Eliminates Selective-Fade Distortion and Garbling Able to Receive One Sideband at a Time to Minimize Interference.
SE-3 MK III D Introducing a whole new world of shortwave and broadcast reception with the new Sherwood SE-3 Mk IV phase-locked AM product detector. Even with the evolution of modern-day communications receivers, with their improved stability, selectivity, and dynamic range, AM reception under fading signal conditions has remained a compromise. With a diode or synchro-phase detector, severe distortion occurs every time the carrier drops out in a selective fade. Carrier nulls (fades) of 20 to 30 dB or more are common, which result in total loss of intelligibility. Under these conditions, the desired signal has become a double-sideband reduced-carrier transmission which cannot be detected in the normal manner.
Exalted-carrier selectable-sideband (ECSS) reception can be used to combat the fading problem. Distortion is not present, (assuming a properly operating receiver/detector), but a beating problem occurs because you cannot keep the receiver right on frequency. In effect, one type of distortion is traded for another. (Even a synthesized receiver, tunable in 1 Hz steps, would be mistuned a fraction of Hertz, resulting in a swishing sound every few seconds.) While tuning errors of some tens of Hertz may be tolerated during voice reception, music sounds more unpleasant because the harmonic relationship so critical to musical overtones is destroyed.
The only complete solution to the problem is phase-locked detection with a product detector. The BFO in the SE-3 is locked to transmitted signal’s carrier, and then completely replaces it in the detection process. Thus demodulation is fully independent of the signal’s carrier strength at any given moment. A selective null can roll through the signal, changing the tonal balance of the audio, but proper detection will still take place.
The SE-3 is a self-contained buffer, detector, and audio amplifier system. All that is needed is an i-f output (from the receiver) and a speaker or headphones. The amplifier is rated at 5 watts, and is adequate for most installations. If a low efficiency acoustic-suspension speaker is preferred for music reproduction, an auxiliary output is provided for an external amplifier. Tape recordings can also be made from this output, if desired. The headphone jack will accommodate either mono or stereo phones without the need for an adapter.
The SE-3 has two main front-panel controls: A PLL (phase-locked loop) control and audio gain/power on-off. Infinitely variable for the equivalent of passband tuning from LSB to USB, or anywhere in between. Any filter bandwidth is usable from 1.3 kHz to 8 kHz on standard units. An AM / SSB switch is provided for SSB/RTTY/CW reception.
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