David Nebenzahl wrote:
o First of all, superhet receivers have only one stage of detection and
filtering, not two, after the last IF stage, right? (I suppose there may
be some filtering in or around the mixer stage, but I don't think that's
what they're claiming, which I assume is filtering out the carrier.) So
where do they get "two stages of detection and filtering"?
Not really. While there only needs to be one stage of filtering, it is
common in high end receivers (ham radio and millitary, not audiophile)
to have multiple stages of filtering. So for example, if you have a tripple
conversion receiver you often see filters at the final two.
They are cascaded, meaning you might have a 2.4kHz filter at the 2nd if, and
a 1.8kHz one at the third. Or a 600Hz at the second and a 250Hz at the third.
They are also used as "roofing" filters for DSP filters. If you have a
DSP filter capable of adjustable bandwidth from 6kHz to 100Hz, you may see
a roofing filter (6Hz) in front of it. Since up until a few years ago IF DSP
filters were limited to low frequencies, such as 455kHz, you would see the
roofing filter in the second IF, say 8.8mHz or 10.7mHz, and the DSP at 455kHz.
Now you see them in both places.
o Is their explanation of how DCR works even correct? I don't understand
the business of mixing the signal with a LO signal: why would you do
that? They're a little vague: does "synchronized in frequency to the
carrier" mean *exactly* the same frequency as the carrier (???), or some
other frequency to produce a sum or difference frequency? (In which
case, we're back to IF, aren't we, so what's "direct conversion" about this?
If you mix two signals, you get 4, the originals, plus the sum and the
difference. Sounds familar correct. Instead of mixing two signals 455kHz,
or 10.7 mHz, or whatever apart, a DC receiver mixes them with much closer
frequencies, for example, 800Hz for CW (morse code) or even for audio.
In that case you mix the locally generated signal and the distant one
put them through an audio filter and viola, you have a signal. The problem
with that is they are notoriously poorly selective, and if you are scanning
the ham bands in a large city, you may end up with Rush Limbaugh every few
kilohertz. :-(
Now they are most often used in cheap (and I mean cheap) ham radios usually
kits. Since mixer chips, if filters, etc are so easily and cheaply available,
most radios in the $100 plus range are single stage superhets instead.
If I were in front of a firing squad and had to try to describe DCR
without actually knowing what it is, I'd guess(tm)(R) that it's a bunch
of tuned RF stages followed by a detector.
Rarely. No one bothers with the TRF stages. They tend to be expensive and
unless you have a junk box full of multistage capacitors too costly to make.
Anyhow, I think I've shown that even if I'm way off base, Wikipedia
articles tend to be extremely badly written, if not outright full of
doubtful information. What else would one expect of the "encyclopedia"
that any PlayStation-playing, junk-food wolfing pimple-faced
junior-high-school student can edit?
Actually the wikipedia is one of the better sources of information available
today. The paper version of the encyclopedia that used to be given away with
CD ROM drives in the mid 1990's (not Encarta, the other one) was worse.
It's just that it's uneven. Some articles are very well researched and
documented. Others are just an exposition of a (or conflicting) point of
view.
That led me, years ago to come up with Mendelson's Corollary to Godwin's
whatever. In it's simplest form replace "calling someone a NAZI" with quoteing
from the wikipedia. :-)
http://en.wikipedia.org/wiki/Godwin%27s_law
Meanwhile there are many good electronics books which have goneout of
copyright and are being preserved by people scanning them. Some are
available free for dowload, some are sold often for the cost of a blank
CD and shipping.
They make great reading and reference. However to keep it modern, the nook
does not display scanned PDF files well, it has too small a screen, and no
zoom and rotate. The iPad does it wonderfully. I can't comment on the color
nook or any form of the kindle, if someone else can, please do.
Geoff.
--
Geoffrey S. Mendelson N3OWJ/4X1GM
Those who cannot remember the past are condemned to misquote it.