One of the architectures of choice in the current state of the art in modern communication transceivers is Zero Intermediate Frequency (Zero-IF) architecture. Several of the drawbacks of this architecture can be minimized when the voltage controlled oscillator (VCO) is running at a non-integer ratio of the input/output frequency. This in turn requires a non-integer frequency division. Two current prior art approaches to non-integer division circuits are (a) feed-forward (118, 114) and (b) regenerative (148, 144), as illustrated in
Both circuits have been in use for decades, but they are somewhat limited in their applications. The resulting division ratio at the output (124, 154) is
and
for the feed-forward and the regenerative versions, respectively. The only degree of freedom, other than the divider (118 or 148), is whether the upper sideband or the lower sideband of the mixing operation is selected (either by filtering or by implementing an image-rejection mixer). The feed-forward circuit (100) has been somewhat enhanced in prior art by adding another divider (186), as illustrated in
however, this circuit is limited to fractions less than one, as both N and P are positive integers.
In fact, many ratios cannot be reached at all with current designs, without cascading circuits, wasting both power and area. Another disadvantage of cascading circuits is the spurious low frequency emissions generated, that will require filtering.
One of the embodiments of the present invention avoids the drawbacks in prior art designs in the range of non-integer ratios achievable, and it avoids cascading circuit drawbacks, such as wasting power/area or generating low frequency spurious emission.
a) illustrates prior art non-integer frequency division via Feed-Forward mechanism; (b) illustrates prior art non-integer frequency division via Regenerative mechanism; and (c) illustrates prior art non-integer frequency division via Feed-Forward mechanism with additional divider in-line with input feed.
frequency ratio configuration.
An embodiment of this invention (200) combines feed-forward and feedback frequency division circuits in an innovative way, to provide greater flexibility in choosing the division ratio with little added complexity and often with only a minor area and power penalty. This double-feedback embodiment implements both the feed-forward (218) and the feedback (226) integer divider loops having integer dividers in the same circuit, for example, via a feedback-path mixer (230) and a signal-path mixer (214), as shown in
Note that the depiction of this embodiment shows the VCO source (210) and an optional output buffer amplifier (222), which are also depicted in current prior art designs (110, 140, 170, 122, 152, and 182). However, other signal sources may be used, instead of a VCO, and other circuitry or loads may be employed at the output, instead of a buffer amplifier.
The source signal frequency fi feeds to feed-forward divider N (218) and signal-path mixer (214). The frequency at the output (220) of feed-forward divider is
The output (216, 224) frequencies denoted by fo feed into the feedback divider M (226), whose output (228) has frequencies
and subsequently feeds into the feedback-path mixer (228). The output (232) of the feedback-path mixer has frequencies
and it feeds to the signal-path mixer (214). The output (216) of the signal-path mixer becomes
Equating the signal-path mixer output frequencies to fo yields the frequency ratio expression:
The resulting frequency division ratio in equation (2) enables two degrees of freedom, in addition to other attributes such as upper side band (USB) or lower side band (LSB) mode selection for each mixer.
Note that any variation or expansions of this double-feedback frequency divider embodiment with feed-forward and feedback divider loops is also considered part of this invention.
For example, an embodiment of this invention (300), as depicted in
This embodiment provides an extra degree of freedom (having 3 parameters M, N, and P) to tailor the output frequency ratio.
An embodiment of this invention uses an additional divider (by K) (434) between feedback-path mixer (430) and signal-path mixer (414), as illustrated in
Therefore, when in a configuration, such as depicted in
In an embodiment of this invention, e.g., as depicted in
An example of such an embodiment is a 5/3 frequency multiplier (500), as illustrated in
An implementation of this novel Feedback/Feed Forward 5/3 frequency division circuit was simulated in a Cadence environment using TSMC® 90 nm devices. The sub-circuits used were all taken from existing designs made by the PTK Radio Frequency Integrated Circuits (RFIC) design team; however, the sub-circuits were not optimized for current consumption, spurious response, or other parameters, and were only tweaked to demonstrate the actual division ratio.
For the simulation, each of the three dividers (518, 526, and 534) comprises of two Current Mode Logic (CML) flip-flops (with tail supply), connected in feedback configuration to provide frequency divide-by-2 with quadrature outputs. The feedback-path and signal-path mixers were each comprised of two AC-coupled Gilbert cell image rejection mixers. The feedback-path mixer was biased through a resistive load, while the signal-path mixer was biased through an LC tank, for better filtering. Table 1 summarizes some of the simulated frequency content at the output.
The simulated circuit output (with an input of 4.2 GHz) converged very quickly (˜7 ns) to a 7 GHz output (=5/3×4.2 GHz) with almost 0.5Vpk-pk differential amplitude. The spurious response, as expected (see
An embodiment of this invention provides much greater range of possible division ratios available for design over existing art. Many of these ratios cannot be practicably achieved with current designs without cascading circuits, wasting both power and area. The output frequency ratio in an embodiment of this invention, e.g., the 5/3 division ratio shown in
An embodiment of the double feedback frequency divider invention may be implemented in variety of ways.
For example, in order to select desired solution at the output of mixers (based on USB/LSB) in mode selection, an embodiment of this invention may use any one or more of the following or their equivalents:
For example, in order to generate in-phase/quadrature (IQ) signals (for most Image Rejection Mixers), an embodiment of this invention may use any one or more of the following or their equivalents:
For example, an embodiment of this invention may use any one or more of the following mixer implementations or their equivalents:
For example, an embodiment of this invention may use any one or more of the following divider implementations or their equivalents:
The embodiments of this invention may be used in variety of systems and applications, such as in transceivers requiring non-harmonic relation between the VCO frequency and the input/output frequency, wireless systems and products, such as, WiFi or WiMAX (or combined) products, or any other signal processing systems or products. The embodiments of this invention provide more flexibility in frequency planning and system/chip design, such as in power/area savings and the reduction of spurious low frequency emissions. Given this broader range of implementation possibilities, an embodiment of this invention provides more options in choosing VCO frequencies to minimize interference, or provides for the dual-use of the same VCO for multiple transmit/receive bands.
Any variations of the above teachings are also intended to be covered by this patent application.