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basics of an RF circuit?

Discussion in 'Electronic Basics' started by andrew_h, Feb 17, 2006.

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  1. andrew_h

    andrew_h Guest

    What are the basic components of an RF transmitter, and there
    respective roles ? (I'm learning electronics so excuse the
    newbie-language!). Any help would be great.

    Transistor(s) - used to amplify the signal ? Possibly what else?
    Resistors - to limit current entering transistor base, collector and
    biasing ?
    Capacitors - ??? filter out higher frequencies than needed? Why are
    caps sometimes connected to the collector of a transistor?
    Crystal - I understand the concept of this, i.e. say 27 Mhz - does it
    filter the inductor frequency to precisely what the crystal is labelled
    Inductor (which antenna eventually links to) - does this generate radio
    frequency (which must then be amplified by transistors)?
    Signal Diode - ?

    Sorry again if I sound like a newbie, I'd like to learn as much as I
    can bout RF circuits.

    Any answers would be appreciated.

  2. Phil Allison

    Phil Allison Guest

  3. Get yourself a copy of the Amateur Radio Relay League (ARRL) Handbook. It's
    published every year or so, and has been since about when radio was first
    invented, so you shouldn't have trouble finding old editions for not too
    much money.

    The ARRL Handbook is chock full of very good introductory material on basic
    electronics, RF electronics, and practical applications. It is precisely
    aimed at enabling people like you - intelligent people with an interest but
    not much academic background in electronics - to learn and do interesting
    things with radio electronics.

    But it is just not possible to answer your questions in a brief Usenet post,
    in a way that will actually give you useful understanding. Asking "what
    does a capacitor do in an RF circuit" is a lot like asking "what does a +
    sign do in a mathemetical equation". The nature of a capacitor is simple
    (at least theoretically - at RF, things don't always behave like perfect
    components), but it can perform many different functions depending on the
    circuit that it's in, limited only by the creativity of the designer. The
    magic happens in the relationship between the components, not in the
    components themselves.
  4. Bob Myers

    Bob Myers Guest

    The components you listed could be considered the basics of
    practically ANY electronic product. It would be better to ask
    about the basic "blocks," or fundamental circuits which would
    be used together to create a practical transmitter.

    For RF communications, the most common "basic building blocks"
    would be:

    Oscillator: This is a circuit which generates a sinusoidal output
    (a "sine wave signal"), or more broadly SOME form of
    periodic waveform, with no input other than power. At a
    minimum, it is a transistor or some other active element which
    is basically being used as an amplifier, plus some form of
    frequency-selective positive feedback path; that latter item may
    be either a combination of components such as capacitors and
    inductors which act as a filter, or a quartz crystal - which ALSO
    acts as a filter, just through a slightly different mechanism.
    There are various named classes of oscillators, which are most
    often distinguished by the form this filter and feedback path

    Modulator: A circuit which somehow varies the signal produced
    by the oscillator, in accordance with the variation of an input
    signal (i.e., the signal carrying the information you want to transmit).
    There are various forms of modulation, and so quite a few different
    forms of modulators. This is too complex a subject to even begin
    to cover here.

    Output amplifier: A circuit which increase the power of the
    modulated signal for transmission, and which drives the antenna.
    It is possible in some types to combine this "output amplifier"
    function with the "modulator" function, but you'll learn more about
    that as you get into the specifics of transmitters.

    There can certainly be other circuits and components involved;
    there can be additional amplification between these stages,
    tunable oscillators (for varying the transmit frequency), additional
    filtering here and there, and of course all of this is going to require
    an appropriate power supplier. I would strongly second the
    recommendation that you find a good book covering the basics
    of RF and aimed at the amateur or hobbyist - as noted, the
    ARRL Radio Amateur's Handbook is certainly among the best
    choices here.

    Bob M.
  5. Amen to that. Anybody interested in RF should have a copy.
  6. Bill Bowden

    Bill Bowden Guest

  7. andrew_h

    andrew_h Guest

    Thanks for everyones post. Yes, I realised after posting that the
    questions were extremely vague - I know a capacitors have hundreds of
    uses, it was stupid to put the question like that.

    I had meant that in a basic 27 Mhz transmitter, like that used for
    radio controlled cars (late 70s), what the BASIC foundations were - Bob
    Myers answered this beautifully. I had thought that the xxx pF
    capacitors etc that were connected to the transistors were used for a
    particular purpose that was common to alot of RF circuits.
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