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April 2012 2006 by Fabian Kung Wai Lee 1
10 - RF Oscillators
The information in this work has been obtained from sources believed to be reliable.The author does not guarantee the accuracy or completeness of any informationpresented herein, and shall not be responsible for any errors, omissions or damagesas a result of the use of this information.
April 2012 2006 by Fabian Kung Wai Lee 2
Main References
• [1]* D.M. Pozar, “Microwave engineering”, 2nd Edition, 1998 John-Wiley & Sons.• [2] J. Millman, C. C. Halkias, “Integrated electronics”, 1972, McGraw-Hill.• [3] R. Ludwig, P. Bretchko, “RF circuit design - theory and applications”, 2000
Prentice-Hall.• [4] B. Razavi, “RF microelectronics”, 1998 Prentice-Hall, TK6560. • [5] J. R. Smith,”Modern communication circuits”,1998 McGraw-Hill.• [6] P. H. Young, “Electronics communication techniques”, 5th edition, 2004
Prentice-Hall.• [7] Gilmore R., Besser L.,”Practical RF circuit design for modern wireless
systems”, Vol. 1 & 2, 2003, Artech House.• [8] Ogata K., “Modern control engineering”, 4th edition, 2005, Prentice-Hall.
2
April 2012 2006 by Fabian Kung Wai Lee 3
Agenda
• Positive feedback oscillator concepts.• Negative resistance oscillator concepts (typically employed for RF
oscillator).• Equivalence between positive feedback and negative resistance
oscillator theory.• Oscillator start-up requirement and transient.• Oscillator design - Making an amplifier circuit unstable.• Constant |Γ1| circle.• Fixed frequency oscillator design.• Voltage-controlled oscillator design.
April 2012 2006 by Fabian Kung Wai Lee 4
1.0 Oscillation Concepts
3
Introduction
• Oscillators are a class of circuits with 1 terminal or port, which produce a periodic electrical output upon power up.
• Most of us would have encountered oscillator circuits while studying for our basic electronics classes.
• Oscillators can be classified into two types: (A) Relaxation and (B) Harmonic oscillators.
• Relaxation oscillators (also called astable multivibrator), is a class of circuits with two unstable states. The circuit switches back-and-forth between these states. The output is generally square waves.
• Harmonic oscillators are capable of producing near sinusoidal output, and is based on positive feedback approach.
• Here we will focus on Harmonic Oscillators for RF systems. Harmonic oscillators are used as this class of circuits are capable of producing stable sinusoidal waveform with low phase noise.
April 2012 2006 by Fabian Kung Wai Lee 5
April 2012 2006 by Fabian Kung Wai Lee 6
2.0 Overview of Feedback Oscillators
4
April 2012 2006 by Fabian Kung Wai Lee 7
Classical Positive Feedback Perspective on Oscillator (1)
• Consider the classical feedback system with non-inverting amplifier,
• Assuming the feedback network and amplifier do not load each other, we can write the closed-loop transfer function as:
• Writing (2.1a) as:
• We see that we could get non-zero output at So, with Si = 0, provided 1-A(s)F(s) = 0. Thus the system oscillates!
+
+
E(s) So(s)Si(s)A(s)
F(s)
( ) ( )( ) ( )sFsA
sA
iSoS
s −=1
( ) ( ) ( )sFsAsT =PositiveFeedback Loop gain (the gain of the system
around the feedback loop)
Non-inverting amplifier
(2.1a)
(2.1b)
( ) ( )( ) ( ) ( )sSsS isFsA
sAo −= 1
Feedback network
High impedance
High impedance
April 2012 2006 by Fabian Kung Wai Lee 8
Classical Positive Feedback Perspective on Oscillator (1)
• The condition for sustained oscillation, and for oscillation to startup from positive feedback perspective can be summarized as:
• Take note that the oscillator is a non-linear circuit, initially upon power up, the condition of (2.2b) will prevail. As the magnitudes of voltages and currents in the circuit increase, the amplifier in the oscillator begins to saturate, reducing the gain, until the loop gain A(s)F(s) becomes one.
• A steady-state condition is reached when A(s)F(s) = 1.
( ) ( ) 01 =− sFsA
( ) ( ) 1>sFsA ( ) ( )( ) 0arg =sFsA
For sustained oscillation
For oscillation to startup
Barkhausen Criterion (2.2a)
(2.2b)
Note that this is a very simplistic view of oscillators. In reality oscillatorsare non-linear systems. The steady-state oscillatory condition correspondsto what is called a Limit Cycle . See texts on non-linear dynamical systems.
5
April 2012 2006 by Fabian Kung Wai Lee 9
Classical Positive Feedback Perspective on Oscillator (2)
• Positive feedback system can also be achieved with inverting amplifier:
• To prevent multiple simultaneous oscillation, the Barkhausen criterion (2.2a) should only be fulfilled at one frequency.
• Usually the amplifier A is wideband, and it is the function of the feedback network F(s) to ‘select’ the oscillation frequency, thus the feedback network is usually made of reactive components, such as inductors and capacitors.
+
-
E(s) So(s)Si(s)-A(s)
F(s)
( ) ( )( ) ( )sFsA
sA
iSoS
s −=1
Inverting amplifier
Inversion
Classical Positive Feedback Perspective on Oscillator (3)
• In general the feedback network F(s) can be implemented as a Pi or T network, in the form of a transformer, or a hybrid of these.
• Consider the Pi network with all reactive elements. A simple analysis in [2] and [3] shows that to fulfill (2.2a), the reactance X1, X2 and X3 need to meet the following condition:
April 2012 2006 by Fabian Kung Wai Lee 10
+
-
E(s) So(s)-A(s)
X1
X3
X2
( )213 XXX +−=
If X3 represents inductor, thenX1 and X2 should be capacitors.
(2.3)
6
Classical Feedback Oscillators
• The following are examples of oscillators, based on the original circuit using vacuum tubes.
April 2012 2006 by Fabian Kung Wai Lee 11
+
-
+
-
+
-Hartleyoscillator
Clapposcillator
Colpittoscillator
+
-
Armstrong oscillator
April 2012 2006 by Fabian Kung Wai Lee 12
Example of Tuned Feedback Oscillator (1)
A 48 MHz Transistor Common -Emitter Colpitt Oscillator
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0 2.0
-1.0
-0.5
0.0
0.5
1.0
1.5
-1.5
2.0
time, usec
VL,
VV
B, V
+
-
E(s) So(s)Si(s)-A(s)
F(s)
VL
VB
VC
LL1
R=L=2.2 uH
V_DCSRC1Vdc=3.3 V
CCD1C=0.1 uF
CCc1C=0.01 uF
CCc2C=0.01 uF
CCEC=0.01 uF
CC2C=22.0 pF
CC1C=22.0 pF
RRLR=220 Ohmpb_mot_2N3904_19921211
Q1
RRER=220 Ohm
RRCR=330 Ohm
RRB2R=10 kOhm
RRB1R=10 kOhm
( ) ( )ωω FA
t0
1
7
April 2012 2006 by Fabian Kung Wai Lee 13
Example of Tuned Feedback Oscillator (2)
A 27 MHz Transistor Common-Base Colpitt Oscilator
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0 2.0
-400
-200
0
200
400
-600
600
time, usec
VL,
mV
VE
, mV
+
+
E(s) So(s)Si(s)A(s)
F(s)
VL
VE
VB
VC
RR1R=1000 Ohm
CC1C=100.0 pF
CC2C=100.0 pF
LL1
R=L=1.0 uH C
C3C=4.7 pF
RRB2R=4.7 kOhm
RRER=100 Ohm
RRCR=470 Ohm
V_DCSRC1Vdc=3.3 V
CCc1C=0.1 uF
CCc2C=0.1 uF
CCD1C=0.1 uF
pb_mot_2N3904_19921211Q1
RRB1R=10 kOhm
April 2012 2006 by Fabian Kung Wai Lee 14
Example of Tuned Feedback Oscillator (3)
VLVC
VB
CCc2C=0.1 uF
CCc1C=0.1 uF
CCEC=0.1 uF
sx_stk_CX-1HG-SM_A_19930601XTL1Fres=16 MHz
CC2C=22.0 pF
CC1C=22.0 pF
V_DCSRC1Vdc=3.3 V
CCD1C=0.1 uF
RRLR=220 Ohmpb_mot_2N3904_19921211
Q1
RRER=220 Ohm
RRCR=330 Ohm
RRB2R=10 kOhm
RRB1R=10 kOhm
A 16 MHz Transistor Common-EmitterCrystal Oscillator
8
Limitation of Feedback Oscillator
• At high frequency, the assumption that the amplifier and feedback network do not load each other is not valid. In general the amplifier’s input impedance decreases with frequency, and it’s output impedance is not zero. Thus the actual loop gain is not A(s)F(s) and equation (2.2) breakdowns.
• Determining the loop gain of the feedback oscillator is cumbersome at high frequency. Moreover there could be multiple feedback paths due to parasitic inductance and capacitance.
• It can be difficult to distinguish between the amplifier and the feedback paths, owing to the coupling between components and conductive structures on the printed circuit board (PCB) or substrate.
• Generally it is difficult to physically implement a feedback oscillator once the operating frequency is higher than 500MHz.
April 2012 2006 by Fabian Kung Wai Lee 15
April 2012 2006 by Fabian Kung Wai Lee 16
3.0 Negative Resistance Oscillators
9
April 2012 2006 by Fabian Kung Wai Lee 17
Introduction (1)
• An alternative approach is needed to get a circuit to oscillate reliably.
• We can view an oscillator as an amplifier that produces an output when there is no input.
• Thus it is an unstable amplifier that becomes an oscillator!
• For example let’s consider a conditionally stable amplifier.
• Here instead of choosing load or source impedance in the stable regions of the Smith Chart, we purposely choose the load or source impedance in the unstable impedance regions. This will result in either |Γ1 | > 1 or |Γ2 | > 1.
• The resulting amplifier circuit will be called the Destabilized Amplifier.
• As seen in Chapter 7, having a reflection coefficient magnitude for Γ1or Γ2 greater than one implies the corresponding port resistance R1 or R2 is negative, hence the name for this type of oscillator.
April 2012 2006 by Fabian Kung Wai Lee 18
Introduction (2)
• For instance by choosing the load impedance ZL at the unstable region, we could ensure that |Γ1 | > 1. We then choose the source impedance properly so that |Γ1 Γs | > 1 and oscillation will start up (refer back to Chapter 7 on stability theory).
• Once oscillation starts, an oscillating voltage will appear at both the input and output ports of a 2-port network. So it does not matter whether we enforce |Γ1 Γs | > 1 or |Γ2 ΓL | > 1, enforcing either one will cause oscillation to occur (It can be shown later that when |Γ1 Γs | > 1 at the input port, |Γ2 ΓL | > 1 at the output port and vice versa).
• The key to fixed frequency oscillator design is ensuring that the criteria |Γ1 Γs | > 1 only happens at one frequency (or a range of intended frequencies), so that no simultaneous oscillations occur at other frequencies.
10
April 2012 2006 by Fabian Kung Wai Lee 19
Recap - Wave Propagation Stability Perspective (1)
• From our discussion of stability from wave propagation in Chapter 7…
Z1 or Γ1
bsbsΓ1
bsΓs Γ1
bsΓs Γ12
bsΓs 2Γ1
2
bsΓs 2Γ1
3
Source 2-portNetwork
Zs or ΓsPort 1 Port 2
s
s
sssss
ba
bbba
ΓΓ−=⇒
+ΓΓ+ΓΓ+=
11
22111
1
...
bsΓs 3Γ1
3
bsΓs 3Γ1
4
a1b1
Compare with equation (2.1a)
ssbb
s
s
sssss
bb
bbbb
ΓΓ−Γ=⇒
ΓΓ−Γ
=⇒
+ΓΓ+ΓΓ+Γ=
1
11
1
11
231
2111
1
1
...
( ) ( )( ) ( )sFsA
sA
iSoS
s −=1
Similar mathematicalform
April 2012 2006 by Fabian Kung Wai Lee 20
Recap - Wave Propagation Stability Perspective (2)
• We see that the infinite series that constitute the steady-state incident (a1) and reflected (b1) waves at Port 1 will only converge provided |Γ sΓ1| < 1.
• These sinusoidal waves correspond to the voltage and current at the Port 1. If the waves are unbounded it means the corresponding sinusoidal voltage and current at the Port 1 will grow larger as time progresses, indicating oscillation start-up condition.
• Therefore oscillation will occur when |Γ sΓ1 | > 1.
• Similar argument can be applied to port 2 since the signals at Port 1 and 2 are related to each other in a two-port network, and we see that the condition for oscillation at Port 2 is |ΓLΓ2 | > 1.
11
Oscillation from Negative Resistance Perspective (1)
• Generally it is more useful to work with impedance (or admittance) when designing actual circuit.
• Furthermore for practical purpose the transmission lines connecting ZLand Zs to the destabilized amplifier are considered very short (length → 0).
• In this case the impedance Zo is ambiguous (since there is no transmission line).
• To avoid this ambiguity, let us ignore the transmission line and examine the condition for oscillation phenomena in terms of terminal impedance.
April 2012 2006 by Fabian Kung Wai Lee 211ZZ ≅
Zs ZoZ1
DestabilizedAmp. andLoad
sZZ ≅
Very short Tline
April 2012 2006 by Fabian Kung Wai Lee 22
Source Network
Port 1
Zs Z1
( ) ss
sss
VZZ
ZV
XXjRR
jXRV
1
1
11
11
+=⋅
++++=
Oscillation from Negative Resistance Perspective (2)
• We consider Port 1 as shown, with the source network and input of the amplifier being modeled by impedance or series networks.
• Using circuit theory the voltage at Port 1 can be written as:
(3.1)
jXs
Rs
jX1
R1
V ZL
Z2
Vamp
Port 2
Amplifier with load ZL
12
Oscillation from Negative Resistance Perspective (3)
• Furthermore we assume the source network Zs is a series RC network and the equivalent circuit looking into the amplifier Port 1 is a series RL network.
• Using Laplace Transform, (3.1) is written as:
April 2012 2006 by Fabian Kung Wai Lee 23
Rs
Cs
R1
L1
V ZL
Z2
Vamp
Vs
Zs Z1
( ) ( )sVsLRR
sLRsV s
sCs s
⋅+++
+=1
11
11
ωσ js +=where
(3.2a)
(3.2b)
Oscillation from Negative Resistance Perspective (4)
• The expression for V(s) can be written in the “standard” form according to Control Theory [8]:
• The transfer function V(s)/Vs(s) is thus a 2nd order system with two poles p1, p2 given by:
• Observe that if (R1 + Rs) < 0 the damping factor δ is negative. This is true if R1 is negative, and |R1| > Rs.
• R1 can be made negative by modifying the amplifier circuit (e.g. adding local positive feedback), producing the sum R1 + Rs < 0.
April 2012 2006 by Fabian Kung Wai Lee 24
( ) ( )( )
( )22
211
1211
1 2
1
11
1nn
ns
CLLRR
s ss
sLRsC
ss
sLRs
Ls
V
V
s
s ωδωω
+++=
+++⋅= +
Frequency Natural Factor Damping11
1 1
2==== +
s
s
s
CLnC
L
RR ωδ
(3.3a)
where
122,1 −±−= δωδω nnp (3.4)
(3.3b)
13
Oscillation from Negative Resistance Perspective (5)
• Assuming |δ|<1 (under-damped), the poles as in (3.4) will be complex and exist at the right-hand side of the complex plane.
• From Control Theory such a system is unstable. Any small perturbation will result in a oscillating signal with frequency that grows exponentially.
• Usually a transient or noise signal from the environment will contain a small component at the oscillation frequency. This forms the ‘seed’ in which the oscillation builts up.
April 2012 2006 by Fabian Kung Wai Lee25
0|1 <+o
RRs ω ×
×
Re
Im
0
Complex pole pair
Complex Plane
t
A small disturbanceor impulse ‘starts’ theexponentially growingsinusoid
Time Domain
v(t)
12 −δωn
Oscillation from Negative Resistance Perspective (6)
• When the signal amplitude builds up, nonlinear effects such as transistor saturation and cut-off will occur, this limits the β of the transistor and finally limits the amplitude of the oscillating signal.
• The effect of decreasing β of the transistor is a reduction in the magnitude of R1 (remember R1 is negative). Thus the damping factor δwill approach 0, since Rs+ R1 → 0.
• Steady-state sinusoidal oscillation is achieved when δ =0, or equivalently the poles become
• The steady-state oscillation frequency ωo corresponds to ωn,
April 2012 2006 by Fabian Kung Wai Lee 26
sCnCLn XXLsns
=⇒=⇒= 11
112
1 ωωω
njp ωδ
±==02,1
01 =+⇒o
sXX ω
14
Oscillation from Negative Resistance Perspective (7)
• From (3.3b), we observe that the steady-state oscillation frequency is determined by L1 and Cs, in other words, X1 and Xs respectively.
• Since the voltages at Port 1 and Port 2 are related, if oscillation occur at Port 1, then oscillation will also occur at Port 2.
• From this brief discussion, we use RC and RL networks for the source and amplifier input respectively, however we can distill the more general requirements for oscillation to start-up and achieve steady-state operation for series representation in terms of resistance and reactance:
April 2012 2006 by Fabian Kung Wai Lee 27
0|1 <+o
RRs ω
0|1 =+o
XX s ω
0|1 =+o
RRs ω
0|1 =+o
XX s ω
(3.6a)
(3.6b)
(3.5a)
(3.5b)
Steady-stateStart-up
Illustration of Oscillation Start-Up and Steady-State
• The oscillation start-up process and steady-state are illustrated.
April 2012 2006 by Fabian Kung Wai Lee 28
0 10 20 30 40 50 60 70 80 90 100 110 120
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Destabilized Amplifier
ZLZs
t
R1+Rs
0
Oscillationstart-up
Steady-state Z1Zs
We need to note that this is a very simplistic view of oscillators. Oscillators are autonomous non-linear dynamical systems, and the steady-state condition is a form of Limit Cycles .
15
April 2012 2006 by Fabian Kung Wai Lee 29
Source Network
Port 1
Zs Z1
Summary of Oscillation Requirements Using Series Network
• By expressing Zs and Z1 in terms of resistance and reactance, we conclude that the requirement for oscillation are.
• A similar expression for Z2 and ZL can also be obtained, but we shall not be concerned with these here.
jXs
Rs
jX1
R1
V ZL
Z2
Vamp
Port 2
0|1 <+o
RRs ω
0|1 =+o
XX s ω
0|1 =+o
RRs ω
0|1 =+o
XX s ω
(3.6a)
(3.6b)
(3.5a)
(3.5b)
Steady-state Start-up
The Resonator
• The source network Zs is usually called the Resonator, as it is clear that equations (3.5b) and (3.6b) represent the resonance condition between the source network and the amplifier input.
• The design of the resonator is extremely important.
• We shall see later that an important parameter of the oscillator, the Phase Noise is dependent on the quality of the resonator.
April 2012 2006 by Fabian Kung Wai Lee 30
16
Summary of Oscillation Requirements Using Parallel Network
• If we model the source network and input to the amplifier as parallel networks, the following dual of equations (3.5) and (3.6) are obtained.
• The start-up and steady-state conditions are:
April 2012 2006 by Fabian Kung Wai Lee 31
jBsGs jB1G1
V ZL
Z2
Vamp
Port 1
0|1 =+o
GGs ω
0|1 =+o
BBs ω
0|1 <+o
GGs ω
0|1 =+o
BBs ω
Steady-state Start-up
(3.7a)
(3.7b)
(3.8a)
(3.8b)
Series or Parallel Representation? (1)
• The question is which to use? Series or parallel network representation? This is not an easy question to answer as the destabilized amplifier is operating in nonlinear region as oscillator.
• Concept of impedance is not valid and our discussion is only an approximation at best.
• We can assume series representation, and worked out the corresponding resonator impedance. If after computer simulation we discover that the actual oscillating frequency is far from our prediction (if there’s any oscillation at all!), then it probably means that the series representation is incorrect, and we should try the parallel representation.
• Another clue to whether series or parallel representation is more accurate is to observe the current and voltage in the resonator. For series circuit the current is near sinusoidal, where as for parallel circuit it is the voltage that is sinusoidal.
April 2012 2006 by Fabian Kung Wai Lee 32
17
Series or Parallel Representation? (2)
• Reference [7] illustrates another effective alternative, by computing the large-signal S11 of Port 1 (with respect to Zo) using CAD software.
• 1/S11 is then plotted on a Smith Chart as a function of input signal magnitude at the operating frequency.
• By comparing the locus of 1/S11 as input signal magnitude is gradually increased with the coordinate of constant X or constant B circles on the Smith Chart, we can decide whether series or parallel form approximates Port 1 best.
• We will adopt this approach, but plot S11 instead of 1/S11. This will be illustrated in the examples in next section.
• Do note that there are other reasons that can cause the actual oscillation frequency to deviate a lot from prediction, such as frequency stability issue (see [1] and [7]).
April 2012 2006 by Fabian Kung Wai Lee 33
April 2012 2006 by Fabian Kung Wai Lee 34
4.0 Fixed Frequency Negative Resistance
Oscillator Design
18
April 2012 2006 by Fabian Kung Wai Lee 35
Procedures of Designing Fixed Frequency Oscillator (1)
• Step 1 - Design a transistor/FET amplifier circuit.
• Step 2 - Make the circuit unstable by adding positive feedback at radio frequency, for instance, adding series inductor at the base for common-base configuration.
• Step 3 - Determine the frequency of oscillation ωo and extract S-parameters at that frequency.
• Step 4 – With the aid of Smith Chart and Load Stability Circle, make R1< 0 by selecting ΓL in the unstable region.
• Step 5 (Optional) – Perform a large-signal analysis (e.g. Harmonic Balance analysis) and plot large-signal S11 versus input magnitude on Smith Chart. Decide whether series or parallel form to use.
• Step 6 - Find Z1 = R1 + jX1 (Assuming series form).
April 2012 2006 by Fabian Kung Wai Lee 36
Procedures of Designing Fixed Frequency Oscillator (2)
• Step 7 – Find Rs and Xs so that R1 + Rs<0, X1 + Xs=0 at ωo. We can use the rule of thumb Rs=(1/3)|R1| to control the harmonics content at steady-state.
• Step 8 - Design the impedance transformation network for Zs and ZL.
• Step 9 - Built the circuit or run a computer simulation to verify that the circuit can indeed starts oscillating when power is connected.
• Note: Alternatively we may begin Step 4 using Source Stability Circle, select Γs in the unstable region so that R2 or G2 is negative at ωo .
19
April 2012 2006 by Fabian Kung Wai Lee 37
Making an Amplifier Unstable (1)
• An amplifier can be made unstable by providing some kind of local positive feedback.
• Two favorite transistor amplifier configurations used for oscillator design are the Common-Base configuration with Base feedback and Common-Emitter configuration with Emitter degeneration.
April 2012 2006 by Fabian Kung Wai Lee 38
Making an Amplifier Unstable (2)
Vout
Vin
L_StabCircleL_StabCircle1LSC=l_stab_circle(S,51)
LStabCircle
S_StabCircleS_StabCircle1SSC=s_stab_circle(S,51)
SStabCircle
StabFactStabFact1K=stab_fact(S)
StabFact
RReR=100 Ohm
S_ParamSP1
Step=2.0 MHzStop=410.0 MHzStart=410.0 MHz
S-PARAMETERS
DCDC1
DC
CCLBC=0.17 pF
CCbC=10.0 nF
LLB
R=L=22 nH
RRLBR=0.77 Ohm
CCc2C=10.0 nF
CCc1C=10.0 nF Term
Term1
Z=50 OhmNum=1
LLC
R=L=330.0 nH
LLE
R=L=330.0 nH
V_DCSRC1Vdc=4.5 V
TermTerm2
Z=50 OhmNum=2
RRb1R=10 kOhm
RRb2R=4.7 kOhm
pb_phl_BFR92A_19921214Q1
Positive feedbackhere
Common BaseConfiguration
This is a practical modelof an inductor
An inductor is addedin series with the bypasscapacitor on the baseterminal of the BJT. This is a form of positiveseries feedback.
Base bypasscapacitor
At 410MHz
20
April 2012 2006 by Fabian Kung Wai Lee 39
Making an Amplifier Unstable (3)
freq410.0MHz
K-0.987
freq410.0MHz
S(1,1)1.118 / 165.6...
S(1,2)0.162 / 166.9...
S(2,1)2.068 / -12.723
S(2,2)1.154 / -3.535
Unstable Regions
s22 and s11 have magnitude > 1
ΓL PlaneΓs Plane
April 2012 2006 by Fabian Kung Wai Lee 40
Making an Amplifier Unstable (4)
Vout
pb_phl_BFR92A_19921214Q1
CCe1C=15.0 pF
CCe2C=10.0 pF
RRb1R=10 kOhm
RRb2R=4.7 kOhm
TermTerm1
Z=50 OhmNum=1
CCc1C=1.0 nF
RReR=100 Ohm
CCc2C=1.0 nF
L_StabCircleL_StabCircle1LSC=l_stab_circle(S,51)
LStabCircle
S_StabCircleS_StabCircle1SSC=s_stab_circle(S,51)
SStabCircle
StabFactStabFact1K=stab_fact(S)
StabFact
S_ParamSP1
Step=2.0 MHzStop=410.0 MHzStart=410.0 MHz
S-PARAMETERS
DCDC1
DC
LLC
R=L=330.0 nH
V_DCSRC1Vdc=4.5 V
TermTerm2
Z=50 OhmNum=2
Positive feedback here
Common EmitterConfiguration
Feedback
21
April 2012 2006 by Fabian Kung Wai Lee 41
Making an Amplifier Unstable (5)
freq410.0MHz
K-0.516
freq410.0MHz
S(1,1)3.067 / -47.641
S(1,2)0.251 / 62.636
S(2,1)6.149 / 176.803
S(2,2)1.157 / -21.427
UnstableRegions
S22 and S11 have magnitude > 1
ΓL Plane Γs Plane
April 2012 2006 by Fabian Kung Wai Lee 42
Precautions
• The requirement Rs= (1/3)|R1| is a rule of thumb to provide the excess gain to start up oscillation.
• Rs that is too large (near |R1| ) runs the risk of oscillator fails to start up due to component characteristic deviation.
• While Rs that is too small (smaller than (1/3)|R1|) causes too much non-linearity in the circuit, this will result in large harmonic distortion of the output waveform.
V2
Clipping, a sign of too much nonlinearity
t
Rs too small
t
V2
Rs too large
For more discussion about the Rs = (1/3)|R1| rule,and on the sufficient condition for oscillation, see [6], which list further requirements.
22
April 2012 2006 by Fabian Kung Wai Lee 43
Aid for Oscillator Design - Constant |ΓΓΓΓ1| Circle (1)
• In choosing a suitable ΓL to make |ΓL | > 1, we would like to know the range of ΓL that would result in a specific |Γ1 |.
• It turns out that if we fix |Γ1 |, the range of load reflection coefficient that result in this value falls on a circle in the Smith chart for ΓL .
• The radius and center of this circle can be derived from:
• Assuming ρ = |Γ1 |:L
L
S
DS
Γ−Γ−=Γ
22
111 1
222
2211
**22
2
centerTSD
SDS
ρρ
−
+−=2
22222112RadiusSD
SS
ρρ
−=
By fixing |Γ1 | and changing ΓL .
(4.1a) (4.1b)
April 2012 2006 by Fabian Kung Wai Lee 44
Aid for Oscillator Design - Constant |ΓΓΓΓ1| Circle (2)
• The Constant |Γ1 | Circle is extremely useful in helping us to choose a suitable load reflection coefficient. Usually we would choose ΓL that would result in |Γ1 | = 1.5 or larger.
• Similarly Constant |Γ2 | Circle can also be plotted for the source reflection coefficient. The expressions for center and radius is similar to the case for Constant |Γ1 | Circle except we interchange s11 and s22, ΓL and Γs . See Ref [1] and [2] for details of derivation.
23
April 2012 2006 by Fabian Kung Wai Lee 45
Example 4.1 – CB Fixed Frequency Oscillator Design
• In this example, the design of a fixed frequency oscillator operating at 410MHz will be demonstrated using BFR92A transistor in SOT23 package. The transistor will be biased in Common-Base configuration.
• It is assumed that a 50Ω load will be connected to the output of the oscillator. The schematic of the basic amplifier circuit is as shown in the following slide.
• The design is performed using Agilent’s ADS software, but the author would like to stress that virtually any RF CAD package is suitable for this exercise.
April 2012 2006 by Fabian Kung Wai Lee 46
Example 4.1 Cont...
• Step 1 and 2 - DC biasing circuit design and S-parameter extraction.
DCDC1
DC
S_ParamSP1
Step=2.0 MH zStop=410.0 MHzStart=410.0 MH z
S-PARAME TERS
StabFactStabFact1K=stab_f act (S )
S t abFac t
LLC
R=L=330.0 nH
LLE
R=L=220.0 nH
LLB
R =L=12.0 nH
S_StabCircleS_StabCircle1source_s tabc ir=s_stab_c irc le(S ,51)
SStabCircle
L_StabCircleL_StabCircle1load_s tabcir=l_s tab_c irc le(S,51)
LSt abCircle
TermTerm 1
Z=50 OhmNum=1
CCc1C=1.0 nF
TermTerm 2
Z=50 OhmNum=2
CCc2C=1.0 nF
RR eR =100 Ohm
CC bC =1.0 nF
V_DCSRC1Vdc=4.5 V R
Rb1R=10 kO hm
RRb2R=4.7 kOhm
pb_phl_BFR 92A_19921214Q1
Port 1 - Input
Port 2 - Output
AmplifierPort 1 Port 2LB is chosen care-fully so that theunstable regionsin both ΓL and Γs
planes are largeenough.
24
April 2012 2006 by Fabian Kung Wai Lee 47
Example 4.1 Cont...
freq410.0MHz
K-0.987
freq410.0MHz
S(1,1)1.118 / 165.6...
S(1,2)0.162 / 166.9...
S(2,1)2.068 / -12.723
S(2,2)1.154 / -3.535
Unstable Regions
Load impedance here will resultin |Γ1| > 1
Source impedance here will resultin |Γ2| > 1
April 2012 2006 by Fabian Kung Wai Lee 48
Example 4.1 Cont...
• Step 3 and 4 - Choosing suitable ΓL that cause |Γ1 | > 1 at 410MHz. We plot a few constant |Γ1 | circles on the ΓL plane to assist us in choosing a suitable load reflection coefficient.
LSC
|Γ1 |=1.5
|Γ1 |=2.0
|Γ1 |=2.5
ΓL = 0.5<0
This point is chosenbecause it is onreal line and easilymatched.
ΓL Plane
Note: More difficult to implement loadimpedance nearedges of Smith Chart
ZL = 150+j0
25
Example 4.1 Cont...
• Step 5 – To check whether the input of the destabilized amplifier is closer to series or parallel form. We perform large-signal analysis and observe the S11 at the input of the destabilized amplifier.
April 2012 2006 by Fabian Kung Wai Lee 49
LSSPHB1
Step=0.2Stop=-5Start=-20SweepVar="Poutv"LSSP_FreqAtPort[1]=Order[1]=5Freq[1]=410.0 MHz
LSSP
RRLR=150 Ohm
VARVAR1Poutv=-10.0
EqnVar
P_1TonePORT1
Freq=410 MHzP=polar(dbmtow(Poutv),0)Z=50 OhmNum=1
CCc2C=1.0 nF
CCc1C=1.0 nF
LLB
R=L=12.0 nH
CCBC=1.0 nF
V_DCSRC1Vdc=4.5 V
RRER=100 Ohm
LLE
R=L=220.0 nH
RRB2R=4.7 kOhm
RRB1R=10 kOhm
LLC
R=L=330.0 nH
pb_phl_BFR92A_19921214Q1
We are measuringlarge-signal S11 lookingtowards here
Large-signal S-parameterAnalysis controlin ADS software.
Example 4.1 Cont...
• Compare the locus of S11 and the constant X and constant B circles on the Smith Chart, it is clear the locus is more parallel to the constant X circle. Also the direction of S11 is moving from negative R to positive R as input power level is increased. We conclude the Series form is more appropriate.
April 2012 2006 by Fabian Kung Wai Lee 50
Region where R1 or G1 is negative
Poutv (-20.000 to -5.000)
S(1
,1)
Direction of S11 as magnitudeof P_1tone source is increased
Compare
Locus of S11 versus P_1tone power at 410MHz(from -20 to -5 dBm)
Boundary ofNormal Smith Chart
Region where R1 or G1 is positive
26
April 2012 2006 by Fabian Kung Wai Lee 51
Example 4.1 Cont...
• Step 6 – Using the series form, we find the small-signal input impedance Z1 at 410MHz. So the resonator would also be a series network.
• For ZL = 150 or ΓL = 0.5<0:
• Step 7 - Finding the suitable source impedance to fulfill R1 + Rs<0, X1 + Xs=0:
851.7257.101
1
479.0422.11
1
11
22
111
jZZ
jS
DS
o
L
L
+−=Γ−Γ+=
+−=Γ−Γ−=Γ
851.7
42.33
1
1
1
−≅−=
≅=
XX
RR
s
s
R1X1
April 2012 2006 by Fabian Kung Wai Lee 52
Example 4.1 Cont...
Common-Base (CB) Amplifier
with feedback
Port 1 Port 2Zs = 3.42-j7.851
ZL = 150
• The system block diagram:
27
April 2012 2006 by Fabian Kung Wai Lee 53
Example 4.1 Cont...
pFC
C
44.49851.7
1
1851.7
==
=
ω
ω
CB Amplifier3.42
27.27nH49.44pF
50
Zs= 3.42-j7.851 ZL=150
@ 410MHz3.49pF
• Step 5 - Realization of the source and load impedance at 410MHz.
Impedance transformation network
April 2012 2006 by Fabian Kung Wai Lee 54
Example 4.1 Cont... - Verification Thru Simulation
Vpp = 0.9VV = 0.45V
Power dissipated in the load:
mW
R
VP
LL
025.250
45.05.0
2
1
2
2
==
=
BFR92A
Vpp
28
April 2012 2006 by Fabian Kung Wai Lee 55
Example 4.1 Cont... - Verification Thru Simulation
• Performing Fourier Analysis on the steady state wave form:
484 MHz
The waveform is very clean withlittle harmonic distortion. Althoughwe may have to tune the capacitorCs to obtain oscillation at 410 MHz.
April 2012 2006 by Fabian Kung Wai Lee 56
Example 4.1 Cont... – The Prototype
0 10 20 30 40 50 60 70 80 90 100 110 120
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Voltage at the base terminal and 50 Ohms load resistor of thefixed frequency oscillator:
Output portVout
Vbb
V
nsStartup transient
29
Example 4.2 – 450 MHz CE Fixed Frequency Oscillator Design
• Small-signal AC or S-parameter analysis, to show that R1 or G1 is negative at the intended oscillation frequency of 450 MHz.
April 2012 2006 by Fabian Kung Wai Lee 57
S_ParamSP1
Step=10.0 MHzStop=800.0 MHzStart=100.0 MHz
S-PARAMETERS
TermTerm1
Z=50 OhmNum=1
CC2C=4.7 pF
RRLR=150 Ohm
CCc2C=330.0 pF
V_DCSRC1Vdc=3.0 V
LLC
R=L=220.0 nH
RRER=220 Ohm
RRBR=47 kOhm
DC_BlockDC_Block1 C
C1C=2.2 pF
pb_phl_BFR92A_19921214Q1
200 300 400 500 600 700100 800
-500
-400
-300
-200
-100
-600
0
-1500
-1000
-500
-2000
0
freq, MHz
rea
l(Z(1
,1))
imag(Z
(1,1))
200 300 400 500 600 700100 800
-0.010
-0.005
-0.015
0.000
0.005
0.010
0.015
0.000
0.020
freq, MHz
rea
l(Y(1
,1))
imag(Y
(1,1))
Selection of loadresistor as in Example 4.1.
There are simplified expressions to find C1
and C2, see reference [5].Here we just trial and error to get some reasonable values.
Destabilized amplifier
Example 4.2 Cont…
• The large-signal analysis to check for suitable representation.
April 2012 2006 by Fabian Kung Wai Lee 58
Poutv (-5.000 to 15.000)
S(1
,1)
LSSPHB1
Step=0.2Stop=15Start=-5Sw eepVar="Poutv"LSSP_FreqAtPort[1]=Order[1]=7Freq[1]=450.0 MHz
LSSP
CC2C=4.7 pF
P_1TonePORT1
Freq=450 MHzP=polar(dbmtow (Poutv),0)Z=50 OhmNum=1
RRLR=150 Ohm
CCc2C=330.0 pF
V_DCSRC1Vdc=3.0 V
LLC
R=L=220.0 nH
RRER=220 Ohm
RRBR=47 kOhm
DC_BlockDC_Block1 C
C1C=2.2 pF
VARVAR1Poutv=-10.0
EqnVar
pb_phl_BFR92A_19921214Q1
Direction of S11 as magnitudeof P_1tone source is increasedfrom -5 to +15 dBm
Compare
Since the locus of S11 is close in shape toconstant X circles, and it indicates R1 goes fromnegative value to positive values as input power is increased, we use series form torepresent the input network looking towardsthe Base of the amplifier.
Boundary ofNormal Smith Chart
S11
30
Example 4.2 Cont…
• Using a series RL for the resonator, and performing time-domain simulation to verify that the circuit will oscillate.
April 2012 2006 by Fabian Kung Wai Lee 590.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50.0 5.0
0.2
0.4
0.6
0.0
0.8
freq, GHz
ma
g(V
fL)
m1
m1freq=mag(VfL)=0.733
450.0MHz
VLVC
VB
LL1
R=10L=39.0 nH
VtPWLSRC2V_Tran=pw l(time, 0ns,0V, 2ns,0.1V, 4ns,0V)
t
TranTran1
MaxTimeStep=1.0 nsecStopTime=100.0 nsec
TRANSIENT
CCc1C=1.0 nF
CC2C=4.7 pF
RRLR=150 Ohm
CCc2C=330.0 pF
V_DCSRC1Vdc=3.0 V
LLC
R=L=220.0 nH
RRER=220 Ohm
RRBR=47 kOhm
CC1C=2.2 pF
pb_phl_BFR92A_19921214Q1
20 40 60 800 100
-1.0
-0.5
0.0
0.5
-1.5
1.0
time, nsec
VL
, V
Eqn VfL=fs(VL)
vL(t)
|VL(f)|Large couplingcapacitor
Example 4.3 – Parallel Representation
• An example where the network looking into the Base of the destabilized amplifier is more appropriate as parallel RC network.
April 2012 2006 by Fabian Kung Wai Lee 60
Poutv (-7.000 to 12.000)
S(1
,1)
V_DCVCCVdc=3.3 V
RRER=100 Ohm
LLC
R=0.2L=2 nH
RRB1R=1000 Ohm
RRLR=50 Ohm
VARVAR5
fo=2300Poutv=1.0
EqnVar
LSSPHB1
Step=0.2Stop=12Start=-7SweepVar="Poutv"LSSP_FreqAtPort[1]=fo MHzOrder[1]=8Freq[1]=fo MHz
LSSP
CCdec1C=100.0 pF
P_1TonePORT1
Freq=fo MHzP=polar(dbmtow(Poutv),0)Z=50 OhmNum=1
CCc1C=1.2 pF
CCc2C=1.0 pF
CC2C=0.7 pF t
CC1C=0.6 pF t
RRB2R=1000 Ohm
pb_phl_BFR92A_19921214Q1
S11
CompareDirection of S11 as magnitudeof P_1tone source is increasedfrom -7 to +12 dBm
S11 versusInput power
31
Frequency Stability
• The process of oscillation depends on the non-linear behavior of the negative-resistance network.
• The conditions discussed, e.g. equations (3.1), (3.8), (3.9), (3.10) and (3.11) are not enough to guarantee a stable state of oscillation. In particular, stability requires that any perturbation in current, voltage and frequency will be damped out, allowing the oscillator to return to it’s initial state.
• The stability of oscillation can be expressed in terms of the partial derivative of the sum Zin + Zs or Yin + Ys of the input port (or output port).
• The discussion is beyond the scope of this chapter for now, and the reader should refer to [1] and [7] for the concepts.
April 2012 2006 by Fabian Kung Wai Lee 61
April 2012 2006 by Fabian Kung Wai Lee 62
Some Steps to Improve Oscillator Performance
• To improve the frequency stability of the oscillator, the following steps can be taken.
• Use components with known temperature coefficients, especially capacitors.
• Neutralize, or swamp-out with resistors, the effects of active device variations due to temperature, power supply and circuit load changes.
• Operate the oscillator on lower power.
• Reduce noise, use shielding, AGC (automatic gain control) and bias-line filtering.
• Use an oven or temperature compensating circuitry (such as thermistor).
• Use differential oscillator architecture (see [4] and [7]).
32
Extra References for This Section
• Some recommended journal papers on frequency stability of oscillator:
• Kurokawa K., “Some basic characteristics of broadband negative resistance oscillator circuits”, Bell System Technical Journal, pp. 1937-1955, 1969.
• Nguyen N.M., Meyer R.G., “Start-up and frequency stability in high-frequency oscillators”,IEEE journal of Solid-State Circuits, vol 27, no. 5 pp.810-819, 1992.
• Grebennikov A. V., “Stability of negative resistance oscillator circuits”, International journal of Electronic Engineering Education, Vol. 36, pp. 242-254, 1999.
April 2012 2006 by Fabian Kung Wai Lee 63
Reconciliation Between Feedback and Negative Resistance Oscillator
Perspectives• It must be emphasized that the circuit we obtained using negative
resistance approach can be cast into the familiar feedback form. For instance an oscillator circuit similar to Example 4.2 can be redrawn as:
April 2012 2006 by Fabian Kung Wai Lee 64
VL
CCc1C=4.7 pF
RRLR=50 Ohm
CCc2C=1.0 pF
LL1
R=0.1L=15.0 nH t
RRB1R=10000 Ohm t
LLC
R=0.2L=2.2 nH t
CC1C=1.0 pF t
CC2C=0.8 pF t
RRER=100 Ohm t
pb_phl_BFR92A_19921214Q1
V_DCVCCVdc=3.0 V
VL
RRLR=50 Ohm
RRER=100 Ohm t
pb_phl_BFR92A_19921214Q1
CC2C=0.8 pF t
CC1C=1.0 pF t
LL1
R=0.1L=15.0 nH t
CCc1C=4.7 pF
CCc2C=1.0 pF
RRB1R=10000 Ohm t
LLC
R=0.2L=2.2 nH t
V_DCVCCVdc=3.0 V
Amplifier
Feedback Network
Negative ResistanceOscillator
33
April 2012 2006 by Fabian Kung Wai Lee 65
5.0 Voltage Controlled Oscillator
April 2012 2006 by Fabian Kung Wai Lee 66
About the Voltage Controlled Oscillator (VCO) (1)
• A simple transistor VCO using Clapp-Gouriet or CE configuration will be designed to illustrate the principles of VCO.
• The transistor chosen for the job is BFR92A, a wide-band NPN transistor which comes in SOT-23 package.
• Similar concepts as in the design of fixed-frequency oscillators are employed. Where we design the biasing of the transistor, destabilize the network and carefully choose a load so that from the input port (Port 1), the oscillator circuit has an impedance (assuming series representation is valid):
• Of which R1 is negative, for a range of frequencies from ω1 to ω2.
( ) ( ) ( )ωωω 111 jXRZ +=
Lower Upper
34
April 2012 2006 by Fabian Kung Wai Lee 67
About the Voltage Controlled Oscillator (VCO) (2)
Clapp-GourietOscillator Circuitwith Load
Zs
Z1 = R1 + jX1
ZL
April 2012 2006 by Fabian Kung Wai Lee 68
About the Voltage Controlled Oscillator (VCO) (3)
• If we can connect a source impedance Zs to the input port, such that within a range of frequencies from ω1 to ω2:
• The circuit will oscillate within this range of frequencies. By changing the value of Xs, one can change the oscillation frequency.
• For example, if X1 is positive, then Xs must be negative, and it can be generated by a series capacitor. By changing the capacitance, one can change the oscillation frequency of the circuit.
• If X1 is negative, Xs must be positive. A variable capacitor in series with a suitable inductor will allow us to adjust the value of Xs.
( ) ( ) ( )ωωω sss jXRZ +=( ) ( ) ( ) 0 11 << ωωω RRRs ( ) ( ) 1 ωω XX s =
The rationale is that only the initial spectral of the noise signal fulfilling Xs = X1 will start the oscillation.
35
April 2012 2006 by Fabian Kung Wai Lee 69
Schematic of the VCO
RRLR=Rload
ParamSweepSweep1
St ep=100St op=700St art=100SimIns tanceNam e[6]=SimIns tanceNam e[5]=SimIns tanceNam e[4]=SimIns tanceNam e[3]=SimIns tanceNam e[2]=SimIns tanceNam e[1]="Tran1"SweepVar="R load"
P ARAM ET ER SWEEP
VARVAR 1
R load=100X=1.0
EqnVar
TranTran1
MaxTimeS tep=1.2 nsecStopTim e=100. 0 nsec
TRANS IE NT
D CD C1
DC
CC b4C =4.7 pF
V _D CS RC1V dc=-1.5 V
CCb3C=4. 7 pF
di_sms_bas40_19930908D1
LL2
R =L=47.0 nH
CCb2C=10. 0 pF
RR1R=4700 Ohm
CCb1C=2. 2 pF
RRbR=47 kOhm
pb_phl_BFR92A_19921214Q1
RReR=220 O hm
LLc
R=L=220.0 nH
RRoutR=50 O hm
CC c2C =330. 0 pF
V_DCVccVdc=3.0 V
VtP WLVtrigV_Tran=pwl(t ime, 0ns , 0V, 1ns,0.01V, 2ns ,0V)
t
2-port network
Variablecapacitancetuning network
Initial noise source to startthe oscillation
April 2012 2006 by Fabian Kung Wai Lee 70
More on the Schematic
• L2 together with Cb3, Cb4 and the junction capacitance of D1 can produce a range of reactance value, from negative to positive. Together these components form the frequency determining network.
• Cb4 is optional, it is used to introduce a capacitive offset to the junction capacitance of D1.
• R1 is used to isolate the control voltage Vdc from the frequency determining network. It must be a high quality SMD resistor. The effectiveness of isolation can be improved by adding a RF choke in series with R1 and a shunt capacitor at the control voltage.
• Notice that the frequency determining network has no actual resistance to counter the effect of |R1(ω)|. This is provided by the loss resistance of L2 and the junction resistance of D1.
36
April 2012 2006 by Fabian Kung Wai Lee 71
Time Domain Result
0 10 20 30 40 50 60 70 80 90 100
-1.5
-1.0
-0.5
0.0
0.5
1.0
Vout when Vdc = -1.5V
April 2012 2006 by Fabian Kung Wai Lee 72
Load-Pull Experiment
100 200 300 400 500 600 700 800
1
2
3
4
5
• Peak-to-peak output voltage versus Rload for Vdc = -1.5V.
Vout(pp)
RLoad
37
April 2012 2006 by Fabian Kung Wai Lee 73
Vout
Controlling Harmonic Distortion (1)
• Since the resistance in the frequency determining network is too small, large amount of non-linearity is needed to limit the output voltage waveform, as shown below there is a lot of distortion.
April 2012 2006 by Fabian Kung Wai Lee 74
Controlling Harmonic Distortion (2)
• The distortion generates substantial amount of higher harmonics.
• This can be reduced by decreasing the positive feedback, by adding a small capacitance across the collector and base of transistor Q1. This is shown in the next slide.
38
April 2012 2006 by Fabian Kung Wai Lee 75
Controlling Harmonic Distortion (3)
Capacitor to control positive feedback
CCcbC=1.0 pF
RRLR=50 Ohm
RRoutR=50 Ohm
RReR=220 Ohm
LLc
R=L=220.0 nH
I_ProbeIC
pb_phl_BFR92A_19921214Q1
TranTran1
MaxTimeStep=1.2 nsecStopTime=280.0 nsec
TRANSIENT
DCDC1
DC
I_ProbeIload C
Cc2C=330.0 pF
LL2
R=L=47.0 nH
RRbR=47 kOhm
CCb1C=6.8 pF
CCb2C=10.0 pF
V_DCSRC1Vdc=0.5 V
CCb4C=0.7 pF
CCb3C=4.7 pF
di_sms_bas40_19930908D1
RR1R=4700 Ohm
V_DCVccVdc=3.0 V
VtPWLVtrigV_Tran=pwl(time, 0ns,0V, 1ns,0.01V, 2ns,0V)t
The observantperson wouldprobably noticethat we can alsoreduce the harmonicdistortion by introducinga series resistance inthe tuning network.However this is notadvisable as the phasenoise at the oscillator’soutput will increase (more about this later).
Control voltageVcontrol
April 2012 2006 by Fabian Kung Wai Lee 76
Controlling Harmonic Distortion (4)
• The output waveform Vout after this modification is shown below:
Vout
39
April 2012 2006 by Fabian Kung Wai Lee 77
Controlling Harmonic Distortion (5)
• Finally, it should be noted that we should also add a low-pass filter (LPF) at the output of the oscillator to suppress the higher harmonic components. Such LPF is usually called Harmonic Filter.
• Since the oscillator is operating in nonlinear mode, care must be taken in designing the LPF.
• Another practical design example will illustrate this approach.
April 2012 2006 by Fabian Kung Wai Lee 78
The Tuning Range
• Actual measurement is carried out, with the frequency measured using a high bandwidth digital storage oscilloscope.
0 0.5 1 1.5 2 2.5395
400
405
410
f
Vdc
MHz
Volts
D1 is BB149A,a varactormanufactured byPhillipsSemiconductor (Now NXP).
40
Phase Noise in Oscillator (1)
• Since the oscillator output is periodic. In frequency domain we would expect a series of harmonics.
• In a practical oscillation system, the instantaneous frequency and magnitude of oscillation are not constant. These will fluctuate as a function of time.
• These random fluctuations are noise, and in frequency domain the effect of the spectra will ‘smear out’.
April 2012 2006 by Fabian Kung Wai Lee 79
( ) ( )( ) ( )( )tttmVtv noisenoiseoosc θθω +++= cos
ffo 2fo 3foIdeal oscillator output
ffo 2fo 3fo
t
t
Real oscillator output
Smearing
April 2012 2006 by Fabian Kung Wai Lee 80
Phase Noise in Oscillator (2)
• Mathematically, we can say that the instantaneous frequency and magnitude of oscillation are not constant. These will fluctuate as a function of time.
• As a result, the output in the frequency domain is ‘smeared’ out.
t
v(t)
t
v(t)
ffo
ffo
T = 1/fo
Contains both phaseand amplitude modulationof the sinusoidal waveformat frequency fo
( )[ ]2
81log10
offset
o
L ff
QAFkT
PML ⋅⋅∝Leeson’s expression
Large phase noise
Small phase noise
41
April 2012 2006 by Fabian Kung Wai Lee 81
Phase Noise in Oscillator (3)
• Typically the magnitude fluctuation is small (or can be minimized) due to the oscillator nonlinear limiting process under steady-state.
• Thus the smearing is largely attributed to phase variation and is known as Phase Noise.
• Phase noise is measured with respect to the signal level at various offset frequencies.
• Phase noise is measured in dBc/Hz @ foffset. • dBc/Hz stands for dB downfrom the carrier (the ‘c’) in 1 Hz bandwidth.• For example -90dBc/Hz @ 100kHz offset from a CW sine wave at 2.4GHz.
- 90dBc/Hz
100kHz
ffo
t
v(t)
Signal level
Assume amplitude limiting effectOf the oscillator reduces amplitude fluctuation
( ) ( )( )ttVtv noiseoosc θθω ++≅ cos
April 2012 2006 by Fabian Kung Wai Lee 82
Reducing Phase Noise (1)
• Requirement 1 : The resonator network of an oscillator must have a high Q factor. This is an indication of low dissipation loss in the tuning network (See Chapter 3a – impedance transformation network on Q factor).
X1
Xtune
-X1
∆ff
2∆|X1|
TuningNetwork withHigh Q
X1
Xtune
-X1
∆ff
2∆|X1|
TuningNetwork withLow Q
Ztune = Rtune +jXtune
Variation in Xtune
due to environmentcauses small changein instantaneousfrequency.
42
April 2012 2006 by Fabian Kung Wai Lee 83
Reducing Phase Noise (2)
• A Q factor in the tuning network of at least 20 is needed for medium performance oscillator circuits at UHF. For highly stable oscillator, Q factor of the tuning network must be in excess or 1000.
• We have looked at LC tuning networks, which can give Q factor of up to 40. Ceramic resonator can provide Q factor greater than 500, while piezoelectric crystal can provide Q factor > 10000.
• At microwave frequency, the LC tuning networks can be substituted with transmission line sections.
• See R. W. Rhea, “Oscillator design & computer simulation”, 2nd edition 1995, McGraw-Hill, or the book by R.E. Collin for more discussions on Q factor.
• Requirement 2 : The power supply to the oscillator circuit should also be very stable to prevent unwanted amplitude modulation at the oscillator’s output.
Reducing Phase Noise (3)
• Requirement 3 : The voltage level of Vcontrol should be stable.
• Requirement 4 : The circuit has to be properly shielded from electromagnetic interference from other modules.
• Requirement 5 : Use low noise components in the construction of the oscillator, e.g. small resistance values, low-loss capacitors and inductors, low-loss PCB dielectric, use discrete components instead of integrated circuits.
April 2012 2006 by Fabian Kung Wai Lee 84
43
Example of Phase Noise from VCOs
• Comparison of two VCO outputs on a spectrum analyzer*.
April 2012 2006 by Fabian Kung Wai Lee 85
*The spectrumanalyzer internaloscillator mustof course hasa phase noise ofan order of magnitudelower than our VCOunder test.
VCO output with high phase noise VCO output
with low phase noise
More Materials
• This short discussion cannot do justice to the material on phase noise.
• For instance the mathematical model of phase noise in oscillator and the famous Leeson’s equation is not shown here. You can find further discussion in [4], and some material for further readings on this topic:
– D. Schere, “The art of phase noise measurement”, Hewlett Packard RF & Microwave Measurement Symposium, 1985.
– T. Lee, A. Hajimiri, “The design of low noise oscillators”, Kluwer, 1999.
April 2012 2006 by Fabian Kung Wai Lee 86
44
April 2012 2006 by Fabian Kung Wai Lee 87
More on Varactor
• The varactor diode is basically a PN junction optimized for its linear junction capacitance.
• It is always operated in the reverse-biased mode to prevent nonlinearity, which generate harmonics.
• As we increase the negativebiasing voltage Vj , Cj decreases, hence the oscillation frequency increases.• The abrupt junction varactor has highQ, but low sensitivity (e.g. Cj varieslittle over large voltage change).• The hyperabrupt junction varactorhas low Q, but higher sensitivity.
Vj
Vj0
Cj
Linear region
Reverse biased
Forward biasedCjo
Sep 2013 2006 by Fabian Kung Wai Lee 88
A Better Variable Capacitor Network
• The back-to-back varactors are commonly employed in a VCO circuit, so that at low Vcontrol, when one of the diode is being affected by the AC voltage, the other is still being reverse biased.
• When a diode is forward biased, the PN junction capacitance becomes nonlinear.
• The reverse biased diode has smaller junction capacitance, and this dominates the overall capacitance of the back-to-back varactor network.
• This configuration helps to decrease the harmonic distortion.
At any one time, at least one ofthe diode will be reverse biased.The junction capacitance of thereverse biased diode will dominatethe overall capacitance of thenetwork.
Vcontrol
Symbolfor Varactor
To suppressRF signals
To negativeresistanceamplifier
Vcontrol
Vcontrol+
+
To suppressRF signals
45
April 2012 2006 by Fabian Kung Wai Lee 89
Example 5.1 – VCO Design for Frequency Synthesizer
• To design a low power VCO that works from 810 MHz to 910 MHz.
• Power supply = 3.0V.
• Output power (into 50Ω load) minimum -3.0 dBm.
April 2012 2006 by Fabian Kung Wai Lee 90
Example 5.1 Cont…
• Checking the d.c. biasing and AC simulation.
S_ParamSP1
Step=1.0 MHzStop=1.0 GHzStart=0.7 GHz
S-PARAMETERS
DCDC1
DC
b82496c3120j000LCparam=SIMID 0603-C (12 nH +-5%)
4_7pF_NPO_0603Cc1
100pF_NPO_0603Cc2
2_2pF_NPO_0603C1
RRER=100 Ohm
3_3pF_NPO_0603C2
RRLR=100 Ohm
TermTerm1
Z=50 OhmNum=1
V_DCSRC1Vdc=3.3 V
RRBR=33 kOhm
pb_phl_BFR92A_19921214Q1
Z11
46
April 2012 2006 by Fabian Kung Wai Lee 91
Example 5.1 Cont…
• Checking the results – real and imaginary portion of Z1 when output is terminated with ZL = 100Ω.
m2freq=m2=-84.412
809.0MHzm1freq=m1=-89.579
775.0MHz
0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.980.70 1.00
-110
-100
-90
-80
-70
-60
-50
-120
-40
freq, GHz
real
(Z(1
,1))
m2
imag
(Z(1
,1))
m1
April 2012 2006 by Fabian Kung Wai Lee 92
Example 5.1 Cont…
• The resonator design.
Vvar
VARVAR1Vcontrol=0.2
EqnVar
CC3C=0.68 pF
LL1
R=L=10.0 nH
ParamSweepSweep1
Step=0.5Stop=3Start=0.0SimInstanceName[6]=SimInstanceName[5]=SimInstanceName[4]=SimInstanceName[3]=SimInstanceName[2]=SimInstanceName[1]="SP1"SweepVar="Vcontrol"
PARAMETER SWEEP
LL2
R=L=33.0 nH
100pF_NPO_0603C2
V_DCSRC1Vdc=Vcontrol V
S_ParamSP1
Step=1.0 MHzStop=1.0 GHzStart=0.7 GHz
S-PARAMETERS
BB833_SOD323D1
TermTerm1
Z=50 OhmNum=1
47
April 2012 2006 by Fabian Kung Wai Lee 93
Example 5.1 Cont…
• The resonator reactance.
m1freq=m1=64.725Vcontrol=0.000000
882.0MHz
0.75 0.80 0.85 0.90 0.950.70 1.00
20
40
60
80
100
0
120
freq, GHz
imag
(Z(1
,1)) m1
-ima
g(V
CO
_ac.
.Z(1
,1))
Resonatorreactanceas a function ofcontrol voltage
The theoretical tuningrange
-X1 of the destabilized amplifier
April 2012 2006 by Fabian Kung Wai Lee 94
Example 5.1 Cont…
• The complete schematic with the harmonic suppression filter.
Vvar
b82496c3120j000L3param=SIMID 0603-C (12 nH +-5%)
b82496c3100j000L1param=SIMID 0603-C (10 nH +-5%)
b82496c3330j000L2param=SIMID 0603-C (33 nH +-5%)
RR1R=100 Ohm
100pF_NPO_0603C4
b82496c3150j000L4param=SIMID 0603-C (15 nH +-5%)
0_47pF_NPO_0603C9
RRLR=100 Ohm2_7pF_NPO_0603
C8
100pF_NPO_0603Cc2
pb_phl_BFR92A_19921214Q1
TranTran1
MaxTimeStep=1.0 nsecStopT ime=1000.0 nsec
TRANSIENT
DCDC1
DC
CC7C=3.3 pF
CC6C=2.2 pF
V_DCSRC2Vdc=1.2 V
CC5C=0.68 pF
BB833_SOD323D1
VtPWLSrc_triggerV_Tran=pwl(time, 0ns,0V, 1ns,0.1V, 2ns,0V)
t
4_7pF_NPO_0603Cc1
RRER=100 Ohm
V_DCSRC1Vdc=3.3 V
RRBR=33 kOhm
Low-pass filter
48
April 2012 2006 by Fabian Kung Wai Lee 95
Example 5.1 Cont…
• The prototype and the result captured from a spectrum analyzer (9 kHz to 3 GHz).
VCOHarmonicsuppression filterFundamental
-1.5 dBm - 30 dBm
April 2012 2006 by Fabian Kung Wai Lee 96
Example 5.1 Cont…
• Examining the phase noise of the oscillator (of course the accuracy is limited by the stability of the spectrum analyzer used).
300Hz
Span = 500 kHzRBW = 300 HzVBW = 300 Hz
-0.42 dBm
49
April 2012 2006 by Fabian Kung Wai Lee 97
Example 5.1 Cont…
• VCO gain (ko) measurement setup:
Spectrum Analyzer
Vvar
PortVoutNum=2
PortVcontrolNum=1
RRcontrolR=1000 Ohm
RRattnR=50 Ohm
b82496c3120j000L3param=SIMID 0603-C (12 nH +-5%)
b82496c3100j000L1param=SIMID 0603-C (10 nH +-5%)
b82496c3150j000L4param=SIMID 0603-C (15 nH +-5%)
0_47pF_NPO_0603C9
2_7pF_NPO_0603C8
100pF_NPO_0603Cc2
pb_phl_BFR92A_19921214Q1
CC7C=3.3 pF
CC6C=2.2 pFC
C5C=0.68 pF
BB833_SOD323D1
4_7pF_NPO_0603Cc1
RRER=100 Ohm
V_DCSRC1Vdc=3.3 V
RRBR=33 kOhm
Variablepower supply
April 2012 2006 by Fabian Kung Wai Lee 98
Example 5.1 Cont…
• Measured results:
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
750
800
850
900
950
fVCO / MHz
Vcontrol/Volts
MHz/Volt 74.40Volt 35.1
MHz 55 =≅ok MHz/Volt 74.40Volt 35.1
MHz 55 =≅ok
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