by Daniel A. Grunberg   --   Kensington, Maryland U.S.A 


INTRODUCTION

The Sony ICFSW7600G (the "G" is important) is the lowest priced
receiver, available with a synchronous detector.  Synchronous
detection is a more precise, high-technology, substitute for the
envelope detection.  Although envelope detection was used in the
old crystal sets and still is used today in most digitally-tuned
superhets, synchronous detection is not merely a high-tech frill
to add to an already high-tech digitally-tuned receiver.  The
7600G's synchronous detector offers two serious advantages over
envelope detectors.  

I'm going to assume that you already know the advantages that
digitally-tuned receivers have over analog receivers.  (If not, 
you might want to read another article called  "Digital Radios
vs. Analog Radios").  For the purposes of this discussion,
amplitude-modulated (AM) signals are signals transmitted with two
sidebands and a carrier, like signals heard on the AM-broadcast
band and on the shortwave and longwave broadcast bands.  The
carrier is used as a reference to which one or the other (both are
not needed) of the sidebands may be compared to recover the audio
being transmitted.  The recovery process is called detection.  For
the purposes of this discussion, single-sideband (SSB) signals are
signals transmitted with only one sideband and with no carrier.  


REDUCTION OF ADJACENT CHANNEL INTERFERENCE ON AM SIGNALS

Receivers that use synchronous detectors first filter the desired
AM signal, and, in effect, convert the AM signal and its carrier
into an SSB signal with no carrier.  Then the receiver replaces the
carrier with a locally generated "carrier", whose frequency is
precisely calculated and carefully controlled.  As part of the
process, the listener uses a switch to select which sideband will
be kept.  When there is no interference from a station on a
different (than the desired station's) frequency, the selection of
either sideband will yield the same good listening.  However, when
there is an interfering signal on a higher frequency, selection of
the lower sideband often will reduce or eliminate the interference
at the receiver's audio output.  In the same way, when there is an
interfering signal on a lower frequency, selection of the upper
sideband often will reduce or eliminate the interference at the
receiver's audio output.  


REDUCTION OF MULTIPATH DISTORTION

Sometimes, the signal broadcast by a distant shortwave transmitter
arrives at a receiver's antenna via several different paths of
unequal lengths.  When that happens, the receiver "hears" a
separate signal arriving over each path, and each separate signal
is "heard" at a slightly different time.  The  multiplicity of
arrival times of similar signals can be thought of as the
radio-frequency equivalent of the audio-frequency Doppler effect
(the apparent change in a car horn's frequency, as the car horn
approaches, arrives, and then leaves a listener, at high automotive
speed).  When a receiver uses an envelope detector, and several of
the differently timed signals are among the stronger signals that
the receiver "hears", multipath distortion (sometimes called
selective fading or QSB) can cause an "in-and-out" effect on the
receiver's audio.  The "in-and-out" effect makes listening to the
signal unpleasant and even can make understanding the signal
difficult or impossible.  Synchronous detection helps to reduce or 
eliminate the "in-and-out" effect.  Since part of its process is
instant-by-instant calculation of the desired signal's carrier
frequency and continuously re-tuning the receiver to track the
carrier, the synchronous detector compensates, instant-by-instant,
for the multipath distortion, and thereby reduces or eliminates 
the "in-and-out" effect at the receiver's audio output.  


BOTTOM LINE

I have read on rec.radio.shortwave that the 7600G's sync detector
works well, and that it's audio is adequate, but not as good as the
Grundig YB-400's audio.  (The YB-400 and the 7600G sell for about
the same price.)  I have read that in all other respects, the 
YB-400 and the 7600G are equal.  Since a synchronous detector could
be a decided advantage for shortwave reception, if I were buying
a portable SHORTWAVE receiver I would carefully consider the
7600G.

This article was last updated on 17 September 1997.