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455kHz IF Transformers

Discussion in 'Electronic Design' started by Bill Bowden, Jan 29, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    There are usually 4 RF transformers in a AM radio with colors of Red
    for the
    LO oscillator, Yellow for the mixer or 1st IF, White for the 2nd IF,
    and Black
    for the 3rd IF. I notice the impedance of these transformers ranges
    from
    50K : 500 for the 1st stage (yellow), 30K : 500 for the second
    (white), and
    20K : 5K for the 3rd (black) stage.

    I think I understand the 500 ohm secondary impedance which matches
    the
    input impedance of the next stage, but I can't figure out why the
    primary
    impedance is different for the 3 stages?

    The 5K secondary impedance of the black (3rd stage) is probably
    higher
    to produce a higher voltage for the detector.

    What exactly does a primary impedance of 20K mean and how would
    that be setup with a bipolar transistor?

    -Bill
     
  2. Guest

    I haven't seen a radio built like that in decades. Are you referring
    to a particular model you're restoring?
     
  3. I have a 1960 app note describing the design of an
    early AM receiver using germanium transistors.
    This was a two band receiver, 540-1640KHz and
    155-280KHz.

    They used a pair of 28K:800ohm transformers and a
    detector transformer whose impedances were not
    specified, merely described as 1.85:1 turns ratio.

    28k:800ohm was the 470KHz output/input impedances
    of the particular transistor used, (Mullard OC45,
    running at about 1mAdc).

    They did note that the output impedance of the mixer
    stage in the 540-1640KHz band generally exceeded
    35k at 470KHz, but they used the 28k:800 transformer
    simply for economy. So that might explain your 50k
    mixer output transformer.

    The impedances around the detector are not specified,
    but the OC45 is running at a higher current so this
    might explain an apparent lower output impedance.
     
  4. Each stage runs at a different collector current, resulting in a
    different optimum collector impedance. In those days they tried to
    get maximum gain out of each stage, so they took extra care, even if
    it required different transformers for each stage.

    As it turns out, the exact impedances are not all that critical, as
    power transfer only varies as the square-root of the mismatch, and
    those old transistors had very loose specs to begin with.
     
  5. Robert Baer

    Robert Baer Guest

    The "standard" (tube) configuration was known as the all-american
    five, meaning 5 tubes used counting the rectifier for the B+.
    4 RF transformers has been common for tube and transistor radios, but
    the impedances were different as necessary to match lo-Z transistors.
     
  6. Bill Bowden

    Bill Bowden Guest

    I'm not sure how to calculate the output impedance of the transistor,
    but I think it's related to the supply voltage and bias current. And I
    see these same transformers used in a variety of radios running on
    different battery voltages.

    So, if the supply voltage is say 6 volts, the bias current for the 50K
    transformer would be around 6/50000 = 120uA

    Seems a little low and the transistor gain may be higher at greater
    bias currents?

    Suppose we use a higher bias current to obtain a higher transistor
    gain. How would that effect operation?

    -Bill
     
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