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Satellite TV Intermediate Frequencies with Coaxial Cable

Discussion in 'General Electronics Discussion' started by Lawrenciumbc, Jul 30, 2016.

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  1. Lawrenciumbc

    Lawrenciumbc

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    Nov 2, 2015
    I understand that IF's are used in TV distribution to down convert for cost and design reasons of filters and amplifiers etc..., but I wondered if there was a limitation with the fact it normally needs to go down coaxial cable?

    Therefore; does general coaxial cable have a 'maximum frequency'? as the TV IF range is normally around 950MHz to 2.1GHz but this doesn't carry all of the channels I believe (the set top box must 'request' which channels it wants as signals are polarised).
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    no, incorrect
    every channel that is present at the LNB is also within the IF range. This is because when you take the channel freq say 12.5GHz and you subtract the Local Oscillator
    freq of 11.3 GHz, then the IF freq = 1.2GHz ( 1200MHz) ... this works the same for every freq within the reception range of the LNB

    polarisation swapping is just a simple DC voltage up the coax from the STB to the LNB to swap the antenna used within the waveguide of the LNB


    Dave
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    maximum useable frequencies of a given cable depend on its construction

    there are 2 main considerations
    1) attenuation / metre at a give frequency
    2) max useable freq for a cable because of its diameter

    these things are closely related and intertwined.
    eg a 1/2" diameter that has very low loss ( over a short length) for 10 GHz, wont work well for say 20 GHz even when the initial attenuation
    losses are not bad. The problem comes with the problem that at 20GHz, the cable starts acting more like an oversized waveguide at 20GHz
    and multi-moding of the EM wave occurs as it travels down the coax. This produces additional attenuation losses.

    OK, so we use a smaller diameter cable on 20GHz, but that then instantly produces additional normal attenuation losses

    It's a very complex subject ... one I have played with for many years and still don't have a total understanding of all the fine details


    Dave
     
  4. Lawrenciumbc

    Lawrenciumbc

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    Nov 2, 2015
    Ahhh yes sorry, that's an oversight on my part, of course an LO will just translate all of the frequencies down. I think I got a bit mixed up there haha!


    A bit more context on my question; I am studying a TV distribution system that uses fibre optic instead of coax. The IF range is 950MHz to 5.45GHz as the LNB that has been designed actually 'stacks' the signals on top of each other. Thanks to your previous comment, I get now that this is simply the two polarisations that are stacked and sent down the fibre (e.g. vertical might be 950MHz to 2.1GHz and horizontal might be the upper end of the range). Essentially, this is so that the fibre can feed multiple users off one LNB as it carries the full range down it (as opposed to having to be switched by the STB for H or V pol). There is another unit at the STB end which converts the optical to electrical for coax to the STB (it also has multiple outputs and behaves like a 'virtual LNB' so the STB is none the wiser).

    I was trying to work out if 'general everyday' grade TV coax cable is typically unable to sustain this 950MHz to 5.45GHz signal/range and that's why fibre has to be used. I know that using fibre brings other advantages such as low attenuation over large distances and is less susceptible to EM interference but hypothetically, is coax generally able to handle this otherwise?
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    yes

    only if the distances are quite short ..... less than ~ 30 metres for up to 2GHz and less than around 10 metres for up to 5GHz
    Attenuation at freq's over 1GHz start becoming really bad. it's the big killer of using coax
    5GHz and up, waveguide is commonly used
     
    Lawrenciumbc likes this.
  6. Lawrenciumbc

    Lawrenciumbc

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    Nov 2, 2015
    Ok that makes sense, so coax is generally fine for conventional TV systems as the coax run is probably less than 30m in most cases (or i guess a signal booster is used), but in the fibre system I described, coax would be impractical to cover most uses as most people's dish/LNB is over 10 meters from their STB (very generally speaking of course).
     
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