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Help - Impedance matching

Discussion in 'Electronic Basics' started by John Smith, Dec 21, 2003.

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  1. John Smith

    John Smith Guest


    Hope this O.K. place to inquire about the subject matter:

    I have a circuit that modulates a diode laser (25mW) at 200Mhz max.

    Now, I am in a situation where I have to move the laser 15 inches away from
    the circuit board where the laser used to be directly soldered on to.

    The coax cable I am thinking to use is ether RG-178 (50 Ohm) or RG-174( 50
    and am understanding the typical small laser diode impedance is about 2 to 4
    Ohm range. I am thinking, I need to come up with ~46 Ohm attached to the
    diode laser in orde to match the impedance.

    What is the easiest way of matching the impedance for my application.
    Would it be possible to use a 50 Ohm off-the-shelf "Impedance Matching
    Pads" ?

    Thank you very much for your suggestion...
  2. bg

    bg Guest

    If you cut the coax to 1/4 wave, the diode resistance will appear at the
    input to the coax. No other matching is neccessary, providing it worked OK
    without the coax.

    John Smith wrote in message ...
  3. Thank you very much. Yes, the device used to work w/o the coax.

    Let's me clarify what I am understanding:
    The total length of the coax should be 1/4 of the wavelength of
    electromagnetic wave modulating at 200Mhz.

    Is that right ?
  4. bg

    bg Guest

    A 1/4 wave section of line at 200 MHZ is very close to 15 inches. There is a
    propagation delay specified for the cable which means that the wave actually
    is traveling X% slower., so the cable will be a bit shorter than the free
    space length.
    300/f mc = wavelength in free space, so 200Mhz = 1.5 meters
    1.5 / 4 = .375 meters
    ..375 meters = 14.7 inches
    You can look up the spec of the cable you plan to use to see how much
    shorter the propagation delay would make it. Unless you are on the ragged
    edge of power output, a 15 inch cable probably won't make any difference.
    And you can use any cable, the Z won't matter.
    William Cowardin wrote in message ...
  5. bg

    bg Guest

    I made a big mistake here, you want 1/2 wave !!!! So double the length to
    29.4 inches. The velocity factor for a simple RG59u like that used for home
    video is about 66% which puts the cable length at about 19 and 1/2 inches.
    This should give you enough cable left over to tweak if neccessary. Sorry
    for the brain cramp!
    William Cowardin wrote in message ...
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