A reliable squelch feature has long been a difficult goal in the low signal to noise ratio and high interference environments typical of HF links. Many passive techniques have been attempted and implemented. These passive techniques range from simple power measurement to elaborate neural network based speech detection systems. All of these approaches suffer from a decrease in reliability as channel conditions degrade due to the presence of noise and interference. However, it is under these degraded channel conditions where a reliable squelch function is most desirable to reduce operator fatigue. In addition, a reliable squelch function can be useful in the implementation of HF/HF or HF/VHF automated re-transmission systems, or as the basis for a robust field radio wakeup feature. This paper will discuss the design and implementation of an HF squelch system based on a burst serial tone waveform that operates in low signal to noise ratio and high narrowband and impulsive interference environments. The paper will present our initial design goals, an implementation overview, and final system performance. Performance data, including the rate of false unsquelch occurrences and the probability of missed unsquelch in various channel conditions will also be presented.
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