Q. FST3257 CMOS Multiplexer as a DBM

Discussion in 'Homebrew and Kit Projects' started by VK4HAT, Sep 21, 2019.

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  1. VK4HAT

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    fst3257-mixer-schematic.jpg

    In various designs on the interwebs i have found a number of permutations of using this IC as a mixer as a DBM in a superhet RX. With bias applied to 1 port only, both ports as above, bias applied to the S port, using 2 switches as above in 1x1 or using all 4 switches in a 2x2 configuration, and with the B bus as input and A bus as output, the reverse of above. What i have not found is the pro's and cons of all these, slightly different ways of doing things.

    Has anyone used this IC as a mixer? Is so what are the pro's and cons of all these minor variations? I am trying to skip the build 12 different versions to characterise all of them to see what is actually is needed and what is folk law. Real information i am having trouble finding.

    And before anyone says, look in the datasheet, go look in the datasheet yourself. Actually if you dont know, dont even answer my question. I have had enough of them today as it is. ;)
     
    YG8RQN likes this.
  2. KA0HCP

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  3. KA0HCP

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  4. VK4HAT

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    The answer is bias at least the RF input to 1/2 Vcc and get S close to its max input rating because it improves IP3.
     

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