1. Field of the Invention
The invention relates to a frequency synthesizer and, in particular, to a frequency synthesizer with a harmonic locked phase/frequency detector.
2. Description of the Related Art
A frequency synthesizer is an electronic system for generating any range of frequencies from a single fixed timebase or oscillator. They are found in many modem devices, including radio receivers, mobile telephones, radio telephones, walkie-talkies, CB radios, satellite receives, GPS systems, etc. Since a millimeter wave band of 57-64 GHz has been announced for general unlicensed use, frequency synthesizers may be used to facilitate the giga-data-rate wireless transmission for short distance indoor communications. For multi-Gb/s wireless transceivers, a very-high-speed frequency synthesizer plays a very important role.
A phase locked loop base frequency synthesizer is a commonly used architecture for designers. A phase locked loop compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies. The error signal is used to drive a voltage controlled oscillator (VCO) which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This input is called the reference and is derived from a crystal oscillator, which is very stable in frequency.
Meanwhile, it is well-known that the active devices in modern advanced CMOS technology suffer from gate leakage and channel-length modulation due to thinner gate oxide thicknesses and shorter channel lengths, of which causes some negative issues in very high-frequency VCO's design. Firstly, to have a wide tuning range for a VCO, a large varactor is often used. However, the parasitic capacitance limits the oscillation frequency. And, resulting gate leakage current degrades the phase noise. Secondly, since cross-coupled devices with short channel lengths may offer finite output resistance, the equivalent quality factor from the LC tank is decreased. This also degrades the phase noise performance and even causes VCO malfunction.