On 1/15/2011 10:14 PM Jeff Liebermann spake thus:

On Sat, 15 Jan 2011 19:47:23 -0800, David Nebenzahl
wrote:

Talk about off topic... sigh.

Hey, at least it's about *electronics* ...

1. TRF:

In the section on modulation, demodulation and other radio-related
stuff one book brings up "the tuned radio-frequency receiver"
before discussing superhet, as one would expect. But they say;

During the evolution of radio, the tuned-radio-frequency (TRF)
receiver was used to receive AM signals. Today, a few special
applications still use TRF receivers.

Now, they go on to explain why TRF is inferior to superheterodyne.
But I'm curious: are there still any radios that use TRF? and why?
(Keep in mind this book was written in 1979).

Yes, but it's not obvious or really TRF. The reason superheterodyne
receivers were invented was that decent narrow band LC or crystal IF
bandpass filters were not tuneable and didn't work well at higher RF
frequencies. About 45MHz was as high as they went before going exotic
with SAW devices.

These daze, dramatically improved semiconductor technology has
produced chips that work at almost any useful RF frequency. No more
need to downconvert when the IF filtering is done by a DSP (digital
signal processor). Instead of TRF, it's now called "direct
conversion". There's no local oscillator, no mixer, for fixed IF
filter, and probably no LC devices anywhere. Just a ceramic bandpass
filter (or duplexer) some gain, an A/D converter, and a DSP for
demodulation. Most GPS, Wi-Fi, and cellular chipsets work this way.
http://en.wikipedia.org/wiki/Direct-conversion_receiver

OK, so this is why I absolutely *hate* Wikipedia. Here's the lead
paragraph in the article:

In telecommunication, a direct-conversion receiver (DCR), also known as
homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design
that demodulates the incoming signal by mixing it with a local
oscillator signal synchronized in frequency to the carrier of the wanted
signal. The wanted modulation signal is obtained immediately by low-pass
filtering the mixer output, without requiring further detection. Thus a
direct-conversion receiver requires only a single stage of detection and
filtering, as opposed to the more common superheterodyne receiver
design, which converts the carrier frequency to an intermediate
frequency first before extracting the modulation, and thus requires two
stages of detection and filtering.

Now, class, how many things are wrong here? (And please correct *me* if
I'm incorrect):

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"?

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 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.

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?


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.