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.If you have any questions, feel free to Email me email@example.com . I'll do my best to confuse you completely (:-). (Comments or corrections also are welcome.)