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Phase Noise and Oscillators

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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Phase Noise and Oscillators

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  1. Phase Noise and Oscillators Stephen Powell

  2. 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

  3. How to build an Oscillator! • Need a feedback loop • Oscillates at frequency ω0 when 1-A(jω0)B(jω0) = 0 (Barkhausen criteria) • Unstable device!

  4. L,C,R forms an impedance “tank” Impedance has same form as the loop equation Voltage across tank oscillates Simple LCR Oscillator

  5. Effect of Resistance Without R With R

  6. Phase Noise • Oscillator output: V(t) = C·sin(ω0t+θ(t)) • Suppose θ(t) = θsin(pt) model one component of white noise… then:

  7. 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

  8. Aggregate Phase Noise • All three types together… ω1/f3 log(L(∆ω)) ω0/2Q Shot Noise Thermal Noise Flicker Noise log(∆ω)

  9. 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!

  10. Lee Model • Based on time-variance • No empirical variables • Corresponds well to observations

  11. Lee Model (continued)… Region of 1/f3 (Flicker Noise) Region of 1/f2 (Shot Noise)

  12. Effects of Phase Noise • In the time domain: Timing Jitter • bad if you want to synchronize a signal, or sample a signal

  13. Effects of Phase Noise… • In the frequency domain: Reciprocal Mixing • bad for interference immunity

  14. 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

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