Microwave Dielectric Resonator Oscillators: Technical Considerations
Dielectric Resonator Oscillators (DROs) are characterized by low phase noise, compact size, good frequency stability with temperature, ease of integration with other hybrid MIC circuitries, simple construction and the ability to withstand harsh environments. These characteristics make DROs a natural choice both for fundamental oscillators and as the sources for oscillators that are phase-locked to reference frequencies, such as crystal oscillators.
1. Choosing the Dielectric Resonator
In general, a dielectric resonator (DR) is used as a series feedback element. The DR is key to the performance of the oscillator in that it defines the Q of the circuit and locks the frequency. The high unloaded Q (Q0) results in the super low noise performance and is defined by both dielectric loss tangent of the material and the environmental losses. Recent developments in ceramic material technology have resulted in performance improvements such as Q0 as high as 12,000 at 12 GHz and small controllable temperature coefficients. Another important parameter defining the DR is the dielectric constant, which ultimately determines the resonator dimensions as well as the cavity (and circuit design) dimensions. At present, commercially available temperature stable DR materials exhibit dielectric constants of about 36 to 40. These dielectric resonators also come in different forms and modes; however, the cylindrical shape transverse electric (TE) mode (so-called Puck) has been widely accepted as the most advantageous one.
2. Coupling the Dielectric ResonatorChoosing the right DR for the application is key to meeting the DRO’s specifications, however, fitting use of the puck is as equally important to the DRO’s performance. Because of the series negative resistance of the FET in its feedback circuit, the DR is coupled in series to the circuit through a 50 Ohms line and used in a band reject filter mode. At the desired frequency, the puck will reflect any incoming power back to the FET, producing a build-up between the active device and the DR. Coupling between the microstrip and the resonator is accomplished by orienting the resonator’s magnetic momentum perpendicular to the microstrip plane at a particular distance (“D”). The position of the resonator relative to the transmission line determines the oscillator’s stability, output power and phase noise. Optimum positioning can be tricky but is greatly aided by linear and nonlinear simulations. Adjusting “D” increases or decreases the amount of coupling. A higher coupling provides more output power and robustness of oscillation build-up, however, it reduces the loaded Q and therefore the phase noise performance. A lower coupling will improve phase noise but reduces the output power, and under certain circumstances, the oscillator could fail to start oscillating. Therefore, when designing a low noise oscillator, the compromise is to set the distance “D” small enough that the oscillator will always start (under both quick and slow turn-on and at all temperatures) and provides enough power, but large enough to get high loaded Q and low phase noise. Finally, as more energy is stored in the DR, the temperature characteristics of the DRO will more closely follow that of the DR. Consequently, a lighter coupling will also provide more control of the oscillator drift over temperature. The phase relationship of the puck to the active device is as equally critical to efficiently creating an oscillation build-up.
3. General Electrical Consideration
In general, a GaAs FET or a Si-bipolar transistor is chosen as the active device for the oscillator portion of the DRO circuit. The Si-bipolar transistor is generally selected for lower phase noise characteristics, while the GaAs FET is required for higher frequencies. The two most challenging aspects of the design will be to meet the low phase noise specifications and the frequency stability over temperature. The important rules of thumb that should be considered to optimize this design for low phase noise are:a. Maximize the loaded Q (QL of the tuned circuit. This goal will be achieved with a very high Q unloaded dielectric resonator that will only be lightly coupled to the circuit to limit loading effects.
b. Choose a device with a low flicker noise.
c. Maximize the power at the input of the oscillator. A light coupling of the DR will ensure that most of the circuit’s available power is stored in the DR and available at the FET’s gate.
In addition, the phase noise is also dominated by Signal to Noise Ratio at the input (SNRI) which depends on the noise figure of the active device and on the power available from the source. Consequently, design rules that make good Low Noise Amplifiers (LNAs) also apply to low phase noise oscillators. Usually, a typical oscillator runs at about 20% efficiency, however, this achievement also depends on how much output power is tapped out of the circuit. A higher output power means higher efficiency, however, this will reduce the circuit’s loaded Q, which in turn degrades the phase noise performance. A light output coupling will increase phase noise but reduce the power available to drive the rest of the system.
4. Mechanical Consideration
In a DRO, the electrical layout is only one aspect of the oscillator design. Mechanical interests also highly influence the local oscillator (LO)’s performance. The cavity’s size and height have loading effects on the LO which can reduce the phase noise performance and create an unwanted frequency drift over temperature. Under best conditions, the DR would be free to resonate in free space, but because of obvious real estate consideration, the LO needs to be constrained within a shielded cavity. The rules-of-thumb dictates that in order for the cavity to have a reasonable thermal and loading effects, the cavity should be at least three pucks high and three puck’s diameter wide. This height requirement is one reason most DRO designers prefer to set their DR on a standoff, so that the housing or PCB on which the DR usually rests does not affect the resonator’s performance. The PCB material’s mechanical integrity also needs careful consideration because of LO drift over temperature and long term aging effects, especially if the cavity is resting on the PCB.
Finally, the fine tuning and adjustment of the DRO will be set through a tuning screw that will increase the DR’s resonant frequency as it closes the electrical field above the puck. This should provide as much 80 MHz of tuning range. However, it is important to notice that tuning the frequency with a tuning screw is achieved at the cost of reduction in both unloaded Q and temperature stability. This worsen of temperature stability is due to the increasing slope of the tuning curve as the metal plate gets closer to the DR surface.
5. Electronic Frequency Tuning
Frequency tuning of a DRO can be achieved by using voltage controlled diodes (varactors). The circuit configuration for coupling the varactors to the DR consists of an additional microstrip line paralleling the transmission line that couples the DR to the active device, and placed on the opposite side of the DR At the DR plane of coupling, the transmission line can be treated as two quarter-wavelength impedance transformers (or, more precisely two impedance inverters) terminated with two tuning varactors. The varactors' capacitive variation at the end of the inverter is transformed into inductive variation at the plane of the coupling by the impedance inverter. By increasing the coupling between the DR and the varactor/microstrip line, the tuning bandwidth of the DRO can also be increased. There is a trade-off for wider tuning bandwidth resulting in degraded phase noise and poorer frequency stability, mainly due to the resultant equivalent degradation in the unloaded Q of the dielectric resonator. On the other hand, it is necessary that the electrical tuning band of the DRO be wider than the anticipated frequency drift of the oscillator versus temperature. Therefore, coupling the dielectric resonator to the tuning line and coupling the tuning circuit to the oscillator circuit must be kept in balance, so that there can be enough tuning range without significantly degrading the phase noise characteristics or temperature performance.