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low frequency signal over coaxial cable

Discussion in 'General Electronics Discussion' started by mmuj, Nov 7, 2011.

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  1. mmuj


    Nov 7, 2011
    Hi All

    I need to use low speed phone line modem chip (cmx868) to communicate over dedicated 8 miles long coaxial cable. cmx868 uses 1.2/2.4kHz signal for tx/rx. as it is low frequency signal do i need worry about coaxial cable characteristics impedance which is 50 ohm?

    2.4kHz signal wave length is over 80 miles (roughly speaking) so signal quater wave length is over 20 miles, more than twice the cable length so in transmission line terminology my coaxial cable length is "short". Apart from copper resisitive losses due i need to worry about capacitance of the cable?

    Thanks in advance

  2. daddles


    Jun 10, 2011
    Even if you wanted to put a DC signal on that cable, you'll still need to deal with the distributed capacitance. You should be able to calculate what the total capacitance will be from the manufacturer's spec of capacitance per unit length. For example, if the capacitance is 100 pF/m, you'll be dealing with a little over one uF for an 8 mile cable.
  3. shrtrnd


    Jan 15, 2010
    Sheez, I'd be worried about the power losses over that length itself.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    When I was with Telecom, we would use lumped inductances that were in series with each leg of the line. This would go some way to cancelling the effects of the capacitance 88mH from memory.

    8 miles is nothing, its just a stone's throw for a 2.4KHz signal. Thats within the normal voice range of a telephone audio cct.

    mmuj .... what sort of voltage level is coming out of that modem chip ? if its only 5 - 10V then you may have to look at increasing it a bit. It would be worth trying as is for a start, after all the old manual phones worked over many many miles with just a couple of those old large 1.5V primary cells.

    Last edited: Nov 9, 2011
  5. mmuj


    Nov 7, 2011
    thanks everyone

    CMX868 can generate upto 0dBm signal output but it goes through line termination causing loss of 6dBm resistor plus transformer loss say 1dBm. Output to the line is -7dBm and for 600 line 0dBm = 0.775V. so i would say less than half a volt is actually appearing on the line. but is nornal modem voltage levels for phone line they work fine even to 15 miles.


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