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What is a fractional N PLL?

What is a fractional N PLL?

The term fractional-N describes a family of synthesizers that allow the minimum frequency step to be a fraction of the reference frequency. A basic modern PLL comprises a reference source, a phase frequency detector, a charge pump, a loop filter, and a Voltage Controlled Oscillator (VCO).

How does a PLL work?

The function of the PLL is to compare the distributed clock to the incoming reference clock, and vary the phase and frequency of its output until the reference and feedback clocks are phase and frequency matched.

Why do we need PLL?

The PLL is widely used for a variety of purposes. It is used to recover the clock signal in some wireless applications. It is used to recover the original signal in frequency modulation radio. It is used to multiply a frequency by a fixed factor.

What is fractional divider?

Fractional-n dividers The VCO stabilizes at a frequency that is the time average of the two locked frequencies. By varying the percentage of time the frequency divider spends at the two divider values, the frequency of the locked VCO can be selected with very fine granularity.

Why charge pump is used in PLL?

Charge pump make use of switching devices for controlling the connection of voltage to the capacitor. Charge pump is one of the important parts of PLL which converts the phase or frequency difference information into a voltage, used to tune the VCO.

How does fractional synthesizer work?

A fractional n synthesizer uses the basic digital PLL loop. It has a VCO, phase detector, loop filter, divider and could even utilise a mixer within the loop as well. This means that the VCO will be operating at a frequency equal to the division ratio times the phase comparison frequency.

What are 3 running conditions of PLL?

This phase difference φ generates a corrective control voltage vc to shift the VCO frequency from f0 to fs and thereby maintain the lock. Once locked, PLL tracks the frequency changes of the input signal. Thus, a PLL goes through three stages (i) free running, (ii) capture and (iii) locked or tracking.

Is PLL analog or digital?

Analog and “Digital” PLLs Digital phase-locked loops are typically smaller than analog PLLs, due to their digital phase detector and loop filter. However, both analog PLLs and digital PLLs contain analog elements. Thus both PLL types: Have a stringent lower limit on the supply voltage.

Which filter is used in PLL?

PLL with a 5th-order Butterworth filter.

What is the function of a hydraulic charge pump?

The purpose of the charge pump is to provide makeup fluid to the system during operation. There are tight tolerances between the pistons and the barrel in the pump and motor. This means that some of the oil inside the pump and motor will bypass the pistons and flow back to the tank through the case drain lines.

How do you create a charge pump?

The basic charge-pump circuit is a switch-mode dc-dc converter that’s often needed in designs requiring more than one dc supply voltage. It’s made up of switches and capacitors. The switches are usually diodes in discrete designs, but are also MOSFETs in IC versions.

What is the advantage does a fractional N synthesizer have over integer-n?

A frac-N allows step sizes on the order of tens of Hertz, while an integer-N may result in tens of kilohertz. The frac-N also will lock faster when compared to a similar integer-N solution. This is because the lower value of N allows a wider loop filter bandwith, which in turn allows a faster lock time.