Oscillation frequency

Basic Knowledge of Crystal Units
No. Item Title Contents
1 Outline Oscillating frequency Oscillating frequency is actual frequency of oscillation circuit which works with crystal.
It is mainly determined by cristal unit and influenced by MCU, external load capacitance, stray capacitance of PCB and so on.
2 Measurement Measurement method Oscillation frequency is calculated by below formula. fL: Load resonant frequency, fr: Resonant frequency, Cs: Load capacitance, C0, C1: Equivalent circuit contents
Formula
Measurement method 1
  1. Measurement equipment and device
    • Oscillation circuit
    • Frequency counter
    • Amplifier (or oscilloscope with output signal)
    • Probe
  2. Prepare measurement equipment as shown in figure below. Please expand the Y-axis range of oscilloscope as wide as possible.
  3. Bring probe to oscillation circuit as close as possible.
  4. Read frequency on frequency counter.
Note
  • Please do not contact probe to oscillation circuit. Actual oscillation frequency becomes low by input capacitance of probe.
  • Please pay attention that accuracy of frequency shown in oscilloscope would not be enough.
  • Please pay attention for sampling rate or number of averaging of degital oscilloscope due to quantization error.
Measurement method 2
  1. Measurement equipment and device
    • Oscillation circuit
    • Spectrum analyzer
    • Antenna (Coaxial cable could be used as antenna)
  2. Prepare measurement equipment as shown in figure below.
  3. Bring antenna to oscillation circuit as close as possible.
  4. Read peak frequency of response in spectrum analyzer.
Note
  • Please do not contact antenna to oscillation circuit.
  • Please measure response with narrow span of spectrum analyzer for accurate measurement.
3 Notes   It is important to measure frequency without any contact to oscillation circuit to prevent influence of probe and so on.
Oscillation frequency will shift if probe touches to oscillation circuit by its capacitance.

Oscillation circuit: Frequency measurement by counter


Oscillation circuit: Frequency measurement by spectrum analyzer

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