A high-speed tone decoder phase-locked loop

THE RECENT GROWTH in popularity of Touch-Tone* dialing and similar types of multi-frequency signaling systems has produced a rapidly increasing demand for low cost, narrow-band tone detectors. Monolithic versions of the tone decoder phaselocked loop (TDPLL) have been developed as one means of meeting this demand. Unfortunately, conventional tone decoder phase-locked loops’ share a common fault. As detection bandwidth gets narrower, detection time gets longer, making it impossible to do narrow-band detection of short tone bursts. Consideration of a block diagram of a conventional TDPLL (Figure 1 ) will clarify this point. A phase comparator, low-pass filter (LPF) and voltage-controlled oscillator (VCO) form a second-order phase-locked loop. A quadrature phase detector multiplies the input signal with a phase shifted VCO output. Once loop capture is achieved, the detector supplies a strong dc component to the output driver. Since loop capture produces an output, detection bandwidth equals capture range, and narrow bandwidth is achieved by using a long time constant in the LPF. However, this decreases the natural frequency and damping of the second-order loop, resulting in long acquisition time. For example, at a bandwidth of f 2.5%, 100 cycles of input may be required for detection.