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how to use a varicap

Discussion in 'Electronic Basics' started by albert, Oct 1, 2003.

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  1. albert

    albert Guest

    Hi,

    I'm implementing a modulator using a VCO. In order to tune to
    different frequencies, a varicap is used. The varicap should be
    connected between two pins of the VCO IC, by varing the reverse
    voltage between the two diodes of the varicap, different values of
    capacitance can be obtained. However, when the pins of the varicap are
    connected to a volatage source, at the same time, the corresponding
    pins of the VCO IC are also connected through the voltage source.
    Consequently, the varicap is bypassed! How to use a varicap on a
    breadboard then? Maybe this is a stupid question, but I really have
    little practical experience.


    Thanks a lot!
     
  2. John Fortier

    John Fortier Guest

    The varycap needs to be isolated from the chip. Use capacitors to achieve
    this. The overall capacitance will be the two isolating capacitors and the
    varicap in series, so you will need to amend your calculations accordingly.
    The connection sequence should be - chip terminal, isolating capacitor,
    varicap, isolating capacitor, other chip terminal.

    You will also need to ensure that your bias voltage is completely free from
    any interfering signal. Since the back biased varicap has a very high
    impedance it's possible to use a high resistance coupled with a decoupling
    capacitor to reduce any bias line interference to a level where it will no
    longer cause problems. Connect the resistors between your biasing voltage
    and the varicap and the decoupling capacitor between the bias voltage supply
    and ground. You will need to use a larger electrolytic and a smaller
    ceramic capacitor in parallel to cover all frequencies which may interfere.
    I would suggest 1 megohm resistors, 10 nF ceramics and 10uF electrolytics.

    If the circuit isn't balanced across the varicap, that is,if one leg of the
    varicap is connected to ground, the above holds true, but only applies to
    the non earth end of the varicap.

    Hope this helps

    John
     
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