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Phase Noise and Oscillators. Stephen Powell. What is an Oscillator?. Produces a signal at a particular frequency They are everywhere! watches, radios, computers, in most electronic circuits Uses? Generates signals for transmission Frequency translation Provides timing references.

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Presentation Transcript
what is an oscillator
What is an Oscillator?
  • Produces a signal at a particular frequency
  • They are everywhere!
    • watches, radios, computers, in most electronic circuits
  • Uses?
    • Generates signals for transmission
    • Frequency translation
    • Provides timing references
how to build an oscillator
How to build an Oscillator!
  • Need a feedback loop
  • Oscillates at frequency ω0 when

1-A(jω0)B(jω0) = 0 (Barkhausen criteria)

  • Unstable device!
simple lcr oscillator
L,C,R forms an impedance “tank”

Impedance has same form as the loop equation

Voltage across tank oscillates

Simple LCR Oscillator
effect of resistance
Effect of Resistance

Without R

With R

phase noise
Phase Noise
  • Oscillator output: V(t) = C·sin(ω0t+θ(t))
  • Suppose θ(t) = θsin(pt) model one component of white noise… then:
origins of phase noise
Origins of Phase Noise
  • Three types of contributing noise:
    • Flicker Noise: power inversely proportional to frequency, AKA 1/f noise
    • Shot Noise: due to random charge carriers, proportional to current
    • Thermal Noise: present in all resistors, wide band, AKA Johnson noise
aggregate phase noise
Aggregate Phase Noise
  • All three types together…

ω1/f3

log(L(∆ω))

ω0/2Q

Shot Noise

Thermal Noise

Flicker Noise

log(∆ω)

modified leeson model
Modified Leeson Model
  • Tries to account for traits seen in spectrum
  • Loosely based on principle of time-invariance
  • F, ω1/f3 are empirically calculated
    • Not good for prediction!
lee model
Lee Model
  • Based on time-variance
  • No empirical variables
  • Corresponds well to observations
lee model continued
Lee Model (continued)…

Region of 1/f3

(Flicker Noise)

Region of 1/f2

(Shot Noise)

effects of phase noise
Effects of Phase Noise
  • In the time domain: Timing Jitter
    • bad if you want to synchronize a signal, or sample a signal
slide13

Effects of Phase Noise…

  • In the frequency domain: Reciprocal Mixing
    • bad for interference immunity
so what did i learn again
So, what did I learn again?
  • Oscillators are unstable (but very important!!) devices
  • Phase noise is bad
    • Spreads the frequency spectrum
    • Causes timing jitter
  • Two models can be used to provide insight in reducing phase noise