From: andrew@rentec.com (Andrew Mullhaupt)
Newsgroups: rec.radio.shortwave
Subject: Re: What is Synchro Detect?
Date: 4 Jan 93 06:17:43 GMT
Organization: Renaissance Technologies Corp., Setauket, NY.

In article <C077BA.21K@eis.calstate.edu> tdenbo@eis.calstate.edu (Tom Denbo) writes:
>I'm not familiar with the term Synchro Detect.  Could someone explain how
>it works and what advantages it has.

There are several versions of synchronous tuning. The basic idea is always
the same - radio signals are effectively composed of parts which need not
all be received in order to determine what the original transmitted signal
was. 

In the case of  amplitude modulation, the transmission is formed by multiplying
a fixed frequency sinusoid by a much lower frequency audio signal. We can
consider the audio modulation to be a superposition of different audio
frequency sinusoids, and since multiplication by the carrier is a linear
operation, the transmission is the corresponding linear combination of the
carrier sinusoid multiplying the audio sinusoids. So it suffices to consider the
case where the audio consists of a single sinusoid, (sort of like a test of
the emergency broadcast system).

Now the rule for multiplying sinusoids is best thought of as a corollary
of Euler's theorem and the multiplication law for the exponential:

	exp i x = cos x + i sin x

	exp ( a + b ) = ( exp a ) (exp b)

from which one derives

	cos Ft cos ft = 1/2 ( cos (F+f)t + cos (F-f)t )

Which implies that if you multiply the carrier (frequency F) by the
audio (frequency f) you get components at frequencies F+f and F-f. 

One point of view is that when you multiply real sinusoids, you add
and subtract the frequencies.

In practice, the modulation is offset so that it doesn't negate the
carrier, i.e. the modulation is really C + cos ft, so the resulting
signal is

	cos Ft (C + cos ft) = C cos Ft + 1/2(cos (F+f)t + cos (F-f)t)

The terms on the right are often referred to as the carrier, upper
sideband and lower sideband.

You can recover the audio part from all the three pieces on the right
or just some of them. Many systems take advantage of this fact. The
first really widspread use of this was in SSB (single sideband) radio,
where the transmitter changes the modulation around so that only one
of the sidebands is transmitted. There is a power economy which makes
this attractive, but the receiver has to effectively re-synthsize the
missing sideband before it 'sounds right'. This is why SSB sounds funny
on a radio that doesn't know this trick. In shortwave listening, the
situation is a little different, since most shortwave broadcasts have
both sidebands. Here what you can do is to resynthesize the carrier
and one of the sidebands to recover the audio from only one sideband.
(A little algebra shows how this happens.) The problem with this is
that you have to get the _phase_ of the synthetic carrier right, which
is usually done by using a PLL _phase locked loop_ circuit. When the
radio is capable of receiving AM signals by reconstituing one or more
of the three parts, this is likely to be called synchronous detection.

Note that many radios use a PLL to synthesize digital frequencies but
this is not the same as synchronous detection.

Depending on which parts of the signal the radio can do without, the
synchronous detection goes by different names. The advantages of 
synchronous detection depend on which capabilities the radio has. The
most frequent advantage is the fact that even though the signal may
experience selective fading (especially if the signal has been bounced 
of the ionosphere) the synthetic carrier does not. Other important
advantages obtain if the radio can ignore one sideband in the case that
it is subject to interference, often from an adjacent station. In this
case, better audio can be received because the effective filter width 
can be larger without encountering the interference. This can be done
by selectable sideband, or by connecting the synchronous detector to
the passband shift. 

Synchronous detection is a great thing in some cases, and reported to
be not so great in others. I think that it makes the Sony 2010 a much
better radio than it would be without, and it's a significant aid on
the Drake R-8, although the R-8 does pretty well without it. There
have been mixed reviews of the early Grundig and Lowe synchronous
detectors, but from what I have read the Lowe HF-150 has an excellent
synchronous detector with most if not all of the useful modes.

I think that one aspect of the variation in quality is due to the
different PLL circuits which could be used. PLL's are highly nonlinear
devices and their tracking characteristics are complicated. I expect
that as engineers gain more experience with synchronous detection and
their performance, we will see uniformly good synchronous detection
at lower and lower cost.

The radios most interesting to me with synchronous detection are
the Sony 2010 and SW-77, the Drake R-8, the Lowe HF-150 and soon
to be HF-250, the Japan Radio NRD-535D, and the new Grundig Satellit.
I've used the R-8 and the 2010 extensively. Anyone who can make
useful comparisons to the others should feel free to comment.

>I'm also looking for a cheap SW rcvr.  Any suggestions?

If you want synchronous detection, the lowest cost radio which makes
sense may be the Sony 2010. Next up from that are the Sony SW-77,
the new Grundig Satellit, and the Lowe HF-150. I don't know if these
can really be called cheap.

I've had good luck with Panasonic's RBF-45 and the little Sangeans
are also quite nice. Whenever considering a radio purchase, it's
a good idea to check in WRTH and Passport.

Later,
Andrew Mullhaupt