RadioBanter - View Single Post

June 3rd 05, 04:47 AM

In article ,
(Michael Black) wrote:

"rkhalona" ) writes:
I've been a SWLer for at least 20 years, have a Ph.D. in EE, have 20+
years of
experience in the telecomm industry (uwave, sats, cellular, UWB...) and
have taught
grad/undergrad comm courses at various U.S. universities.

I agree with another poster that the SW8/R8B sync. detectors are among
the best ever incorporated
into SWL gear. My previous comment about sync. doesn't mean that one
cannot achieve similar
signall quality without it (e.g., using PBT, but how many portables or
low-cost tabletops have PBT
these days?), but the convenience of being able to select sidebands in
sync mode is a big plus.

RK

But it seems you are mixing apples and oranges.

Sync detection means a locally generated "carrier" is present, so
if the signal fades the lack of a strong carrier is not a factor.

It does nothing to prevent fading (which I bring up because someone
recently said something along those lines here) it merely helps when
the signal fades.

Selectable sideband really has nothing to do with synchronous
detection, other than that using the phasing method it's relatively
cheap to implement compared to an expensive IF filter. It's not
really like a few extra parts to a synchronous detector will
add selective sideband, the added parts may be cheap but it adds
complication to the circuit.

When Webb wrote about the synchronous detector in CQ Magazine
about it, it was the whole shebang. But, that was a time
when many receivers had fairly wide IF filters, and lacked
product detectors. At the same time, you'd see SSB adaptors
that used the phasing method, which added that product
detector and reduced the unwanted sideband. Adding
synchronous circuitry to those was relatively simple, so once
you added the sync circuitry you not only got DSBsc reception,
but better SSB reception.

What we often see is lower end receivers tossing it in (because
the phasing method is a cheap way of knocking out the unwanted
sideband, and plus there are ICs that do it all in one
package), but it doesn't make up for the lack of a narrow
IF filter with steep sides (at least not as implemented
in those cheap receivers). It's a means of adding something
without a major cost increase.

I'm not even sure where we've veered off to. I thought
the previous comment was something like synchronous detection
wasn't all that important. I'd say that's true, given that
people lived without it till it became a feature in relatively
recent years. Someone listening to broadcast radio (am or
shortwave) that are relatively strong may be the ones to
benefit the most, because you can get deep fades where
the sidebands are still nice and strong. Signals
that you have to strain to hear, it's far less likely to
be useful, because they are already below a minimum strength.
You'd want to pull in other techniques at that point, and that
includes the narrow IF filter that has good slopes.

I really do not understand this attitude toward synchronous detection.
It's the best thing to happen to AM reception in years. I have it in my
radios at home and wish I had it my car radio. Maybe it's that I live on
the coast that the selective fading is more problematic but SW anytime
or night time AM broadcast reception is greatly improved on weak and
strong signals alike.

The choice of double side band of single is an additional plus that I
would not minimize either. One side band of an AM signal can have have
interference on it and not the other. Often passband tuning takes care
of it but not always. Take the situation of a digital mode signal near
an AM broadcast and the passband tuning will get rid of most but not all
the digital mode interference. Using synchronous selective side band
detection is much more effective.

You can count me as one that would not want to live without it.

Do you own radios with synchronous detection? Try operating one with
sync on and another with sync off. Spend some hours listening to night
time AM or SW and then come back and tell us which you prefer.

--
Telamon
Ventura, California