In various systems including receivers, tuners, radios, cellular telephones and the like, a down conversion process occurs in which a received signal at a very high frequency, such as a radio frequency (RF) is received and processed to obtain a lower frequency signal to be used for further processing. Such lower frequency signals may include an intermediate frequency (IF) signal, such as a low IF signal. In other systems, a RF signal may be directly downconverted to a zero IF signal.
Typically, such down conversion is effected by taking a received signal at a first frequency (typically RF) and mixing it with a local oscillator (LO) frequency to obtain a desired IF or other lower frequency signal. In certain systems, the LO frequency may be generated using a crystal oscillator or other such reference frequency generator. Often, such a frequency is provided to a frequency divider, where the reference frequency is divided by either a fixed or programmable amount. The divided frequency may then be provided to a mixer for mixing with a received signal.
In systems using quadrature signals (i.e., an in-phase (I) signal and a quadrature-phase signal (Q)), the LO signals provided to the mixer are preferably low noise signals, as the noise created in generating the LO frequency such as via a frequency divider and a voltage controlled oscillator (VCO) can adversely affect receiver performance. For example, if a divide by two frequency divider is present, noise and other interference may exist, such as second harmonics of the divided frequency. Accordingly, oftentimes for RF communications a divide by four frequency divider is used. Such frequency dividers also suffer from noise problems and require a high frequency VCO with buffers capable of driving large capacitances at high frequencies, such that the VCO consumes high power. Furthermore, outputs from such a frequency divider may be jittery, causing a modulation in terms of phase that may appear as phase noise.
Similarly, a frequency divider used in developing a LO frequency should be well matched between its I and Q sides, as mismatches can cause a phase error and therefore image rejection degradation. The image rejection of a frequency divider (and more specifically the image rejection of a receiver including the divider) refers to the ability to reject responses resulting from RF signals at a frequency offset from the desired RF carrier frequency by an amount equal to twice the IF of the receiver.
Still further, a frequency divider should be well shielded, as 45° phase-shifted signals are present inside the frequency divider and an asymmetric coupling between the signals can cause degradation. Also, a frequency divider should consume low power, particularly where the circuitry operates at gigahertz frequencies.
However, these different criteria lead to expensive and inefficient designs, particularly with respect to power consumption. Thus a need exists for a frequency divider that can provide desired frequencies while maintaining low power and accurate operation.
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