Interference on Some Sky Channels

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dolsen

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#584695 21-Feb-2012 17:38

manaia:

I looked up the spreadsheet: 1.8GHZ DECT phones are known to cause intereference on two streams, 5 (which has Four) and 11 (which has Prime). The weird thing is that the stream that the interference affects depends on the LNB type (10750 or 11300) so you shouldn't be getting interference on both. My guess is that you have a dual throat, single output 10750 LNB for your MySKY. It's possible that this is failing and causing problems on the other stream. SKY now has quad output LNBs on 11300 to help deal with the interference problem where we can't do a decoder swap. Also you could move your DECT phone as far away from the STB as possible.

Hi Manaia

A couple of questions,

Does your spreadsheet also have other lnb frequencies listed? I'm pretty sure that my Dad has one which is 10600 (it's a fairly old sky install) and am wondering if this will be listed in the spreadsheet. Do you have a record of what lnb is in use at an address?

Are all of the quad output lnbs 11300, or, are these special ones to get around interference in special cases?

tkr001

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#584719 21-Feb-2012 18:23

My quad lnb is 10750

Niel

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#584860 21-Feb-2012 22:23

The LNB converts the incoming signal of ~11GHz to a lower frequency of ~1 to 2GHz. The issue is that this IF band includes 1.8GHz which is used for DECT. The cables are well shielded and seldom suffer from interference, but the STB design age determines which EMC standards it was tested to (and they get stricter every few years). Also with every design the component size shrinks and shielding/filter components become more readily available, so blocking interference becomes easier. Multi-layer PCBs also become more readily available and lower cost, which greatly improves immunity.

The old STBs were designed before DECT which is why the old STBs were not EMC tested or designed with consideration for DECT. EMC standards also take a number of years to catch up to new technology standards, and then there is a grace period.

The LNB local oscillator can run at 2 frequencies to select 2 receiving bands, and the IF output from the LNB includes the DECT band (which is 1.88GHz - 1.90GHz, or since 2005 in the USA slightly higher). I've calculated the satellite frequencies that down converts to the DECT band to be 11.63GHz to 11.65GHz for the low band LNB, and 12.45GHz to 12.50GHz for the high band LNB. I have not looked up which channels transmit in these 2 bands, but are likely to be Four and Prime as indicated by Manaia.

PS: This is my limited understanding of LNBs. Numbers might be different, principal is the same.

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Earbanean

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#586284 24-Feb-2012 14:48

I've tried putting my phone right next to the decoder and making calls on it right by the decoder, but I can't seem to reproduce the problem. From that I would have thought that the phone is not the problem.

Is there any way to explicitly determine whether the phone is causing the intermittent interference?

Niel

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#586429 24-Feb-2012 19:52

Check the signal strength (or better, the bit error rate) on the STB while making the call. The digital signal has a lot of error correction so you might not notice the problem until interference is bad (or until bad weather when the signal is weak).

You can never have enough Volvos!

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