Connect with us

Beginner Design BandPass Filter

Discussion in 'Electronic Design' started by Jim Douglas, Dec 4, 2004.

Scroll to continue with content
  1. Jim Douglas

    Jim Douglas Guest

    I am getting back into electronics, reading as much as I can and
    experimenting. I got to a part in a book that discussed filters and became
    very interested. I am also a Shortwave radio user and decided to create a
    bandpass filter for the SW radio, 3MHZ-30MHZ.

    Now I start researching and there are way tooo many types for a beginner to
    start with. I realize that this filter may not be the best but want to build
    my own. I would like to create a good filter for that range? Recommendations
    on where to start, which filter type is better?
    Is one better than another for a beginner to tweak and build? Is one better
    in the freq range?

    Sorry if this is too basic.


    Jim Douglas
  2. Tim Wescott

    Tim Wescott Guest

    First, a fixed bandpass filter from 3MHz to 30MHz isn't going to do you
    much good unless you have a stupendously crappy radio. If you wish to
    build one none the less you'd be best to just cascade a 30MHz lowpass
    with a 3MHz highpass.

    An adjustable bandpass filter that lets you peak your receiver's
    response at one frequency and avoid strong signals that are pounding
    your receiver's front end may be useful, but making one that goes
    continuously from 3 to 30MHz would create difficulties, but it could be

    I'd suggest you get a copy of the ARRL Handbook and start perusing it --
    it has lots of useful information for this sort of thing and who knows?
    Maybe you'll want to get a license and get on the air.
  3. John Larkin

    John Larkin Guest

    Right. It will cost a bit of gain, too.
    It's called a 'preselector', a tunable bandpass filter. They typically
    have a band switch and a variable capacitor.

  4. Hi Jim,

    You would be better off posting your request to where you are more likely to meet some
    kindred spirits. As Tom has said, you really require a tuneable
    filter for the 3 - 30MHz range but as you haven't given any
    details it is difficult to give specific advice.

    There are broadly two types of front-end tuners, those that
    attempt to match the antenna to the receiver so as to increase
    signal levels and those that are meant to provide more
    selectivity. The first may help you to pull weak signals out of
    the background noise and the second to eliminate problems with
    strong stations on relatively close-in frequencies. Of course,
    the best type is a combination of both as just providing a better
    match will not necessarily give you greater selectivity.

    IMHO, the easiest way would be to experiment with a simple
    parallel circuit tuned to cover one octave (3 - 7MHz) using
    different capacitor values/tapping points until you find a
    combination that improves matters for you - then cast it in stone
    and start on the next octave (7 - 14MHz). Remember that the
    looser the coupling (i.e. the lower the tap is down the coil) the
    greater the 'Q' and the better the selectivity. Link couplings
    are nice too but not as easy to adjust. The easy part is that you
    can all this with everything nailed to a lump of wood.

    Cheers - Joe
  5. With this sort of oscillator, you have to analyze several completely
    separate phases.

    One phase starts the moment the output transistor begins to turn off,
    and collector voltage heads toward ground by the pull down action of
    the 470 ohm resistor. during this phase, D1 first forward biases and
    clamps the base voltage for Q1 to no more than a diode drop and C1
    charges through R2.

    Eventually (this is where a bit of RC analysis comes in) the current
    through D1 goes through zero and C2 changes charge rate because the
    current through R1 in series with R2 sets the charge rate for C2.
    During this phase, the base voltage at Q1 swings between negative one
    diode drop toward positive one diode drop, where Q1 begins to conduct
    as the current through R1 detours from C2 to the base emitter

    Once Q1 conducts enough to start to turn Q2 on and the voltage drop
    across R3 starts to increase, a positive feedback loop forms that very
    quickly drives Q1 and Q2 into saturation because the positive voltage
    change across R3 gets coupled back to the base of Q1 through C1 and
    R2, making the current from R1 insignificant. This phase lasts as
    long as C1 R2 can supply enough current to Q1 to keep both transistors
    well saturated.

    Once C1 charges enough that this current is no longer available, Q1
    and Q2, while still conducting somewhat, fall out of saturation enough
    that the voltage drop across R3 starts to sag. At that moment, the
    current through C1 R2 reverses direction and Q1 shuts down and you are
    at the starting point of this description.

    Each of these phases has to be analyzed to predict both the on and off
  6. (snip)

    Please disregard my last post. I not only jumped between threads, I
    jumped between newsgroups. This was an answer to an entirely
    different post. My apologies.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day