5.6: Summary

This chapter focused on the design of two categories of microwave oscillators: fixed-frequency oscillators and VCOs. All microwave oscillators can be considered to be an amplifier with a feedback network with the amplifier establishing the amplitude of oscillation and the feedback network setting the frequency of oscillation. As well, nearly all microwave oscillators can be considered as reflection oscillators in which the active device, with appropriate feedback, presents a negative resistance, or equivalently, a negative conductance to a linear frequency-selective circuit. Whether design based on negative conductance or negative resistance is used depends on which, resistance or conductance, reduces in magnitude as the signal level increases. Since transistors are essentially voltage-controlled current sources, and the current tends to saturate at high output levels, the natural view is to consider transistor-based oscillators as having a negative conductance that reduces in magnitude as the signal level increases. With negativeconductance reflection oscillators part or all of the feedback network appears as a linear two-terminal, that is one-port, circuit in parallel with the negative conductance from the active device. This two-terminal circuit is often called the resonator or tank circuit.

The chapter presented two case studies of oscillator design. This enabled design decisions to be illustrated that could not be presented in an algorithmic way. While the design case studies considered common-base Colpitts designs, the principles apply to other types of oscillators. Still the Colpitts common-base/common-drain oscillator core has proved to yield microwave VCOs with stellar performance. RFICs necessarily use differential circuits, but even then they are conveniently designed as negative conductance oscillators. Treating the design problem as that of interconnected one-port circuits considerably simplifies making design trade-offs, and so makes it easier to achieve an optimal VCO.

VCO design is the most complicated of microwave designs with significant trade-offs of low phase noise, stability, broad tuning range, rapid turn-on transient, high output power, and high efficiency. Several decades ago fixed-frequency oscillators were most common. With these the feedback network incorporates a high-\(Q\) resonant element that results in the phase noise on the oscillating signal being insignificant in nearly all microwave applications. The high-\(Q\) resonator of the fixed-frequency oscillators largely assures single frequency of operation, and the desired constant amplitude output is assured by ensuring the active device enters saturation. With a VCO, stability is not as easy to achieve. Stability here refers to the oscillator producing a sinewave of a single frequency and constant amplitude.

With VCOs the resonator is nearly always of relatively low \(Q\), as it must be variable. Then a major concern in oscillator design is phase noise appearing on the output signal. Managing phase noise is complicated because the origins of phase noise are not well known and so physically based device models, that would enable the accurate determination of phase noise in computer-based RF circuit simulation, are not available. For noncompetitive VCOs there are many noise sources other than the intrinsic phase noise of an active device that dominate the phase noise of the output signal. For example, it is known that up-conversion of low-frequency noise, and down-conversion of harmonic noise, produce oscillator phase noise. However, once these sources of phase noise are minimized in design, there is a remaining intrinsic phase noise component. It is this phase noise, intrinsic to active devices, that is the concern of competitive VCO design. While the origins of intrinsic phase noise in well-designed oscillators is not completely understood, there are best practices to follow that minimize the coupling of external noise to the oscillating signal. External noise at low frequencies will be up-converted unless care is taken. Good design practice is to eliminate low-frequency noise from the power supply and from the tuning voltage source for VCOs. Some more specific guidelines:

1. Good grounding is required with decoupling capacitors between the supply and ground.
2. Attention must be given to the signal return path for the supply and tuning voltage source to avoid common impedance coupling. Any noise on the tuning voltage in a VCO will result in phase noise on the VCO output. In a microstrip circuit, only minimum metal on the microstrip layer should be removed with floating metal connected to the ground through multiple vias. This suppresses substrate modes, minimizes parasitic coupling, and minimizes electro-thermal passive intermodulation distortion.
3. The oscillator output should drive a resistive load and it is common to use a resistive pad (i.e., attenuator) followed by a bandpass filter. This reduces the effect of the load on the oscillation frequency. Also it is common to isolate the tank circuit from the load as was seen in the case studies presented in this chapter.
4. Internal oscillator paths should be as small as possible to minimize the coupling of noise from the environment.

A VCO can be incorporated in a phase-locked loop which further reduces phase-noise and locks the oscillator to a low-frequency highly stable frequency reference. The oscillation frequency is then controlled by the fractional frequency division in the phase-locked loop.