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Broadband transformer center tap?

Discussion in 'Electronic Basics' started by Hammy, Nov 21, 2009.

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  1. Hammy

    Hammy Guest

    I want to use a transformer on the output of a DAC on a AD9834 DDS IC.
    Heres is a picture of using a transformer on the output of one of
    Analog devices DDS IC's.

    http://i48.tinypic.com/25gvw5t.png

    If I want to transform from 200ohms (which is what the DAC needs to
    stay in compliance) to 50 ohms output Z. Is this correct;

    Turns ratio for transforming 200 -> 50

    Square root of 200/50 = 2

    Primary inductance for 3db at 10kHz is 159uH. I calculated this based
    on examples from application notes coil craft and here.

    http://www.electronics-tutorials.com/basics/wide-band-rf-transformers.htm

    I'm after as low a frequency I can get to at least 10 Mhz using the
    same toroid as this guy. (35T0501-10H)

    10 KHz to 85 MHz. A SIMPLE BROADBAND TRANSFORMER.

    http://www.cliftonlaboratories.com/easy_broadband_transformer.htm

    Now my question about the center tap primary. My calculated primary
    inductance is 159uH as already mentioned for correct transformation do
    I put two 159uH windings on the primary or just one 159uH winding on
    the primary and tap it at the mid point 79.5 uH?
     
  2. Andrew Holme

    Andrew Holme Guest

    Inductance of a solenoid is proportional to number of turns squared.

    Double the number of turns and you get four times the inductance.

    If they are closely coupled, two 40 uH windings in series make a
    centre-tapped 160 uH winding.

    The impedance seen across a centre-tapped transformer winding is four times
    the impedance seen between the centre-tap and either end.
     
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