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What consequence for 75 Ohm output into 50 Ohm input ?

Discussion in 'Electronic Equipment' started by Peter, Feb 24, 2004.

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  1. Peter

    Peter Guest

    How should I recalibrate a HP 3336A 75 Ohm output signal generator (
    with better than .2 dB output resolution ) for use into 50 Ohm DUT's ?

  2. Ian Bell

    Ian Bell Guest

    Perhaps the simplest thing is to make a 75 ohm to 50 ohm T matching network
    with a fixed attentuation.

  3. Jim Yanik

    Jim Yanik Guest

    There are commercial L-pads to convert from 75 to 50ohm,with enough
    bandwidth and low VSWR for much higher frequencies.
    This generator only goes to 21 Mhz,IIRC.
  4. Peter

    Peter Guest

    Well, I mean : how should I read the ( in 75 Ohm output calibrated ) dBm scales
    when the HP 3336 generator's 75 Ohm output is inputted into 50 Ohm devices ?

    I guess there is a calculating factor for it.
    VSWR will be 1.5 ( right ? ) i.e. 4% power transfer loss.
    Is it as simple as that I can multiply the output scales with a 100/104 fraction ?

  5. Jim Yanik

    Jim Yanik Guest

    (Peter) wrote in
    A commercial L-pad should have the db loss marked on it,or in the datasheet
    that originally came along with it.
    Then you just subtract the db loss of the L-pad from the output db's.
  6. Guest

    We called them "min-loss-pads" years ago

  7. bg

    bg Guest

    If you terminate a 75 ohm generator with 75 ohms, the open circuit output
    voltage will drop by 6db right? Now if you terminate the same generator with
    50 ohms the open circuit output should drop by
    7.9588 db. The difference (7.9588 - 6) = 1.9588db.
  8. maxfoo

    maxfoo Guest

    75 ohm gen >----[ 43.3 ohms]--------o---------->50 ohm DUT
    86.6 ohms

    use a 43.3 ohm resistor in series and a 86.6 ohm in shunt to gnd...

    Remove "HeadFromButt", before replying by email.
  9. Jim Yanik

    Jim Yanik Guest

    Then use a power meter to determine the output level and L-pad loss.
    If you use SMD resistors and a PCB,you ought to achieve a decent BW.
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