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matching impedance

Discussion in 'Electronic Design' started by Pazzeo, Feb 19, 2005.

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  1. Pazzeo

    Pazzeo Guest

    Hi all, i have this problem: in my circuit i must insert a laser. The
    problem is that i do matching impedance at 75 ohm. This matching is on a
    wide band: 80- 900 Mhz.
    How do i realize this?

  2. James Meyer

    James Meyer Guest

    First you must measure the laser's impedance for the AC signal over the
    range of frequencies of interest. Make sure that the DC operating point, if
    any, is set to the final value during the measurements.

    Hopefully, the AC signal will be a small percentage of the DC bias. If
    it isn't, then the load will be non-linear and matching will be "problematical"
    at best.

    Once you have the measurements, convert them into an equivalent lumped
    circuit configuration of L, R, and C elements. At that point, if the matching
    circuit isn't obvious to you, post the circuit here and someone will do the math
    for you.

  3. James Meyer wrote...
    While your answer is correct, Jim, it may over-complicate the problem.
    Most lasers operate at currents of at least 40mA, and as a result have
    very low intrinsic dynamic impedances. Taking this as an assumption,
    I matched my wideband 50-ohm RF signal to a Hitachi laser diode with
    a small 47-ohm SMD series resistor, and added a Picosecond Pulse Labs
    bias-T to inject the dc current. My network analyzer and TDR tests
    showed a reasonable 50-ohm termination was obtained. Overall light
    throughput response was flat, dropping by 1dB at 1200MHz. It should
    be noted I took special care to maintain effective double-terminated
    50-ohm transmission lines for both the laser and PIN-diode receiver.
    It's possible the latter was responsible for the 1.2GHz rolloff. I
    noted with amusement that Hitachi's datasheet curves showed a peak
    at 600MHz, and dropped off rapidly thereafter. I surmised this was
    the response of their optical detector, or due to bad laser wiring.

    Hint 1: All my open-wiring distances were kept to under 1mm.
    Hint 2: Learn about making high-performance bias-T networks. Pazzeo
    can read the recent discussion here, and photo postings on a.b.s.e.

    Ahem. Pazzeo can "do the math" by replacing my 50-ohm resistor with
    a 75-ohm part. :>) But as to whether he can make good transmission
    lines and evaluate them, that may be another matter.
  4. Fred Bloggs

    Fred Bloggs Guest
  5. Fred Bloggs wrote... These are a start, but may be poor substitutes for the technology in
    the Picosecond Pulse Labs bias-Tee units. We made a PCB version of my
    lashup, using the MiniCircuits parts, and only got 600MHz performance.
  6. Pazzeo

    Pazzeo Guest

    Hi all, I thank all for your help. But I don't resolve my problem. I would
    send my circuit but it 's very big file.
    My email is true, if you want you can contact me so I can send you by email
    my circuit.

    Thank for your help
  7. Pazzeo

    Pazzeo Guest

    for Winfield Hill
    I search discussion but I don't find anything, can you help me? What is it
    If you can, you contact me so I send you my circuit, my email is true.
    I thank you very very much
  8. Rich Grise

    Rich Grise Guest

    a.b.s.e is an abbreviation for .
    It's another newsgroup where binaries (for example, schematics) can be
    posted. You can attach your schematic to your post, and it's OK, and
    everyone (everyone who gets that group on their server) can see it.

    Also, you can search newsgroups at or - don't use the American beta version,
    because it's an awful interface.

    Good Luck!
  9. JeffM

    JeffM Guest

    ...schematics...can be posted
    Using the beta has always been a matter of learning the tricks.
    Clicking *show options*, *Show original* gives a fixed-font display.
    The *Reply* link under *show options* allows a clean display
    if you want to post from Google and blockquote ASCII graphics.

    They have patched the page display as well.
    For ~3 weeks now there has been a Fixed font - Proportional font
    at the top of the thread page.
  10. Pazzeo

    Pazzeo Guest

    If you want i have posted in a.b.s.e. my schematic circuit.
    Please help me I'm very disperated :(.

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