Fractional-N frequency synthesizers offer numerous advantages in terms of performance compared
to integer-N frequency synthesizers for emerging wireless communications applications.
Frequency synthesizers are used throughout communications systems for tuning the signal
frequencies needed for receiving and transmitting. As silicon CMOS technology has been applied at
higher frequencies, it has helped the expansion of wireless technology to a wide range of
applications.1,2 In particular, these synthesizers have supported applications requiring tuning with
fine resolution—from kHz steps to a few MHz—and low phase noise, on the order of -100 dBc/Hz
offset 10 kHz from the carrier.
Many of these synthesizers have been developed as integrated-circuit (IC) solutions.3 In terms of
circuit architectures, integer-N frequency synthesizers are often challenged in meeting performance
requirements such as loop bandwidth, phase noise, and channel spacing due to the fundamental
design of the integer-N divider modulus.
In contrast, a fractional-N frequency synthesizer can provide the loop bandwidths needed for many
of these emerging wireless applications, with fine channel spacing. In addition, they can achieve low
phase noise without excessive reference spurious levels. Since a fractional-N frequency synthesizer
uses a higher phase/frequency-detector (PFD) comparison frequency and lower division ratio than
an integer-N frequency synthesizer, low-frequency phase noise can be suppressed to a high degree in
a fractional-N synthesizer.5
- Frequency Synthesizer
Available at: http://works.bepress.com/mahmoud_moghavvemi/147/