3/7/2005 Oscillators.doc 1/6 Jim Stiles The Univ. of Kansas Dept. of EECS Oscillators Generally speaking, we construct an oscillator using a gain device (e.g., a transistor) and a resonator. Examples of resonators include LC networks: To make an oscillator, we basically take the output of an amplifier and “feed it back” (i.e., feedback), through the resonator, to the input of the gain device. Under the proper conditions, this device will be unstable—it will oscillate! 01LCω=0cosAtω3/7/2005 Oscillators.doc 2/6 Jim Stiles The Univ. of Kansas Dept. of EECS A: Every resonator has a resonant frequency. The oscillator will oscillate at this frequency! The good news: a perfect resonator will resonate precisely at frequency 0ω. The bad news: there are no perfect resonators! Therefore, the oscillating frequency of an oscillator is a bit ambiguous. A spectral analysis (e.g., power vs. frequency) of an oscillator output reveals that energy is spread over a range of frequencies centered around 0ω, rather than precisely at frequency 0ω. Q: But at what signal frequency 0ω will an oscillator oscillate? f f0 P/Hz Perfect Oscillator f0 f P/Hz Typical Oscillator3/7/2005 Oscillators.doc 3/6 Jim Stiles The Univ. of Kansas Dept. of EECS * The “bandwidth” of this output spectrum is related to the quality of the resonator. * A high-Q resonator provides a spectrum with a narrow width (i.e., spectrally pure). * A low–Q resonator provides an output with a wider spectral width. * Generally, low-Q resonators are lossy, where as high-Q resonators ehibit low loss! LC networks are generally quite lossy, and thus low-Q! Q: Yikes! Are there any high-Q resonators available for constructing microwave oscillators? Crystal Resonators: Like the name suggests, these devices are in fact crystals (e.g. Quartz). The resonant frequency of a crystal resonator is dependent on its geometry and its atomic lattice structure. These resonators are typically used for RF oscillators, where signal frequency is less than 2 GHz. A: Of course! Among my favorite resonators are crystals and dielectric cavities.3/7/2005 Oscillators.doc 4/6 Jim Stiles The Univ. of Kansas Dept. of EECS Dielectric Cavity Resonator – Cavity resonators have a resonant frequency that is dependent on the cavity geometry. Dielectric cavities are popular since they have low loss and can be made very small. Oscillators made with these devices are called Dielectric Resonance Oscillators, or DROs. Typically, these resonators will be used for microwave oscillators, at frequencies greater than 2 GHz Transmission Line Resonator - We can also make a resonator out of transmission line sections. Typically, these are used in stripline or microstrip designs (as opposed to coaxial). Technically, these are LC resonators, as we utilize the inductance and capacitance of a transmission line. As a result, transmission line resonators typically have a lower Q than crystals or cavities, although they exhibit lower loss than “lumped” element LC resonators. Q: So, would we ever use a lumped LC network in a RF/microwave oscillator design? A: Actually, there is one application where we almost certainly would! The main drawback of the resonators described above is that they are fixed. In other words they cannot be tuned ! If we wish to change the oscillating frequency 0ω, we must change (i.e., tune) the resonator.3/7/2005 Oscillators.doc 5/6 Jim Stiles The Univ. of Kansas Dept. of EECS This is tough to do if the resonant frequency depends on the size or shape of the resonator (e.g., crystals and cavities)! Instead, we might use a lumped LC network, where the capacitor element is actually a varactor diode: A varactor diode is a p-n junction diode whose junction capacitance (Cj) varies as a function of diode voltage (vD), when reversed biased. E.G.,: Thus, by changing the diode (reverse) bias voltage, we change the capacitance value, and thus change the resonate (i.e., oscillator) frequency: ()()01DDvLC vω= We call these oscillators Voltage Controlled Oscillators (VCOs). VD Cj 03/7/2005 Oscillators.doc 6/6 Jim Stiles The Univ. of Kansas Dept. of EECS Q: Just exactly why would we ever want to change an oscillator’s frequency ? A: We’ll soon discover that a tunable oscillator is a critical component in a super-heterodyne receiver
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