The difference and usage of passive crystal and active crystal oscillator

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The difference and usage of passive crystal and active crystal oscillator [Copy link]

The difference and usage of passive crystal and active crystal oscillator :


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1. Passive crystal - Passive crystal needs to use the internal oscillator of the DSP chip, and the datasheet has a recommended connection method. Passive crystals do not have voltage issues, and the signal level is variable, that is, it is determined by the oscillation circuit. The same crystal can be applied to a variety of voltages and can be used for a variety of DSPs with different clock signal voltage requirements. The price is usually lower, so for general applications, it is recommended to use crystals if conditions permit. This is especially suitable for manufacturers with rich product lines and large batches.

The disadvantage of passive crystals compared to crystal oscillators is that the signal quality is poor, and usually requires precise matching of peripheral circuits (capacitors, inductors, resistors, etc. for signal matching). When replacing crystals of different frequencies, the peripheral configuration circuits need to be adjusted accordingly. It is recommended to use a quartz crystal with higher precision, and avoid using a ceramic crystal with lower precision as much as possible .

2. Active crystal oscillator - Active crystal oscillator does not require the internal oscillator of the DSP chip , has good signal quality, is relatively stable, and has a relatively simple connection method (mainly to do power supply filtering, usually using a PI-type filter network composed of a capacitor and an inductor, and a small-resistance resistor is used at the output end to filter the signal), and does not require a complex configuration circuit.

The common usage of active crystal oscillators is: one pin is suspended, two pins are grounded, three pins are connected to output, and four pins are connected to voltage. Compared with passive crystals, the disadvantage of active crystal oscillators is that their signal level is fixed, and the appropriate output level needs to be selected. It has poor flexibility and is expensive.

For applications with sensitive timing requirements, I personally think that active crystal oscillators are better, because you can choose more precise crystal oscillators, or even high-end temperature compensated crystal oscillators. Some DSPs do not have internal oscillator circuits, so they can only use active crystal oscillators, such as TI's 6000 series. Active crystal oscillators are usually larger than passive crystals, but now many active crystal oscillators are surface-mounted, and their size is comparable to that of crystals, and some are even smaller than many crystals.

Note:

---- Total frequency error: The maximum frequency error between the crystal oscillator frequency and the given nominal frequency caused by all combinations of specified working and non-working parameters within a specified time.

---- Frequency temperature stability: The maximum allowable frequency deviation without or with an implied reference temperature when operating within the specified temperature range under nominal power and load.

---- Frequency stabilization warm-up time: Based on the stable output frequency of the crystal oscillator, the time required from power-on to the output frequency being less than the specified frequency tolerance.

---- Frequency aging rate: The relationship between the oscillator frequency and time when the oscillator frequency is measured under constant environmental conditions. This long-term frequency drift is caused by slow changes in the crystal components and oscillator circuit components, and can be expressed as the maximum rate of change after a specified time limit (such as ±10ppb/day, 72 hours after power-up), or the maximum total frequency change within a specified time limit (such as: ±1ppm/(first year) and ±5ppm/(ten years)).

---- Frequency voltage control range: The minimum peak change in the crystal oscillator frequency when the frequency control voltage is adjusted from the reference voltage to the specified endpoint voltage.