# Chapter 7 - Chapter 7 Oscillators 1 Low noise amplifier...

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1 Chapter 7 Oscillators

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2 Duplexer Phase Locked Loop Oscillator Low noise amplifier Power amplifier Up- converter Down- converter Rx IF Tx IF Ctrl Bus
3 What’s Microwave Oscillator z An oscillator is a generator of microwave signal; z An oscillator is a power converter from DC power to microwave power; z An oscillator makes use of a negative resistance device to generate microwave signal; z For a transmitter, the oscillator is the source of microwave signal; z For a receiver, the oscillator is the pump signal for the mixer, called local oscillator or LO; z The oscillator’s frequency can be stabilized by incorporating phase locked loop or dielectric resonator (DR).

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4 How an Oscillation Is Built Up An oscillator can be taken as a frequency selective positive feedback amplifier. It keeps on amplifying a special frequency of the noise signal until the amplitude of the signal reaches to the saturant state of the amplifier. Figure 1 depicts the process of an oscillation buildup. Figure 1. The buildup process of an oscillation.
5 Figure 2. The voltage and current in an NR device Negative Resistance Device A negative resistance (NR) device is a device in which the voltage and the current are 180° out of phase with respect to each other, which means: 0 < D D di dv (1) Figure 2 shows the voltage and current of an NR device. The NR indicate that an increase in voltage across an NR device will produce a decrease in current and vice versa, leading to the product of the current and voltage being negative, which corresponds to the concept of power generation. Exercise: Consider the phase relationship between the current and voltage for below load: 1. Resistive load; 2. Capacitive load; 3. Inductive load; 4. Negative resistance load.

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6 I V Figure 3. The I-V characteristic of the NR diodes R Region What Devices for NR The negative resistance can be some bi-terminal devices or tri-terminal devices with suitable feedback networks. For bi-terminal devices we have: i) Gunn effect diode; ii) IMPATT diode; iii) Tunnel effect diode; iv) TUNNETT diode. All the negative resistive diodes have the similar I-V charateristic, as depicted in Figure 3, which has a region where the resistance is negative. For tri-terminal devices, we have: i) Bipolar transistor; ii) Field Effect Transistor (FET). In oscillator, the transistor operate at their unstable region so that can appear negative resistive at the output port.
7 The DC Bias of Oscillator The DC bias of an oscillator supplies the power to be converted into RF/microwave power. The bias of a tri-terminal device oscillator is the same as the bias for tri-terminal device amplifiers. While the diode oscillators have only one bias point. The RF choke is also needed for isolation between RF and DC, which is similar to the transistor amplifier design. The Figure 4 below shows a microstrip Gunn diode oscillator in millimeter wave: Figure 4 The microstrip Gunn oscillator with low pass filter as the bias network.

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8 Figure 5 A two-port oscillator block diagram
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Chapter 7 - Chapter 7 Oscillators 1 Low noise amplifier...

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