NE602 - Calculating input bandpass filter

Discussion in 'Wireless & RF Design' started by atferrari, Feb 21, 2013.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    I intend to experiment the basics with an NE602 double balanced mixer. Frequency of interest 10 MHz.

    From all circuits I found, there are two, more or less close to what I intend to build winding my own coils. No toroids available for the moment.

    My questions:

    a) The double tuned bandpass filter in this design is symetric: the secondaries are connected to the antenna pot and to the pins 1 & 2 of the NE602.

    How is this possible since I am designing for a yet unknown antenna (thus unknown impedance) while having 1500 Ohms at the input of the NE602?

    b) The other design (page 11) connects the secondary to the antenna and the primary of L2 straigth to pins 1 & 2.

    I calculated Xl (= Xc) = 333 Ohms. Is this reasonable to match the 1500 Ohms input of the NE602? What about the secondary of L1 respect to the antenna?

    c) I believe I will end winding two similar tuned transformers adjusted with trimmers.

    Understanding that the secondaries will show an impedance proportional to their number of turns vis a vis that of the primary, what values should I use to calculate my coils?

    d) Could I use a tap to the primary instead of a secondary to adapt impedances?

    e) Today, revising my initial calculations for the possible values of L and C I found myself in doubt: in an LC tank at resonance, reactances are XC = XL, say 330 Ohms each.

    What impedance should I actually measure? Provided that they cancel each other, only actual R is left. Am I right?