This invention generally relates to voltage controlled oscillators (VCOs) or oscillator circuits, and more particularly to low noise and low phase hit tunable oscillator circuits. A voltage controlled oscillator or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. In general, VCOs are designed to produce an oscillating signal at a particular frequency ‘f’ that corresponds to a given tuning voltage. The frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage Vtune applied to a tuning network across a resonator circuit. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred to as the tuning range or bandwidth of the VCO. The tuning sensitivity of the VCO is defined as the change in frequency over the tuning voltage. It is usually desirable to tune the VCO over a wide frequency range within a small tuning voltage range.
The magnitude of the output signal from a VCO depends on the design of the VCO circuit. The frequency of operation is in part determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs are often critical to the performance of PLLs. In turn, PLLs are generally considered essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are particularly important in wireless networks as they enable communications equipment to lock-on to the carrier frequency (onto which communications are transmitted) relatively quickly.
The popularity of mobile communications has renewed interest in and generated more attention to wireless architectures and applications. These applications are typically available on various wireless devices or apparatus including pagers, personal digital assistants, cordless phones, cellular phones, and global positioning systems. These applications may be found on networks that transport either voice or data. The popularity of mobile communications has further spawned renewed interest in the design of relatively low phase noise and phase hit free oscillators that are also tunable over a fairly wide frequency range (e.g., broadband tunable). Phase noise at a certain offset from the carrier frequency, frequency tuning range and power consumption are generally regarded as key figures of merit with respect to the performance of an oscillator. Given the relatively low number of active and passive devices in a VCO circuit, however, designing a VCO is generally assumed to be easy. Similarly, optimizing the foregoing figures of merit is generally regarded as straightforward. But constraints on phase noise and broadband tunability are demanding performance metrics that represent tradeoffs between each other. In addition, the need for a phase hit free solution has been in demand for a relatively long time.
More specifically, despite continuous improvement in oscillators/VCOs technology, the requirements of low power consumption, low phase noise, low thermal drift, low phase hits and compact size continue to make the design of VCOs challenging. The dynamic time average Q-factor (quality factor or Q) of the resonator and the tuning diode noise contribution generally set the noise performance of VCOs. The dynamic loaded Q is inversely proportional to the frequency range or tuning band of a VCO. As such, tradeoffs are continually being made between factors that may affect an oscillator's Q-value such as, for example, power, power consumption, noise performance, frequency stability, tuning range, interference susceptibility, physical space, and economic considerations. Most oscillators utilize some form of a transistor based active circuit or element to satisfy trade-off requirements. But transistors add to the complexity of an oscillator's design due to their inherent non-linearities, noise properties and temperature variations.
In that regard, designers are often hesitant to try new oscillator topologies primarily because they are unsure of how they will perform in terms of phase noise, conduction angle, tuning range, harmonic contents, output power etc. As such, most designers have a small number of favorite oscillator circuits that they adapt to meet changing and future requirements.
Traditionally, RF designers typically use LC (Inductor/Capacitor) resonator tank circuits to achieve low phase noise performance. A perfectly lossless resonant circuit is an ideal choice for an oscillator, but perfectly lossless elements, e.g., inductor, capacitors, are usually difficult to make. Overcoming the energy loss implied by the finite Q of a practical resonator with the energy supplying action of an active element is one potentially attractive way to build oscillators that meet design requirements. In order to guarantee stable sustained oscillation, it is usually desirable to maintain a net negative resistance across the LC resonator tank of an oscillator circuit. A negative resistance generated by the active device (3-terminal bipolar/FET) is usually used to compensate for the positive resistance (loss resistance) in a practical resonator, thereby overcoming the damping losses and reinforcing the stable oscillation over the period.
One of the major challenges in the design of a transceiver system is frequency synthesis of the local oscillator signal. Frequency synthesis is usually done using a PLL. A PLL typically contains a divider, phase detector, filter, and VCO. The feedback action of the loop causes the output frequency to be some multiple of a supplied reference frequency. This reference frequency is generated by the VCO whose output frequency is variable over some range according to an input control voltage as discussed above.
Varying the reference frequency of an oscillating signal source is important to second and third generation wireless systems. In fact, the coexistence of second and third generation wireless systems requires multi-mode, multi-band, and multi-standard mobile communication systems, which, in turn, require a tunable low phase noise and low phase hit signal sources. The demand for mobile communication is increasing rapidly and in this system, tunable VCOs are used as a component of a frequency synthesizer, which provides a choice of the desired channel.
A phase hit can be defined as a random, sudden, uncontrolled change in the phase of the signal source that typically lasts for fractions of a second. It can be caused by temperature changes from dissimilar metals expanding and contracting at different rates, as well as from vibration or impact. Microphonics, which are acoustic vibrations that traverse an oscillator package and circuits, can cause a change in phase and frequency. Microphonics are usually dealt with using a hybrid resonance mode in a distributed (micro/strip-line, stripline, suspended stripline) medium.
Phase hits are typically infrequent. But they cause signal degradation in high-performance communication systems. The effect of phase hits increases with data rate. If a phase hit cannot be absorbed by a device (e.g., a receiver) in a communication system, a link may fail resulting in a data loss. As a result, a continuing task is reducing or eliminating phase hits. While phase hits have plagued communication equipments for years, today's higher transmission speeds accentuate the problem given the greater amount of data that may be lost as a result of a phase hit.
Low phase noise performance and wideband tunability have been assumed to be opposing requirements due to the problem of the controlling the loop parameters and the dynamic loaded Q of the resonator over the wideband operation. The resistive losses, especially those in the inductors and varactors, are of major importance and determine the Q of a tank circuit. There have been several attempts to come to grips with these contradictory but desirable oscillator qualities. One way to improve the phase noise of an oscillator is to incorporate high quality resonator components such as surface acoustic wave (SAW) and ceramic resonator components. But these resonators are more prone to microphonics and phase hits. These resonators also typically have a limited tuning range to compensate for frequency drifts due to the variations in temperature and other parameters over the tuning range.
Ceramic resonator (CRO) based oscillators are widely used in wireless applications, since they typically feature very low phase noise at fixed frequencies up to about 4 GHz. CRO resonator-based oscillators are also known for providing a high Q and low phase noise performance. Typically, a ceramic coaxial resonator comprises a dielectric material formed as a rectangle prism with a coaxial opening running lengthwise through the prism and an electrical connector connected to one end. The outer and inner surfaces of the prism, with the exception of the end connected to the electrical connector and possibly the opposite end, are coated with a metal such as copper or silver. A device formed in this manner essentially comprises a resonant RF circuit, including capacitance, inductance and loss resistance that oscillates in the transverse electromagnetic (TEM) mode if loss resistance is compensated.
CRO oscillators, however, have several disadvantages, including a limited operating temperature range and a limited tuning range (which limits the amount of correction that can be made to compensate for the tolerances of other components in the oscillator circuit). CROs are also typically prone to phase hits (due to expanding and contracting at different rates with variation of the temperature for outer metallic body of the CRO and other components of the oscillator circuit).
As such, a circuit designer must typically consider designing a digitally implemented CRO oscillator to overcome the above problems, otherwise, large phase hits can occur. In addition, since the design of a new CRO oscillator is much like that of an integrated circuit (IC), development of an oscillator with a non-standard frequency requires non-recurring engineering (NRE) costs, in addition to the cost of the oscillators.
Thus, a need exists for methods and circuitry that overcome the foregoing difficulties, and improve the performance of an oscillator or oscillator circuitry, including the ability to absorb phase hits over the tuning range of operation.