Ceramic Resonators (CERALOCK) FAQQPlease explain possible causes of oscillation stop, and appropriate countermeasures.

If oscillation stop occurs, or oscillation is disabled, you must examine the cause of the problem, such as malfunction of the CERALOCK, mismatching between CERALOCK and IC or reset program error. However, if the IC is not operating as an amplifier, oscillation is disabled by any means. Therefore, you must perform IC operation check first. The IC operation can be checked simply by connecting the IC's input terminal to a power supply or ground to confirm whether the output signal is inverted. If the IC serves as an inverter, the output signal must be inverted.

When oscillation is disabled even if the IC serves as an inverter, the CERALOCK (oscillation circuit) must have a problem. Improper circuit constants, or malfunction of the CERALOCK can be considered as the cause of this problem. If the circuit constants are proper, the circuit will develop normal oscillation even if the CERALOCK electrical characteristics become unstable within the ratings (for example, the resonance resistance (R1) may be slightly increased). If the circuit constants are not proper, oscillation may stop due to the fluctuations in CERALOCK electrical characteristics.

The oscillation stability relative to fluctuations in CERALOCK electrical characteristics in an oscillation circuit can be referred to as oscillation excess gain. The oscillation excess gain is correlated with "R1" of the CERALOCK: When "R1" is increased, the oscillation excess gain will be reduced. To check the oscillation stability, measure the characteristics of a single CERALOCK by using an impedance analyzer or network analyzer. In comparison between a defective CERALOCK and a normal one, you can see some difference in their characteristics. If oscillation stop occurs during a temperature change, check their characteristics by changing the operating temperature with a dryer or freezer.

If oscillation stop occurs with the normal CERALOCK, or at a high probability, the circuit constants may be improper. In this case, you must check the circuit constants, because circuit constants may significantly vary depending on the IC being used. Among the circuit constants, load capacitance "CL" (CL1/CL2) is the most important parameter. If "CL" is too high, the circuit gain will be reduced: If "CL" is too small, the circuit cannot provide proper phase difference, which also causes oscillation excess gain to be reduced. To check this parameter, measure the amplitude characteristic of load capacitance. For this purpose, change "CL" (CL1=CL2) from the standard value (e.g. capacitance of 3-terminal product) in the range of 1/10 to 10 times, and observe the change in input (V1) amplitude. When the proper load capacitance is provided, the largest amplitude must be observed.

On the other hand, if feedback resistance (Rf) is too large, feedback is disabled when the PCB's insulation resistance is reduced due to any cause, resulting in oscillation stop. This problem is caused by unstable DC bias. To examine the cause of this problem, measure the bias voltage with a probe connected to the IC's output terminal after removing the CERALOCK, and you will find that the bias voltage is nearly equal to VDD, although it must be VDD/2 in normal conditions.

You can solve this problem by adding an external feedback resistor of approx. 1M ohms to the oscillation circuit.

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