Cable TV splitters

Discussion in 'General Electronics Chat' started by Stanly, Mar 31, 2014.

  1. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi everyone,

    I have two questions about cable TV splitters:

    Question #1: A number showing dB is given. What is that, please? I believe it has something to do with loss of signal, but I would kindly like more detail.

    Question #2: A number showing Hz (MHz or GHz) is given. What is that please? Is more better? My main splitter (where the cable line comes in from outside) has six cable connections going out. It says 5 - 1000 MHz. Is that good. Should I swap it for one that goes even higher than 1000 MHz?

    Thank you.

    Stanly
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    Yes, db refers to less signal coming out of the splitter. A 2 way "splits" the signal. Generally each port will be 3.5db down.

    No, higher isn't better. A splitter will be fairly flat across the range of it's ratings.

    If your signals are well within the ratings, there will be little improvement with a higher freq. one.

    Above is true when impedances are matched properly.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    dB or decibel is a unit of power or amplitude gain or loss based on a logarithmic scale.

    A power loss of 3dB represents a 50% loss, i.e. one-half the original power.

    -6dB means the power has dropped to 1/4 of the original power.

    -9dB is another 1/2 drop, i.e. 1/8 of the original power.
     
  4. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi everyone,

    Thank you inwo and MrChips for your replies.

    I have these questions, please.

    First, for MrChips, let's says that I have one splitter (call it Splitter A) and it has a loss of 3.5 dB, for example. Then let's say that Splitter B has a loss of 11 dB. Splitter A would be better, right?

    Now, for inwo. But how do I know that the signals are indeed well within range? My current splitter (the main cable line is coming in from the outside, and then it splits into six separate cable lines) goes to a maximum of 1000 MHz. What if I could find at my local electronics store a splitter that goes to a maximum of 2 GHz. Wouldn't I be wise to go through the small expense and hopefully the not great effort of replacing the 1000 MHz splitter in favor of the 2 GHz one?

    Thank everyone for listening.

    Stanly
     
  5. w2aew

    Member

    Jan 3, 2012
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    64
    True. The splitter with the lower dB number will give you less signal loss, that is good. To learn more about dB, you want to view my video on the topic:
    https://www.youtube.com/watch?v=1mulRI-EZ80

    If everything on the far side of the 1000MHz splitter is working as it should (CATV, Internet, etc.) then this means your cable service probably doesnt have signals that are significantly above 1000MHz, so the 2000MHz splitters would be a waste of money.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Question #1. Yes, 3.5dB loss means the signal you receive on your TV is going to be better than with a -11dB splitter.

    Question #2. Cable TV goes up to 1000MHz. A 1000MHz splitter will work. There is nothing gained by choosing a 2GHz splitter.
     
  7. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi w2aew and MrChips,

    OK. Very good and clarifying answers. It would make sense to buy and install a new splitter that has less signal loss (a smaller dB reading in absolute value), but it would not make sense to spend the time and effort for the purpose of getting a splitter with a frequency value higher than 1000 MHz.

    And w2aew -- that was a superb video about dB. Very impressive and instructive!

    Thanks to everyone.

    Stanly
     
  8. BReeves

    Member

    Nov 24, 2012
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    Except your not going to find a splitter with less loss than the one you have. Splitters are passive devices and the loss is fixed depending on how many branches. Each aditional branch reduces all the branches, only way you might help signal level is if you have a 6 way splitter but only need 5. Replacing the 6 way with a 5 would reduce the overall loss.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    an amplified splitter would have less loss, but might not work if you have cable internet.
     
  10. Stanly

    Thread Starter New Member

    Mar 29, 2014
    22
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    Hi BReeves,

    Thank you and everyone for your replies.

    So I have these numbers for splitters around my house:

    In a 6-way splitter, the dB loss is marked as 11.5

    In a 3-way splitter, the dB loss is marked as 7

    In a 2-way splitter, the dB loss is marked as 3.5

    I understand that the dB loss is the value of the common logarithm.

    Is there a precise way to show a ratio of number of splitters to dB loss?

    Thank you.

    Stanly
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    I haven't seen this done but think of it this way. Every time you share the signal you have to share the power.

    Splitter power 10xlog(power)
    2-way 1/2 -3dB
    3-way 1/3 -4.7dB
    4-way 1/4 -6dB
    5-way 1/5 -7dB
    6-way 1/6 -7.7dB

    Now there are losses at every split. Add an extra -0.5dB x number of splits to the above.
    This seems to work except for the 2-way splitter.
     
  12. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi everyone,

    So now I will suggest an answer to my own question.

    We will take this ratio:

    (log(dB loss)) / (# of splitters)

    Thus we have the following:

    log(3.5) / 2 = 0.27 (approximately)
    log (7) / 3 = 0.28 (approximately)
    log (11.5) / 6 = 0.18 (approximately)

    So, roughly speaking, every splitter that gets added leaves about one-quarter the signal strength to each line coming out. And we need to keep in mind that the dB loss that is marked on the splitter may well be rounded.

    How does this sound for a relationship of number of splitters to loss of signal strength?

    Stanly
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    What you have written makes no sense mathematically.

    3.5dB is already 10 x log(2.24)

    Hence taking the log of 3.5 makes no sense.

    Did you not understand my analysis?
     
  14. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi MrChips,

    I am sorry, but I guess that I don't understand your analysis.

    You wrote the following, and I will just cite what you had regarding the 3-way splitter:

    So could you please fill the numbers that I have for my 3-way splitter into your general formula. My 3-way has a power loss of 7 dB.

    I guess that I am being dense on this.

    Thank you.

    Stanly
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    A 3-way splitter has to split the power three ways. Each split gets 1/3 the power.

    log(1/3) = -0.477

    Convert Bels to deci-Bels (dB) by multiplying by 10.
    This gives a loss of 4.77dB
    Because of additional losses, I multiply 0.5 x 3 = 1.5
    I add the total loss = 4.77 + 1.5 = 6.27dB
    Your 3-way splitter says 7dB loss. That's close enough for me.
     
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  16. MrChips

    Moderator

    Oct 2, 2009
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    Just an interesting observation for the mathematically challenged.

    -10dB results in 1/10 the original power.
    -20dB is not 1/20 the original power. It is 1/100 the original power.
    -30dB is 1/1000 the original power.

    That's the beauty of the logarithmic scale.

    The same works with audio sound.
    A 1W audio amplifier sounds not too bad. To get 10dB gain you have to increase the power to 10W.
    To get twice the perceived amount of audio power from 10W, you need not 20W but 100W.
    For 30dB, you need 1000W.

    Normal speaking sound level is about 60dB.
    70dB is noisy city traffic.
    80bB is a noisy factory or restaurant.
    90dB is noise from a chain saw. You would need a 1000W audio amp to compete with that.

    If you are operating a chain saw or 1000W audio amp without hearing protection you need your hearing and your brain examined.
     
    Last edited: Apr 1, 2014
  17. Stanly

    Thread Starter New Member

    Mar 29, 2014
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    Hi MrChipps,

    Excellent. I understand your algorithm now. It seems to make sense.

    Thank you very much.

    Stanly
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Keep in mind that splitters do not necessarily divide equally.

    A 3way may be split and split again internally. As a 2way connected to a 2way.

    This will leave one port down 3db (3.5) and two ports down 7 (7.5).

    And not all devices that look like splitters are splitters. Such as taps, diplexers, and tuned combiners.

    Don't forget the bottom range when swapping. Cable with internet often needs down to 10mhz for return path and housekeeping.

    The term db is often, loosely, used to denote signal level in volts (not generally power). "0" db (1mVRMS) is a reference level of 1mv into 75 ohms. Chosen as the minimum level signal for perfect picture in the 50s.

    Design specs usually call for 10db at each tv input. Although most set will do ok at -10.
     
  19. quarkelec

    New Member

    Apr 1, 2014
    3
    0
    thanks for sharing above information.
     
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