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Shared power and data pair

Discussion in 'Electronic Design' started by Greg Neff, Dec 9, 2003.

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  1. Greg Neff

    Greg Neff Guest

    I have to supply both power (48VDC @20ma) and data (1.25Mbps) over a
    single pair. The requirement I'm having trouble with is that the 48V
    supply shall provide a minimum 12K output impedance in the 0.5-2MHz
    range. A 3.9mH choke is the obvious answer, but the SRF of chokes
    this size is way too low. Does anyone have any ideas to help me out
    here? I could string a bunch of small chokes together in series, but
    I would like a better solution. I am thinking virtual inductor, but I
    need some pointers on this...



    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  2. I read in sci.electronics.design that Greg Neff <>
    Can you not use an electronic current source?
     
  3. Putting an inductance of 4 millihenries (with a self resonant
    frequency at or above 1 MHz) in series with the 48 volt supply would
    do the trick. Finding such an inductor might be tricky. If the data
    is differential (and it would be good if it were) would allow the
    inductor to be split into two 2 millihenry units, one in series with
    each line.
     
  4. That was my first thought. Depends on the swing of the signal and how
    much voltage drop he can stand to the power.. but if we make favorable
    assumptions a discrete current source/sink could do it easily.

    Best regards,
    Spehro Pefhany
     
  5. Greg Neff

    Greg Neff Guest

    I have to supply 48VDC +/-10%. The maximum load will be 20ma.


    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  6. Greg Neff

    Greg Neff Guest

    The data is differential and transformer-coupled to the pair. A
    capacitor will keep the transformer dry.

    A SRF of 1MHz is right in the middle of the 0.5MHz to 2MHz range that
    I'm worried about. This is what I'm trying to avoid.


    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  7. A 4 mHy inductor with a self resonance at 1 MHz has 12k ohms impedance
    at both .5 and 2 MHz (inductive at .5 MHz and capacitive at 2 MHz),
    and a higher impedance at all frequencies between, with a peak at 1
    MHz. Is there a problem with this?
     
  8. Greg Neff

    Greg Neff Guest

    Well, I'm not sure. The data is Manchester encoded, so it has
    1.25MBps and 0.625MBps frequency components. I don't want the
    inductor to introduce any peculiar effects due to the change in type
    of impedance at these frequencies. Maybe I'm being paranoid, but I
    want to be safe.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  9. The minimum impedance spec should cover this possibility. If the
    transmission line has an impedance of a few hundred ohms or less, and
    the differential receiver terminates it near that over the necessary
    spectrum, the resonance of the series inductance should produce
    acceptable distortion as long as its impedance is high enough compared
    to the transmission line impedance.
     
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