IMPORTANT NOTICE Effective March 21, 2004, the Sync Detector kit has been discontinued. It became very difficult to get the Sony ICs in a timely fashion so it just wasn't practical any longer. No parts or boards are available. 73 Steve - - - - - - - - - - The Sync Detector Kit Why Serious AM Listeners Need One! AM DETECTORS AM can be a very pleasant form of modulation, useful for Amateur, Aircraft, Shortwave Broadcast, and Standard Broadcast stations. Reception of amplitude modulated signals can be difficult at times, though. Selective fading, adjacent interfering signals, and phase distortion at the receiver can all reduce the quality and readability of AM signals. The usual means of AM demodulation is the envelope detector. In its simplest and most common form, it is a crystal or tube diode. The preceding stages in the radio receiver are entirely devoted to bringing the AM signal to this diode detector. The $1000 radio is still a crystal set at heart! There are better ways to demodulate AM signals and one of the best is Synchronous Detection. In this method, a local oscillator is phase-locked to the original AM carrier signal, and is used in a product detector to produce audio. Additional means can be used to select which of the AM sidebands are used to produce audio, if desired. The synchronous detector has been around since the early days of AM -- but has rarely been used in radio designs. The low cost and "adequate" performance of the crystal diode almost always won out over the complexity and higher cost of the sync detector. For the serious listener of AM, however, the envelope detector is not adequate. Synchronous Detection is the way to go... REDUCING DISTORTION The AM signal can arrive at the receiver in a variety of states. It can have phase-cancellation of the carrier or sidebands (selective fading) caused by multi-path propagation in the ionosphere. It can have adjacent signals causing interference or heterodyne whistles. The envelope detector cannot help with these problems -- in fact, the nature of the envelope detector exaggerates the distortion and noise caused by such problems, making AM very unpleasant to monitor. The synchronous detector will greatly reduce these problems allowing excellent AM listening in what would otherwise be poor conditions. The nasty tearing distortion caused by selective carrier fading is eliminated by use of the phase-locked local oscillator. This oscillator substitutes for the original AM carrier, even when the carrier goes into one of these fades. The envelope detector will hear high distortion, while the sync detector produces clean audio. Selective fades on the sidebands can be reduced by use of a synchronous detector capable of choosing which sideband to use for demodulation. REDUCING INTERFERENCE A synchronous detector that includes the selectable sideband feature will help reduce adjacent signal interference, as well as heterodyne whistles. The choice of the upper sideband will cut out interference from a signal below the desired one. Similarly, use of the lower sideband will reduce interference from a signal above the desired frequency. THE SYNC DETECTOR KIT I have been a ham for 21 years (WD8DAS), and a professional Broadcast Engineer for 15 years. In the course of this activity I often found myself frustrated by the poor AM performance of my various receivers -- mostly due to their envelope detectors. For years I researched the synchronous detector, gathering information and building a number of prototypes for my own use. I even bought one of the very few commercial radios that include a synchronous AM detector. I've become a firm believer in the benefits of the sync detector and have tried to spread the word among my friends and fellow radio hobbyists -- shortwave listeners, hams, and AM broadcast monitors. When I began being asked to build sync detectors for some of these folks, I decided to put together a kit to save folks the trouble of "reinventing the wheel" when they wanted to gain the benefits of selectable sideband synchronous detection. The kit I now offer was developed to provide a design based on modern integrated circuits that has the best features of the sync detectors I've discovered...at a reasonable cost to build! The design is to be used with a tube or solid-state receivers having a 400 - 500 kHz IF -- the most common IF is 455 kHz. The kit of parts, with circuit board and instructions, is $159.00. If you would like a fully assembly and tested detector board, the cost is $229.00. CONSTRUCTION AND HOOKUP The sync detector kit is not complex -- but it does require good soldering skills and knowledge of electronic components. Instructions and drawings are included to make the kit as easy as possible, but order the assembled version of the detector if you are not comfortable with electronic circuit construction. The sync detector is connected to the output of the last IF stage of your receiver. You must be comfortable with disassembly of your receiver, and capable of identifying the correct connection point. It is not difficult, but requires a basic knowledge of electronics, as well as a schematic diagram of your receiver. Do not order either version of the detector if you are not comfortable with this sort of electronic work! Due to the fact that I cannot control the construction and/or installation of the detector, I can only accept sealed detectors or detector kits for refund (less $15.00 shipping and handling). If the sealed bag has been opened, a refund is not available. If you enjoy kit-building and know your way around your receiver, you will have success with this sync detector. Your investment will pay off in many enjoyable hours of clear listening! Synchronous Detector Specifications Sync detector: Sony AM Stereo IC IF Center Freq: 455 kHz typical, adjustable for IFs of 400-500 kHz Sideband Selection: Encapsulated Precision Phase-Shift Network Combiner Input Impedance: 1 megohm (FET source-follower). Input Level: 500 mV typical, 100 mV min, 1 v max. Output Impedance: 10 k ohms. Audio Output Level: 100 mv typical (to be fed to audio stages of receiver or an external amp) Power Requirements: 12 volts DC at 20 ma typical, but voltage of 4 - 14 vdc acceptable Physical Dimensions: 1.0" H x 3.0" W x 4.0" D For an independent evaluation of my Sync Detector kit, see Walt Novinger's review in Electric Radio Magazine, October 1994. Steve Johnston IMPORTANT NOTICE Effective March 21, 2004, the Sync Detector kit has been discontinued. It became very difficult to get the Sony ICs in a timely fashion so it just wasn't practical any longer. No parts or boards are available. 73 Steve