Re: How transmitters work.
- From: et472@xxxxxxxxxxxxxxxxxxx (Michael Black)
- Date: 20 Apr 2005 19:20:46 GMT
"Jason" (theologygeek@xxxxxxxxx) writes:
> Please forgive the pure ignorance of this question. I understand that
> this is very basic stuff that I should probably know, but I have never
> been to class to learn about this stuff, nor have I found any tutorials
> that explain this stuff in simple everyday terms. I passed by
> technician and general class amateur license exam by memorizing the
> right buzzwords for this stuff, but not actually understanding the
> concepts. I admit, I am ashamed of this fact. Here's what I have
> gathered so far.
>
> 1. DC current flows through a crystal.
No. A crystal is made of quartz, which isn't a conductor. It will
never pass DC. Electrically, a crystal is the equivalent of an inductor
and capacitor in series, hence it's ability to control frequency.
Really, you need to get a book about radio and start reading. There
is a big difference between asking questions based on points you
don't understand, and asking us to write that material down here.
You don't have the basics to understand how it works, and you
have made too many leaps in your questions.
Here's one place to start. Take an amplifier of some sort (even
a computer with speakers connected to a soundcard), and plug
in a microphone. Hold the microphone near the speakers, and
start turning up the volume. You'll start getting a squeal,
that's the positive feedback, ie oscillation.
The signal goes through the amplifier and then is fed back to
the input, not just once but an infinite amount. It's no longer
amplifying an external signal, it is oscillating.
The frequency of the howl is determined by the inherent characteristics
of the setup, things like the response of the microphone and the speaker,
and the response of the amplifier, and even the microphone's distance
from the speaker. You don't have much control over the frequency, but
it is a good demonstration of oscillation.
Usually, one wants something more definite. So instead of the
speaker/microphone combination, the output of the amplifier feeds
the input directly. And between the output and the input is
some selective element so the feedback is at that frequency. Put
a bandpass filter in there that has a center frequency of1KHz,
and the thing will oscillate at 1KHz.
The crystal acts as the selective element, to ensure that the
oscillator runs at that frequency.
A crystal is one of the better filtering devices available, it
doesn't generate unwanted signals.
But the active elements are not so perfect, and they will often
add unwanted signals. Indeed, the design of some transmitters
will count on generating multiple signals, with the wanted
one filtered out.
Michael
> 2. The resonance of the crystal causes the current to form waves around
> the frequency of crystal
> 3. This new "pulsating" current is then amplified and then sent through
> the crystal again. (This is called positive feedback)
> 4. The pulsating current is sent through a low-pass filter to "filter
> out" unwanted frequencies. (That's why I said "around the frequency"
> of the crystal)
> 5. From the filter, the current flows to an antenna which converts the
> current to RF radiation.
>
> Comments and help would be greatly appreciated.
>
.
- References:
- How transmitters work.
- From: Jason
- How transmitters work.
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