Crystal Units FAQ QWhat are the causes and countermeasures for deviation in the oscillation frequency of a crystal unit?

The following three factors can be considered as the main causes when the oscillation frequency of a crystal unit deviates from the nominal value.

• Drive level exceeds the specification value.
• Load capacitance mismatch
• Abnormal oscillation

These factors occur due to mismatches between circuit conditions and crystal unit specifications, which can be improved by optimizing the circuit constants and component selection.

If the oscillation frequency deviates, isolate the causes more easily by checking in the following order.

1. Measure the oscillation frequency and confirm the difference with the nominal frequency.
2. Check if the drive level is within the specification range.
3. Check if the load capacitance of the circuit matches the load capacitance of the crystal unit specification.
4. Check the oscillation waveform to confirm if an abnormal oscillation is present.

<Cause (1): Drive level exceeds the specification value>

The power consumed by the oscillator is greater than the standard value.
If the drive level is too high, the oscillation frequency may shift higher or the equivalent series resistance (R1) may increase, which can cause a frequency deviation to occur.

Main countermeasures

• It is recommended that you reduce the drive level by increasing the value of the limiting resistor. In this case, the oscillation margin will decrease at the same time, so make sure that the oscillation margin does not fall below 5x for consumer applications and 10x for automotive applications. Furthermore, since the oscillation amplitude will also decrease, be sure to prevent the amplitude from becoming extremely small.
• It is recommended that you increase the impedance of the oscillation circuit by adjusting the load capacitance to a lower value to reduce the circuit current. In this case, the oscillation frequency increases as the load capacitance of the oscillation circuit decreases. It is recommended that you confirm if the oscillation frequency falls within the desired frequency range after changing the load capacitance.
• For the measurement method of the drive level, refer to “Basic Knowledge of How to Measure the Drive Level”

<Cause (2): Mismatch between the load capacitance on the circuit side and the specification value of the crystal unit>

The actual load capacitance of the oscillation circuit differs from the value in the crystal unit specification.
Since the frequency of the crystal unit is specified for a given load capacitance, the oscillation frequency deviates due to the mismatch in the circuit capacitance.

Main countermeasures

• Change the load capacitance to adjust the actual oscillation frequency. If you wish to lower the actual oscillation frequency, increase the capacitance value and vice versa. Note that increasing the load capacitance decreases the oscillation margin. In addition, be sure to note that the oscillation amplitude may decrease due to a drop in the oscillation circuit impedance.
• It is recommended that you change to a crystal unit with a different load capacitance. If the crystal unit is changed to one with a load capacitance higher than that specified in the specifications, the oscillation frequency increases. Conversely, changing to a crystal unit with a smaller load capacitance decreases the oscillation frequency.

Ex.) With the goal of setting the oscillation frequency to 30 MHz, a crystal unit with a nominal frequency of 30 MHz and a load capacitance of 6 pF was installed, at which point the actual oscillation frequency was 30 ppm lower than 30 MHz. Based on this result, it was believed that the actual load capacitance of the oscillation circuit was greater than 6 pF, and the crystal unit was changed to one with a load capacitance of 8 pF. As a result, the oscillation frequency was approximately 5 ppm lower relative to 30 MHz, which improved the deviation of the oscillation frequency.

<Cause (3): Oscillation is not normal (abnormal oscillation)>

The crystal unit is oscillating at a frequency other than the original resonance frequency.
Depending on the microcontroller inverter characteristics and circuit conditions, modes other than the fundamental wave of the crystal unit or circuit-induced oscillations may occur, which causes the oscillation frequency to deviate from the standard value.

Main countermeasures

• Check if the inverter of the microcontroller is an unbuffered type. Depending on the microcontroller characteristics, such as when the built-in inverter is not an unbuffered type, it may not oscillate near the crystal unit’s nominal frequency regardless of whether the crystal unit is oscillating.
• Change to a microcontroller with the appropriate inverter configuration as needed. Fundamentally, measures such as changing the built-in inverter of the microcontroller to an unbuffered type must be taken on the microcontroller side.
• In some cases, the limiting resistance or load capacitance may be adjusted to reduce the frequency of abnormal oscillations.

• What is the drive level?

  Drive level indicates the power consumed by the crystal unit.
  If the drive level is too high, it may cause changes in the frequency characteristics.
  For drive level measurement methods, refer to “Basic Knowledge of How to Measure the Drive Level”

• What is load capacitance?

  It is the capacitive component connected to the oscillation circuit. The load capacitance impacts the oscillation frequency and stability (oscillation margin) of the oscillation circuit.
  For the load capacitance, refer to “Crystal Units: Basic Knowledge of Technical Terms.”

• What is abnormal oscillation?

  It is a phenomenon whereby the oscillation circuit oscillates in a mode other than the original resonance mode (fundamental wave) of the crystal unit, which can cause frequency deviations or malfunctions.

• What is an unbuffered type inverter?

  It is a type of inverter that uses only one C-MOS.
  The unbuffered type is suitable for oscillation circuits using a C-MOS inverter.
  Multiple C-MOS components connected in multiple ways to form a single inverter is called a “buffered type” of inverter.
  Buffered types and Schmitt trigger types may oscillate regardless of the resonance characteristics of the crystal unit, which makes them unsuitable for oscillation circuits.

• During the design process
  It serves as a reference for considering the necessary circuit conditions and component constants when designing an oscillation circuit.

• During prototyping and evaluation
  It can be used as a confirmation item or evaluation procedure when oscillation does not occur in a prototype circuit.

• Troubleshooting
  It is helpful for isolating the cause when a failure to oscillate or oscillation stoppage occurs.

• Mass production defects
  It can be used for analyzing the causes and preventing a recurrence of oscillation failures and unstable operation occurring in mass-produced goods.