Designing All-Pole Filters for High-Frequency Phase-Locked Loops

Since the phase-locked loop (PLL) circuit was proposed in the 1930s, it is being used for a lot of situations when precise frequency and phase references are required. Among these applications, synchronous telecommunication networks experienced a strong development in order to support the explosive information traffic that the modern society demands. Consequently, bandwidth became a decisive parameter, implying higher and higher frequencies for the clock signals exchanged between the nodes of the networks and detected by PLLs. The necessity to improve clock precision that follows the bandwidth increase provoked the improvement of the filter component of the PLLs, avoiding instability and high-frequency components in the reference signals. Here, a technique of designing this kind of filter is presented, considering second-order filters, implying third-order PLLs. Simulations show that following this technique produces very fast tracking processes, enabling precise operation even for very high frequencies.