PLL has its own feedback loop and locks its frequency to the input frequency. By adding another feedback loop through a magnetic circuit to PLL, the combined systems have been found to have various interesting functions, which could not be obtained separately. They are, for example, a multistable system, linear or bistable current to frequency converter, and analog memory. The first one is a combined system of PLL, a magnetic core, and a frequency divider. The second one consists of PLL and a magnetic modulator, The third one is realized by PLL and a magnetic delay line.