A general purpose hangup-mode detector.

Traditional Time Division Multiple Access (TDMA) systems have shied away from using phase-locked-loops (PLL) primarily for the reason that they suffer from a phenomena called hangup, where during the acquisition phase, the PLL gets stuck at a false-lock point for a long period of time, thus rendering the concept of quick multiple access very inefficient in terms of the overhead bits required. The Beam Radio (BR) TDMA system deployed a PLL for clock recovery and used a stable transmit oscillator and a Voltage Controlled Crystal Oscillator (VCXO) together with a hangup detector to achieve the bit efficiencies and quick multiple access required by a TDMA system. The circuit described here detects and corrects for hangup in multi-level transmission systems. It was used in the BR system, where the baseband signal is a four-level Pulse Amplitude Modulated (PAM) signal, but is general enough to be used in other multi- level modulation formats.