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E3 237 Integrated Circuits for Wireless Communication
Lecture 14: Oscillators
Gaurab BanerjeeDepartment of Electrical Communication
Engineering,
Indian Institute of Science,
[email protected]
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Outline
• Basic Concepts
• Feedback Based
• One Port View
• Ring Oscillators
• Basic Oscillator Topology
• Voltage Controlled Oscillators• Voltage Controlled
Oscillators
• Negative gm Oscillators
• Phase Noise
• LO Distribution
• Quadrature generation
• SSB Generation
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Oscillators
Used in RF transceivers for upconversion and downconversion
mixing. Used in phase locked loops (PLLs) for frequency synthesis
and channel selection.
• Evolution of VCOs: Free running oscillators cannot be used in
transceivers -> feedback loop required to stabilize
frequency.
• Loop dynamics and noise requires specific channel spacing
• Finer spacing obtained by synthesis schemes (e.g. ΣΣΣΣ−−−−∆∆∆∆
fractional-N synthesis)
• In order of complexity: Oscillator -> VCO-> PLL ->
ΣΣΣΣ−−−−∆∆∆∆ fractional-N synthesizer
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Feedback Oscillator
• Barkhausen’s criteria for steady state oscillations at w0:• |
H(jw0) | =1 (loop gain = +1)
Self sustaining at s0 if H(s0) = +1.
• | H(jw0) | =1 (loop gain = +1)
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Feedback Oscillator• Nonlinearity of amplifier stabilizes the
amplitude of oscillation.
• Startup requires gain >1 (usually more)
• At steady state, the amplifier saturates with low gain at
peaks (large signal) so that the average loop gain is 1.
• Sometimes results in distorted waveforms.
• Automatic level control for linear oscillators
• Not very popular because of extra circuits and noise
• Most practical amplifiers will do this anyway (large signal
gain compression or saturation)
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1-Port Oscillator
• Energy lost in Rp during every cycle replenished by the
negative resistance in the active circuit, i.e, R1 = -Rp
• Popular in microwave oscillator design.
• Usually, the same oscillator can be described by a 1-port or a
2-port approach
• Does R1 = -Rp imply that the resistive noise is cancelled?
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Ring Oscillator
• Odd number of invertors in single ended implementation,
differential implementations can have even number of stages with
“flipped” outputs.
• DC feedback is negative
• The time period of oscillations is determined by the delays of
individual stages.
• “Analog” view: Three identical cascaded amplifiers in
feedback, oscillation frequency is where individual stage phase
shift = 60o
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Outline
• Basic Concepts
• Feedback Based
• One Port View
• Ring Oscillators
• Basic Oscillator Topology
• Voltage Controlled Oscillators• Voltage Controlled
Oscillators
• Negative gm Oscillators
• Phase Noise
• LO Distribution
• Quadrature generation
• SSB Generation
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Basic Oscillators – One Port View
At drain:
At gate:
Also,
=>
=> Negative impedance!
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Basic Oscillators – One Port View
•The circuit can oscillate if an inductor is placed between the
gate and the drain.
•Ground one of the three terminals -> signal ground, does not
have to be DC ground.
Drain grounded Gate grounded Source grounded
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Basic Oscillator Using Positive Feedback
Drain-Gate feedback Drain-Source feedback
• At resonance, the tank is resistive -> current and voltage
are in phase.
• The drain voltage can be fed back to the gate or source ->
depends on polarity.
• Resistive loading of source (1/gm1), lowers the tank Q ->
Use a transformer!
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Tapped capacitor/inductor transformers
RL
RIN
C1
C2
RL
RIN
L1
L2
RL
RL
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Transformer Based Oscillators
Less popular -> Two inductors and
large CB
Tapped Capacitors : Colpitts Oscillator Tapped Inductors :
Hartley Oscillator
