Summarize two questions about crystal oscillators

Problem with parallel resistors

In some solutions, when the crystal oscillator is connected in parallel with a 1MΩ resistor, the program runs normally, but without the 1MΩ resistor, the program runs lagging and cannot run.

Cause Analysis:

In passive crystal oscillator applications, two external capacitors can fine-tune the clock frequency generated by the crystal oscillator. The parallel 1MΩ resistor can help the crystal oscillator start oscillating. Therefore, when the program starts slowly or does not run, it is recommended to connect a 1MΩ resistor in parallel to the crystal oscillator.

This 1MΩ resistor is to make the device, which is originally a logic inverter, work in the linear region to obtain gain. There is no gain in the saturation region, and the crystal oscillator cannot oscillate without gain. In short, connecting the 1M resistor in parallel increases the negative impedance (-R) in the circuit, which increases the gain and shortens the crystal oscillator start-up time, making it easier for the crystal oscillator to start oscillate.

In other words, assuming there is no disturbance signal in the circuit, the crystal oscillator cannot start to oscillate. In fact, many circuits in inverting gate circuits can start to oscillate without adding this resistor, because general circuits have disturbance signals, but some inverting gate circuits cannot start to oscillate without adding this resistor, because the disturbance signal strength is not enough.

It should be pointed out that the impedance of the oscillation circuit will also change in a low-temperature environment. When the impedance increases to a certain level, the crystal oscillator will have difficulty starting or not vibrating. At this time, we also need to connect a 1MΩ resistor in parallel to the crystal oscillator. It is recommended that in order to increase the stability of the oscillation circuit, a 100Ω resistor is connected in series to the crystal oscillator, which can reduce the frequency deviation of the crystal oscillator.

Note: The parallel resistance cannot be too small, and the series resistance cannot be too large. Otherwise, it will not be easy to vibrate when the temperature is low.

Problems with load capacitance and external capacitance

It is often encountered that people confuse the load capacitance of the crystal oscillator with the external capacitance, and some even mistakenly think that these refer to the same parameter. What needs to be pointed out here is: if you think this way, you are totally wrong.

The following is an analysis and differentiation for you:

Load capacitance refers to an important internal electrical parameter of the crystal oscillator. Under normal circumstances, the common crystal load capacitances on the PCBA of electronic equipment that are not very sensitive to power consumption are 15PF, 18PF, and 20PF.

Then, for electronic products such as watches, mobile phones, and Bluetooth headsets that have obviously high demand for low power consumption, crystal oscillators with smaller load capacitance are often used on PCBA, such as 6PF, 7PF, 9PF, 10PF, and 12PF.

The load capacitance of the crystal oscillator has been locked through the processing technology according to the needs during the production process and cannot be changed during the application.

The external capacitor of the crystal oscillator refers to the electronic component on the PCBA board that is connected in series with the crystal oscillator frequency input pin and output pin respectively. The value of the external capacitor is determined by the crystal load capacitance and the circuit board stray capacitance (including IC capacitance), and is usually the sum of the two.

In oscillator circuit applications, the relationship between the crystal oscillator load capacitance, stray capacitance and external capacitance is as follows:

picture

CL: Load capacitance of quartz crystal resonator.

CS: refers to stray capacitance, including the stray capacitance value inside the IC, the capacitance between the circuit board wiring, the parasitic capacitance between the layers of the PCB board, etc.

C1 and C2: respectively refer to the two external capacitors of the quartz crystal resonator in circuit applications.

There are two application purposes of external capacitors:

Fine-tune the crystal oscillator frequency so that it is as close to the target frequency as possible. Rule: The larger the external capacitor is, the more negative the crystal oscillator output frequency is. On the contrary, the smaller the external capacitor is, the crystal oscillator output frequency tends to change in a positive direction.

An external capacitor can stabilize the oscillation circuit. This is why it is recommended to add a capacitor of the same value to the crystal oscillator frequency input pin and output pin respectively.

※Finally, two points need to be reminded:

1. The role of the external capacitor is only to fine-tune the crystal oscillator frequency. If the frequency deviation is too large when the crystal oscillator is working, it is necessary to consider the reasons for the accuracy of the crystal oscillator itself, such as whether the crystal oscillator accuracy cannot meet the chip requirements and be changed to a higher-precision crystal oscillator.

2. External capacitors are only used for passive crystal oscillator applications. In the circuit application of active crystal oscillator, no external capacitor is required.