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.5-600 MHz LT5512 impedance matching

Discussion in 'Electronic Design' started by [email protected], May 23, 2007.

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  1. Guest

    I'm working on a homebrewed receiver and am looking at using the
    LT5512
    as the mixer. I'm not sure of how to arrange an impedance match and
    unbalance to balance conversion over a wide range. Perhaps people
    can comment on the following so I can see if I'm on the right track.

    The LT5512 datasheet says:

    Frequency Impedance
    10MHz 18.2 + j0.14
    240MHz 18.1 + j2.8
    450MHz 18.1 + j5.2

    One simple approach seems to be:


    50 ohm input - ----------- -----------|---------- IN+
    ) ( ) ( ___
    ) ( ) (- ground --- 6 pF LT5512
    ) ( ) ( |
    ground - ----------- -----------|---------- IN-
    3:1 1:1
    Coilcraft Coilcraft
    WBC3-1TL WBC1-1TL

    The 3:1 transformer converts the unbalanced 50 ohm input to 16.67
    ohms.
    The 1:1 center tap transformer converts the unbalanced 16.67 ohms to a
    balanced 16.67 ohms. The center tap also supplies a DC ground for
    biasing the LT5512 inputs. The 6 pF capactor reactance is:

    Frequency Reactance
    10MHz 2652.58
    240MHz 110.52
    450MHz 58.95

    placing it in parallel with the LT5512 inputs results in:

    Frequency Impedance
    10MHz 1/(1/(18.2 + j0.14) + 1/2652.58) = 18.21
    240MHz 1/(1/(18.1 + j2.8) + 1/110.52) = 17.58
    450MHz 1/(1/(18.1 + j5.2) + 1/58.95) = 16.7

    Questions:

    1) How off base is my simple analysis?

    2) Is there a better way to do this?

    3) How significant is the 1.6 ohm mismatch at 10 MHz?

    -- John
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  2. Tom Bruhns

    Tom Bruhns Guest


    There's no need to worry about such a small mismatch. Especially if
    you will be connecting the antenna directly to the input, you're
    pretty much guaranteed that the source won't be 50 ohms at any but a
    very few spot frequencies. It will have reactance and resistance that
    varies all over the map. But even if it's driven from an amplifier, I
    can guarantee that between the transformer(s), the layout, and the
    amplifier itself, you won't have a match within 10% over very much of
    that wide band.

    You can use the freeware RFSim99 to play lots of "what-if" games,
    including seeing how much loss there really is for various impedance
    mismatches. RFSim99 also comes with a program called "spur search"
    that you may find useful in indentifying mixer spurs (and intermod
    products).

    In any event, you likely will end up being more interested in the
    noise figure than in the power match; you'll want to match for lowest
    noise, which is essentially never the same as matching for best power
    transfer.

    Another thing you should be careful about is selectivity: if you let
    everything from the AM broadcast band up through UHF TV into the
    input, you will be letting in some big signals along with the
    presumably small ones you will be interested in, and distortion
    products will limit what you'll be able to hear. It's customary to
    put some selectivity ahead of the first less-than-stellar-linearity
    parts such as that mixer. If you had a +65dBm input-referred third
    order intercept, I'd say you could probably do without the
    selectivity, but much less than that and you'll benefit from
    selectivity. There are times when you can't do that, like if you want
    to digitize a wide bandwidth all at once, but it doesn't sound like
    that's what you are after.

    There are some good books on modern receiver design. Maybe your
    library has one or could get one for you. Even a quick review of the
    key issues would probably benefit you, if you haven't already done
    that.

    Cheers,
    Tom
     
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