A prescaler is a circuit which generates an output signal related to an input signal by a fractional scale factor. Pre-scaler circuits are useful in many applications such as clock generation in digital circuits and phase-locked loop (PLL) circuits. It is usually desired to divide a clock signal by an integer N. Prescalers are used in the feedback loop between the output of a voltage-controlled oscillator (VCO) and the phase frequency detector in phase locked loop frequency synthesizers to generate higher frequencies. A typical known prescaler is arranged to divide either at an N factor or at an (N+1) factor. A prescaler has two available divide ratios and has an input control line that allows a control circuit to set a first mode where the prescaler divides by a first divide ratio, or set a second mode where the prescaler divides by a second divide ratio. Programmable divider circuits may have frequency limitations, so if higher frequencies are used, a prescaler circuit may be used. A prescaler may divide at a fixed ratio, or for more flexibility, may allow for dividing at one of two available divide ratios. A dual modulus prescaler is a divider whose division ratio or modulus can be switched from one value to another by a control signal. The dual modulus prescaler has a mode control that allows a control circuit to set a first mode where the prescaler divides by a first divide ratio, or set a second mode where the prescaler divides by a second divide ratio. A multi-modulus prescaler is one in which the division ratio can be switched among the multiple division ratios by an external control signal to extend the upper frequency range of the synthesizer but still allow the synthesis of lower frequencies.
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