# basics of an RF circuit?

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

1. ### andrew_hGuest

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
at?
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.

Thanks,
Andrew

3. ### Walter HarleyGuest

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

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 MyersGuest

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
take.

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"
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. ### Charles SchulerGuest

Amen to that. Anybody interested in RF should have a copy.

7. ### andrew_hGuest

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