USD EEE 194RF - Basic Oscillator Model (25 pages)

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Basic Oscillator Model Oscillator has positive feedback loop at selected frequency Barkhausen criteria implies that the multiplication of the transfer functions of open loop amplifier and feedback stage is HF HA 1 Barkhausen criteria aka loop gain equation LC Oscillators Lower RF Frequencies LC Oscillators Lower RF Frequencies LC Oscillators Lower RF Frequencies Can also design with BJTs High RF Microwave Oscillators Take advantage of our knowledge of stability Rollett Stability Factor k 1 Microwave Oscillator Signal Flow b1 bs in 1 s in Conditions of oscillation Unstable if s in 1 or s L 1 Creating Oscillator Condition Frequently begin with common base or common gate configuration Convert common emitter s parameters to common base similarly for FETs Add inductor in series with base or gate as positive feedback loop network to attain unstable Rollett factor k 1 Unstable Condition Oscillation 1 Convert transistor common base s to Z tr 2 Z L 1 1 j L 1 1 3 Z Osc Z L Z tr 4 Convert Z Osc to s Osc 5 Plot stability circles Inductor Value for Oscillation Must repeat previous calculation of Rollet Factor for each value of L In this example L 5 nH s11 0 935613 s12 0 002108 s21 1 678103 s22 0 966101 Unstable Transistor Oscillator Design 1 2 3 4 Select potentially unstable transistor at freq Select appropriate transistor configuration Draw output stability circle in L plane Select appropriate value of L to produce largest possible negative resistance at input of transistor yielding L 1 and Zin 0 5 Select source tuning impedance Zs as if the circuit was a one port oscillator by RS RIN 0 typically RS RIN 3 RIN 0 and XS XIN 6 Design source tuning and terminating networks with lumped or distributed elements Dielectric Resonator Oscillator DRO DRO Networks DR based input matching network of the FET oscillator Varactor Diodes Voltage Variable Caps Gunn Elements For Oscillators Gunn Oscillator with DRO Mixer Basics Heterodyne receiver system incorporating a mixer Basic mixer concept