Patent ID: 6181209
Filing Date: 2001-01-30
Classification: H03L

Abstract:
An all-digital frequency following system capable of generating an output frequency which is an integral multiple of an input frequency, comprising:a frequency counter having a first input end receiving said input frequency as a reference signal which resets said frequency counter at the start of each cycle of the input frequency and a second input end receiving the output frequency of said frequency following system as a feedback to thereby count the number of pulses in the output frequency, said frequency counter holding the output count when the count reaches a preset upper limit, wherein said frequency counter is a 7-bit counter comprising flip-flop means for generating the 7-bit count in response to the output frequency and logic means capable of generating a hold signal when the 7-bit count reaches the preset upper limit, the hold signal causing said flip-flop means to maintain their output bits at current logic state;a digital comparator which latches the output of said frequency counter at the start of each cycle of the input frequency and then compares the latched value with a first predetermined value; said digital comparator generating a first logic level trigger signal if the latched value is greater than the first predetermined value, and a second logic level trigger signal if the latched value is less than the first predetermined value;an up/down counter whose count range is set between an upper limit and a bottom limit and whose initial count is set at a second predetermined value which is increased by one when the first logic level trigger signal is received from said digital comparator and decreased by one when the second logic level trigger signal is received from said digital comparator; when the count of said up/down counter reaches either the upper limit or the bottom limit, the output of said up/down counter is held at that limit; anda digitally-controlled oscillator capable of generating an output signal serving as the output frequency whose value is in proportion to the output of said up/down counter.