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Frequency Stability

FCC standards for FM Broadcast (p 337) Allocation: 88.0 – 108.0 Mhz Channel Spacing/Width: 200 khz Carrier Stabilility: + 2 khz = 20 ppm. Frequency Stability. Temperature Stability.

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Frequency Stability

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  1. FCC standards for FM Broadcast (p 337) Allocation: 88.0 – 108.0 Mhz Channel Spacing/Width: 200 khz Carrier Stabilility: + 2 khz = 20 ppm Frequency Stability

  2. Temperature Stability Example: Nominal frequency is 21.400 Mhz at 25oC. Measured frequency at 45oC is 21.407 Mhz. Df = 7 khz, DT = 20oC. Estimate the frequency at -15oC f(T) = fnom+TCfnom(T-Tnom) = 21.4 + Df = 21.4 + (16.35 x 10-6)(21.4)(-15 - 25) = 21.386 Mhz

  3. Improving Temperature Stability • Control Temp coefficient of components. • Neutralize or swamp out device variations. • Low Power Operation (benign environment) • Active TC compensation/tracking • Control/reduce circuit noise • Buffer output to isolate load variations • Thermal isolation/oven. • Use a crystal

  4. Quartz Crystal Structure Y Y _ _ + + _ _ + + + _ _ + + _ _ + _ _ _ + _ + + + _ _ _ + + _ _ _ + + + + - + _ _ _ X X + + + _ _ + _ + + _ _ _ _ + + + _ _ + + + _ _ + _ + _ + + _ _ + + _ _ + + _ _ + + _ _ + + Undeformed lattice Strained lattice Quartz Crystal

  5. Piezoelectric Effect External Field Internal Field

  6. Crystal Equivalent Circuit Neutralization of Case Capacitance for Series Operation Ln C0 2 – 10 pf Lx 1 – 10 h Cx ~ 10-15 f Rx 50 – 200 W

  7. Crystal Resonances X Reactance of Crystal Blank Parallel Resonance with case capacitance -XC w Series Resonance

  8. Crystal Resonances Reactance of Crystal Blank with Package Capacitance X Very steep slope (large dynamic inductance) Overtone Operation Parallel Resonance with case capacitance Series Resonance w Very small frequency difference. Tank Circuit is tuned to be resonant at desired overtone.

  9. Application to Oscillators(series mode) Hartley Colpitts C Rx C E E

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