this shorter to see more detail.  Put the sweep in single shot mode and press go.  This takes pictures in time so you can see the bursts coming one after the other.  Note that most are good, some are too high level, some are low level, a few may have wrong shape.  Sometimes if you have several burst receivers on adjacent frequencies you will discover one remote with too high level that sends low level spectral regrowth interference into the adjacent burst receivers.  

Try using the spectrum analyser in its normal frequency sweep mode but with peak hold ON.  Wait 10 minutes and you should build up an overall spectrum plot of the peaks of many bursts.  Watch for any remote that is causing interference.  If you can put a single remote site direct to one burst receiver you can concentrate on that remote only until its burst spectrum looks right.

If you leave the spectrum analyser on peak hold for an hour or so you can see occasional interference or see a sweeper move across.  

It is worth observing the full L band IF spectrum, from 0 Hz to 2.1 GHz.  Look closely at the range 0 Hz to 200 kHz. It should be quiet.  If you see interference at low frequencies like 50 Hz, 60 Hz, 25 kHz etc then suspect problems with the DC power supply to the LNB or the DC supply of any IF booster amplifiers.  This can cause phase noise which causes high BER of all burst reception.  Problems can also occur in the 10 MHz reference oscillator.  Power supply smoothing and good capacitors are critical to low phase noise in the LNB and the 10 MHz reference supply. There must be no 50 Hz or 60 Hz mains loop via the LNB cable.

Best regards, Eric.
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« Last Edit: May 1st, 2008 at 9:02am by Eric Johnston »  
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