Before I blunder onward, permit me to say that I've locked horns with
the censors on Wikipedia (actually Wikibooks) and got a first hand
taste of why some of the articles are rather marginal. However, it's
still the best reference around for obtaining general information
about almost any topic and tends to be more understandable than
someone peer reviewed research paper or marginally reviewed book
extract. In topics that I am familiar, I can usually find something
in the article that could use improvement. I usually include a
Wikipedia reference primarily so that those unfamiliar with the topic
can get a general understanding.
Nothing is perfect. Finding errors is not sufficient grounds for
hatred. A lawyer friend has a similar problem with the various laws
and legal decisions. All of them could use improvement and almost any
written law can be misread and misinterpreted. Unlike Wikipedia,
readers are unable to repair the written legal system. Despite
chronic deficiencies, the legal system is still functional, and there
are few attorneys that *hate* the written law simply because there are
mistakes. Some tolerance would be helpful here.
The original paragraph is poorly written and you managed to
misinterpret it. I'll try to do better.
A direct conversion receiver usually uses only a single stage for both
detection and filtering. In any receiver, there is only one stage of
detection. The articles reference to
"...and thus requires two stages of detection and filtering"
should read
"...and thus requires separate stages of detection and filtering". Is
that better?
Yes, it's correct. Mixing with a local oscillator (or reference
signal) on the operating frequency, as in a homodyne receiver, is
considered direct conversion. That includes extracting the carrier
from the receive signal, and subsequently mixing the carrier with the
receive signal to extract the modulation (such as in an I-Q
demodulator). Mixing with a signal that is NOT on the operating
frequency, is heterodyne conversion. Note that it doesn't matter
where the signal is mixed in a (super)heterodyne receiver. One active
stage can do everything as in an autodyne receiver.
The mixing is to extract the modulation. It's called a product
detector for AM and SSB.
<
http://en.wikipedia.org/wiki/Product_detector>
<
http://www.google.com/images?q=product+detector>
If you extract the carrier signal from the receive signal, mix these
together, and low pass the result, you get demodulated AM or SSB. With
slope detection, you can also demodulate FM. Extracting the carrier
is simply lots of amplification so that the amplitude modulation is
clipped (limited) and thus removed. For SSB, a PLL is often helpful
for weak signals, but not really necessary as just IF noise will
usually suffice to produce a usable carrier component.
Yes. Exactly the same frequency. In order for a product detector or
direct conversion receiver to work, it needs to multiply (mix) the
carrier (with no modulation components present) with the receive
signal. What's left is the modulation.
No. The distinction is that direct conversion uses a mixing signal
that is exactly the same as the receive signal. If the signal was
offset, it would be consider (super)heterodyne conversion.
Sure. The detector is allowed to use the received signal to perform
the demodulation, it's still direct conversion. Think of TRF (tuned
RF) type of receiver as a special case of direct conversion, where the
demodulator is rather simplistic.
There's plenty of room for improvement. There may be mistakes but
they are NOT badly written.
They make an effort to reduce errors. Where it become difficult is
that there are so many areas of technology where there are multiple
points of view with large areas of overlap and controversy. As long
as the source of such "doubtful" information is specified, multiple
points of view are presented, and the author is fairly neutral, I
don't have any problem with presenting controversial information.
The next time you research a topic, instead of using Google or
Wikipedia, try using Google Scholar instead.
<
http://scholar.google.com>
This should give you a wide selection of papers and articles written
by qualified experts who probably don't own a Playstation. Some of
the papers have been peer reviewed and are thus deemed correct and
often even authoritative.
If that is insufficiently accurate, try searching for terms using
Google patent search.
<
http://www.google.com/patents>
If you think Wikipedia is full of inaccurate information, wait until
you read some of the hogwash found in some patents. Try searching for
"perpetual motion" for a good start.
