MICROWAVE AND RF DESIGN Case Study: Osc1 Design of a Reflection Oscillator Presented by Michael Steer Reading: Chapter 20, Section 20.4 Index: CS_Osc1 Based on material in Microwave and RF Design: A Systems Approach, 2 nd Edition, by Michael Steer. SciTech Publishing, 2013. Presentation copyright Michael Steer
Case Study Osc1: Design of a Reflection Oscillator 5 V Output 18 GHz Common- Gate Colpitts Reflection Oscillator L r C r R r Resonator ADA Slides copyright 2013 M. Steer. 1
Oscillator type L = 3 L B C 1 C 2 G V DD Z 3 OUT D S Z 1 Z 2 Colpitts Feedback Oscillator Common Gate Oscillator 2
Colpitts oscillator L 3 L 3 C 1 Feedback output C 1 C 2 C 2 Colpitts Feedback Network f osc = π 1 2 LCT C = C $ C = T 1 2 CC 1 2 C + C 1 2 Transistor Configuration V DD L 3 OUT G D C 1 S C 2 Common Gate Collpitts Oscillator 3
Common gate Colpitts oscillator G V DD L 3 OUT D C 1 S C 2 Parasitic capacitances of active device are incorporated in Colpitts feedback elements. Avoids mulifrequency oscillation. Aids stability. Periodic output signal. Output is not chaos. 4
Oscillator type V DD L 3 OUT G D S C 1 L 3 C 1 C 2 Feedback output C 2 Common Gate Collpits Oscillator Vtune Lchoke Output C R L R L B C B R L C R and L R are called a resonator but resonate at a frequency far below the oscillation frequency so the network looks like an effective capacitance. Modified Common Base Colpitts Oscillator 5
Reflection oscillator here 5 V Vtune Lchoke C R L R L B C B Output R L Output L r C r R r Resonator 6
Reflection oscillator Active device 5 V L 1 C 1 50 Ω Output 5 V TL 1 L1 C 1 Z 0= 70 Ω ο L =24.6 @ 16.7GHz 50 Ω X Γ d Γ r PORT 1 Z 0= 50 Ω L r C r R r Resonator Γ d 7
Reflection oscillator operation Y d = G d +jb d Device Tank One-port oscillator Y r = G r +jb r Susceptance B r B d As the amplitude of the oscillation increases, the magnitude of the device conductance, G d decreases while the conductance of the tank circuit, G, is constant. Frequency, f Conductance G r G d As the frequency of the oscillation increases the susceptance of the tank circuit, B, changes while, B d (ideally) does not change. Amplitude A 8
Reflection oscillator Active device 5 V TL 1 Z 0= 70 Ω ο L = 24.6 @ 16.7 GHz L 1 C 1 50 Ω Output L r = 5.6 nh, R r = 10 Ω, C r = 445 fh, L 1 = 15 nh, and C 1 = 10 pf. OscProbe X Γ d Γ r L r C r R r Resonator The resonant frequency of the resonator is 3.188 GHz but this is not the oscillation frequency. It presents the required slope of the susceptance with respect to frequency at the oscillator frequency of 17.76 GHz. 9
Active device and resonator admittances L r 5 V R r L 1 C 1 X 50 Ω C r Γ d Γ r Admittance (S) 0.2 0.1 0.0-0.1 G d B d Gr -0.2 15 16 17 18 19 20 Frequency (GHz) B r 10
Reflection coefficient of the resonator, Γ r, and of the active device Γ d 7 dbm 15 GHz 10 dbm 6 dbm 20 dbm 15 dbm 1/Γ r 20 GHz 0.5 1.0 1.5 2.0 At oscillation Γ d Γ r = 1. i.e. Γ d = 1/Γ r Γ d rotates clockwise with respect to frequency. Γ r rotates clockwise with respect to frequency. 1/Γ r rotates counterclockwise with respect to frequency. 20 GHz Γ r 15 GHz Intersection is at a singe point, thus single frequency oscillation. Cross over at 17.76 GHz and 6 dbm 11
Reflection coefficient of the resonator, Γ r, and of the active device Γ d 10 dbm 20 dbm 15 dbm 20 GHz Want Γ and 1/Γ dd r loci to be parallel at oscillation power. Adjust slope of resonator susceptance to achieve this, also manages tuning range. Γ r 7 dbm 6 dbm 1/Γ r L r R r C r 15 GHz Cross over at 17.76 GHz and 6 dbm 12
Waveforms Voltage (V) 2.0 1.5 1.0 0.5 0.0-0.5 Output Source L r 5 V R r C r -1.0 0 41 82 112 Time (ps) 13
Phase noise -40 Phase noise (dbc/hz) -60-80 -100 f noise -120 1 khz 10 khz 100 khz Frequency 1MHz Frequency of oscillation 14
5 V Final oscillator TL 1 Z 0= 70 Ω ο L = 24.6 @ 16.7 GHz L 1 C 1 50 Ω Output L r = 5.6 nh, R r = 10 Ω, C r L 1 = 15 nh, and C 1 = 10 pf. = 445 fh, L r R r C r Resonator Resonant frequency of resonator is 3.188 GHz Oscillation frequency is 17.76 GHz. Common base Colpitts oscillator but sometimes simply called a reflection oscillator with transmission line in base providing feedback. Resonator enables slope of the admittance to be set. Leads to lower phase noise and overall improved oscillator performance. Resonator is isolated from load. So load has limited effect on resonant frequency. Need to follow with an attenuator (usually) and bandpass filter. 15