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5 posts

Wannabe Geek


#56959 29-Jan-2010 19:18
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Apologies for the long post.
I have a DSE-7500 Freeview STB which has previously worked for both Freeview carriers on a Sky dish in Rothesay Bay. I have now installed it in our new house in Hellcrest (NthShore) using the previous owner's Sky dish. It has a dual LNB with single output (Sharp 2 horn type supplied by Sky LO=10750)

Sky came in without trouble. I then installed a 2 way splitter and added the DSE7500. It initially received Carrier 2 (12456 - C3,C4 etc) but not Carrier 1 (12483 -TVNZ). Sky still ok.
I am fortunate enough to have access to a portable signal analyser which measures sig strength, BER and shows the spectrum so I used it to fine tune the pointing of the dish. It was almost spot on. I was getting 80dbuv at the LNB output but now have 83.5dbuv.
However, now the DSE-7500 tunes only Carrier 1 and only intermittently to Carrier 2! Sky is still good.

After reading various posts on this forum I thought it might be a matter of 'skew' of the LNB so I tried different positions and watched the results on the analyser. There was a position where I could reliably get Carrier 2 but then I lost carrier 1 and the BER on Sky went too high.

I set up the analyser to look at 4 frequencies, 2 for SKY and 2 for F/V. All frequencies have similar signal strength but it is pretty clear that when the skew of the LNB is good for SKY and Carrier 1 the quality of Carrier 2 is poor and I guess that is why the STB will not tune to Carrier 2. Here are typical results at the output of the 2-way splitter at the end of the cable from the LNB. One o/p to the F/V box the other to the S22. The LNB is powered by the F/V box (18V measured).

Freq     Sig       ChBER   PvBER   MER
12519  71.1db  6.2-04  0.0-8   9.5
12421  71.5     4.5-04  0.0-8   9.8
12483  71.4     4.5-04  0.0-8   10.3 F/V Carrier 1
12456  72.5     1.4-03  0.0-8    8.5  F/V Carrier 2

So my questions are:
* Why is carrier 2 such bad quality when it is on the same transponder and supposed to have the same characteristics as Carrier 1?
* Is this likely to be a LNB problem?
* What other explanation.

Any advise from the forum would be greatly appreciated.

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147 posts

Master Geek


  #294511 29-Jan-2010 21:58
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I would start by checking the cable run. If you have not already done so, I would compare the signal at the dish with the signal at the receiver end of the cable.

Swapping out the LNB would certainly be the next step. If you can get your hands on a good quality LNB, such as the 11300 Sharp LNB, used in early Sky installs, is one of the best.

A bit of a hassle as you need to change the LNB clamp as well.



5 posts

Wannabe Geek


  #294561 30-Jan-2010 07:23
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Thanks Kiwisat. I have already tried a different cable direct to the LNB - no better. Your point about the different LNB clamp also came up as I checked alternatives. There must be adaptors though.
Is it hard to change the LO setting in a Sky HDi box? Do you have any knowledge about what Sky think if they find a non-Sky supplied LNB on the dish?

 
 
 
 


932 posts

Ultimate Geek

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  #294577 30-Jan-2010 09:25
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Gypsy: Apologies for the long post.
I have a DSE-7500 Freeview STB which has previously worked for both Freeview carriers on a Sky dish in Rothesay Bay. I have now installed it in our new house in Hellcrest (NthShore) using the previous owner's Sky dish. It has a dual LNB with single output (Sharp 2 horn type supplied by Sky LO=10750)

Sky came in without trouble. I then installed a 2 way splitter and added the DSE7500. It initially received Carrier 2 (12456 - C3,C4 etc) but not Carrier 1 (12483 -TVNZ). Sky still ok.
I am fortunate enough to have access to a portable signal analyser which measures sig strength, BER and shows the spectrum so I used it to fine tune the pointing of the dish. It was almost spot on. I was getting 80dbuv at the LNB output but now have 83.5dbuv.
However, now the DSE-7500 tunes only Carrier 1 and only intermittently to Carrier 2! Sky is still good.

After reading various posts on this forum I thought it might be a matter of 'skew' of the LNB so I tried different positions and watched the results on the analyser. There was a position where I could reliably get Carrier 2 but then I lost carrier 1 and the BER on Sky went too high.

I set up the analyser to look at 4 frequencies, 2 for SKY and 2 for F/V. All frequencies have similar signal strength but it is pretty clear that when the skew of the LNB is good for SKY and Carrier 1 the quality of Carrier 2 is poor and I guess that is why the STB will not tune to Carrier 2. Here are typical results at the output of the 2-way splitter at the end of the cable from the LNB. One o/p to the F/V box the other to the S22. The LNB is powered by the F/V box (18V measured).

Freq     Sig       ChBER   PvBER   MER
12519  71.1db  6.2-04  0.0-8   9.5
12421  71.5     4.5-04  0.0-8   9.8
12483  71.4     4.5-04  0.0-8   10.3 F/V Carrier 1
12456  72.5     1.4-03  0.0-8    8.5  F/V Carrier 2

So my questions are:
* Why is carrier 2 such bad quality when it is on the same transponder and supposed to have the same characteristics as Carrier 1?
* Is this likely to be a LNB problem?
* What other explanation.

Any advise from the forum would be greatly appreciated.


It's not, the difference is neglible.
Have you checked the levels at the input of the rec?



5 posts

Wannabe Geek


  #294631 30-Jan-2010 13:37
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Hi Brunzy, your comments caused me to review the figures I quoted. I made an error in translating my notes for the critical signal 12456. I showed PvBER of 0.0-8 BUT in fact it was 7.5-08. That shows it is the only 1 of the 4 which has errors after error correction.

But just to confucse matters, this morning I could tune to both carriers reliably and the BER on 12456 is back to much the same as 12483. Nothing has changed other than the weather is cooler. I will measure throughout the day to see if there is a return to the poorer result.

The measurements have been made at various points - direct from the LNB, at the input to the Rx without a splitter and then with a splitter. Whilst the absolute values are a bit different carrier 2 showed a consistently poorer BER and MER when I could not get the Rx to tune to Carrier 2.

Well I guess there is as much black magic as there is science in this game.
 

7025 posts

Uber Geek

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  #294741 30-Jan-2010 21:42
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Did you put the splitter in?
Is it a good quality one, and have you checked your connections/for any signs of problems with the f type plugs near the sheilding etc?



5 posts

Wannabe Geek


  #295369 2-Feb-2010 13:11
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Thanks Jaxson for your extra comments. I am not using the splitter now, I am taking the LNB directly to the DSE Freeview box and connecting the Sky box to the loop through of the DSE.

After experimenting with variations of the LNB skew and checking/cleaning all F connectors in the circuit I seem to have found a reliable setting.

I am still curious to know why Carrier 2 seems more critical for LNB skew. Perhaps there is another signal on the V polarity which can interfere.

Thanks to those who offered ideas for the solution.

7025 posts

Uber Geek

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  #295372 2-Feb-2010 13:34
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I'd seriously just use the SBS channels on the vertical transponer to align the skew. It's so weak you have to be pretty pin point accurate with that one so the rest should fall into place. If you're still not having any luck, then I'd check the overall alignment too.

We're not meaning to sound insulting, just it pays to go back and double check everything. If you're not perfectly lined up in the first place you'll waste a lot of time adjusting the skew.

After that it's a step back to look at cable, connectors, splitters, lnb quality and recevier quality along with ensuring a clean line of sight in the first place. Oh, and also condition of the dish, has it been knocked/warped etc. You'll get there, but it pays to make things as simple as possible first, so remove splitters/amplifiers etc.

 
 
 
 




5 posts

Wannabe Geek


  #295389 2-Feb-2010 14:20
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Hi Jaxton
What are the SBS channels you refer to? I thought Sky used all H polarity now.

No offence taken from your "back to basics" suggestions. I agree. In case you missed it, I did realign the dish very accurately using the S22 analyser and I have tried using a different cable to the F/V STB direct from the LNB. There have been many iterations of this alignment process and YES some of the results have been misleading due to poorly crimped F connectors or other side issues.

I have had to return the S22 now so I will have to rest with what I have achieved.

As a side comment and along the lines of your last par about checking all small details - I noticed the dish mount pole was not vertical. Whoever installed it on the sloping tile roof had not used the part of the base bracket which would allow the pole to be vertical. Hence when any adjustments of the dish were made for Az/El there would always be an interaction and of course a small change in LNB skew. I re-jigged the dish mount to ensure a vertical pole and then made a realignment of Az/El. There is no doubt in my mind it is maximized. I got 83.5dbuv direct from the LNB with the LNB powered by the S22 instrument. Does that sound right to you? After that I adjusted the skew for least BER across a selection of Sky and F/V frequencies. It is not cler-cut but with trial and error it seems to have come up with a useful result. The plane of the LNB is approx 10o/c to 4o/c when looking at the dish - this fits with a comment about correct skew for the Sharp LNB which I read elsewhere.

I know it is a lot of messing around for something I could have paid a specialist for but I do like to learn from these experiences too.

Thks for your input




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