The primary spurious frequencies generated in integer-N frequency synthesizers come from reference spurious signal products. The step size and loop bandwidth relationship dictate the spurious attenuation level that can be tolerated for a given synthesizer design. In contrast, spurious signal products in a fractional-N structure emanate from the fractional modulus. Fractional spurious signals appear around the voltage-controlled-oscillator (VCO) carrier frequency regardless of which frequency it is programmed to. The spacing between the first three spurious products is usually equal to the step size or one-half of the channel step size. In a fractional-N frequency synthesizer, the comparison frequency or step size is typically high, which leads to large loop-filter attenuation of the reference spur (even with a wide loop bandwidth implementation).

Boundary spurious products appear in an integer-N structure when the synthesizer’s VCO is programmed to frequencies near harmonic multiples of the comparison frequency. But all fractional-N synthesizers also exhibit these spurious products. These spurious signals are at lower amplitude levels than primary integer-N spurious products located at a harmonic of the comparison frequency. Whether or not these spurious products represent problems in a synthesizer design depends on the loop bandwidth, the comparison frequency, the system spurious specification, and the required frequency band plan (the actual LO channel frequency).

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In a performance comparison, a fractional-N structure provides better step resolution and a faster locking process than an integer-N structure. With the latter, widening the loop bandwidth in order to increase the locking process and step size can cause reference spurious frequencies to emerge. In an integer-N synthesizer, the VCO divider (divider N modulus) integer is also large due to the low comparison frequency, resulting in poor phase-noise performance compared to a fractional-N synthesizer. Due to their fast switching times with fine resolution and acceptable phase noise, fractional-N synthesizers have been widely used in 2.5G and 3G wireless handsets.

4. This pre-layout schematic diagram was created with the help of Protel design software.