Which is better, active crystal oscillator or passive crystal oscillator?

Cecelia Qiu Cecelia Qiu 13 December 2021
Which is better, active crystal oscillator or passive crystal oscillator?

In the crystal industry, we call crystals passive crystals, crystal oscillators we call active crystal oscillators, passive crystals we usually call quartz crystal resonators, active crystal oscillators we call quartz crystal oscillators, these two products Used in electronic products such as telecommunications equipment and satellites. Equipment products, as reference signals and satellites. More stable time or frequency control. In recent years, crystal oscillator terminal technology has developed rapidly in clock modules, satellite communications, image processing, base station navigation, and especially in the clock module industry.

Which is better, active crystal oscillator or passive crystal oscillator?

1. What is an active crystal oscillator

The active crystal oscillator (scillator) has 4 pins and is a complete oscillator. In addition to the quartz crystal, there are transistors and resistance-capacitance components. Therefore, it is large in size and only needs a power supply to output a better waveform. The package of the active crystal oscillator has 4 pins, which are VCC (voltage), GND (ground), OUT (clock signal output), and NC (empty pin). The popular point of the active crystal oscillator is to directly connect the quartz wafer and the clock oscillator IC. The ideal effect can be achieved through the voltage control of the crystal oscillator itself, which can be matched with an external capacitor without causing frequency shift.

Many engineers will encounter such problems in the process of designing products: First, the matching of the crystal oscillator in the circuit is not ideal, which affects the use effect; Second, the temperature drift of the crystal oscillator is too large, and even affects the product performance.

Nowadays, the upgrading of electronic products is particularly frequent. Cost must be an important factor considered by the manufacturer. The application of crystal oscillator also considers the cost. Therefore, when engineers design circuits, the price of quartz crystal oscillators (active crystal oscillators) is 5-10 times higher than ordinary passive crystals, which leads to more. The circuit uses passive crystal oscillators; only some high-end products such as industrial control and high-speed communication products prefer active crystal oscillators, so the above-mentioned common problems occur. The reason is that the effect of the passive crystal oscillator not only depends on the index of the crystal oscillator itself, but also depends on the matching of the oscillator circuit design.

Active crystal oscillators don't need to worry about this problem, because many passive crystal oscillators cannot be matched because of external capacitors. The active crystal oscillator has voltage control performance and can automatically match the external capacitance through the internal IC wafer. The second point is the excessive temperature drift of the crystal oscillator, commonly known as temperature drift. This phenomenon is very fatal. If the product is in normal working condition, the frequency drift caused by temperature change suddenly becomes too large, the product will stop working seriously and be damaged. The passive crystal itself is a crystal wafer and has no temperature compensation function. There is a temperature-compensated crystal in the active crystal.

The active crystal oscillator does not need the internal oscillator of the CPU, the signal is stable, the quality is better, and the connection method is relatively simple (mainly do a good job of power filtering, usually a capacitor and an inductor are used to form a filter network, and the output end is filtered by a small resistance resistor Signal is sufficient), no complicated configuration circuit is required.

2. What is a passive crystal oscillator

Passive crystal oscillator generally refers to quartz crystal resonator, quartz crystal resonator (English: quartz crystal unit or quartz crystal resonator, often marked as Xtal, Extenal Crystal Osillator, external crystal oscillator, because the crystal unit is often used as an external circuit), referred to as quartz Crystal or crystal oscillator is an electronic component that uses the piezoelectric effect of quartz crystal (also known as crystal) to generate high-precision oscillation frequency, and is a passive component. The component is mainly composed of quartz wafer, base, shell, silver glue, silver and other components. According to the lead status, it can be divided into two types: in-line (with leads) and surface mount (without leads). Currently, the main package types commonly used are HC-49U, HC-49/S, GLASS, UM-1, UM-4, UM-5 and SMD.

Passive crystal oscillator is a non-polar component with 2 pins. It needs the help of a clock circuit to generate an oscillating signal, and it cannot oscillate by itself. Passive crystal oscillators have poor signal quality and usually require precise matching of peripheral circuits (capacitors, inductances, resistances for signal matching, etc.). When replacing crystals with different frequencies, the peripheral configuration circuits also need to be adjusted accordingly. Generally, it is recommended to use high-precision quartz crystals, as far as possible not to use low-precision ceramic crystals.

3. The difference between active crystal oscillator and passive crystal oscillator

3.1 signal quality

Active crystal oscillator does not require the internal oscillator of DSP, the signal quality is good, relatively stable, and the connection method is relatively simple (mainly to do a good job of power filtering, usually a PI type filter network composed of a capacitor and an inductor, and a small resistor at the output The value of the resistance can filter the signal), and no complicated configuration circuit is required. Passive crystal oscillators need to use the on-chip oscillator of DSP. Passive crystal oscillators have no voltage problems, and the signal level is variable, that is to say, it is determined according to the oscillator circuit.

The same crystal can be applied to a variety of voltages, and can be used in a variety of DSPs with different clock signal voltage requirements. Therefore, for general applications, if conditions permit, it is recommended to use a crystal. This is especially suitable for producers with abundant product lines and large quantities. The usual connection method of active crystal oscillator: 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 crystals is that their signal level is fixed, and the appropriate output level needs to be selected, which is less flexible. It is recommended to use high-precision quartz crystals, and avoid using low-precision ceramics as much as possible.

3.2 Price

Active crystal oscillators are more expensive, but the active crystal oscillator itself can vibrate. Regardless of whether it is a passive crystal oscillator or an active crystal oscillator, it has its own advantages and disadvantages. If the cost of the product is considered, it is recommended to choose a passive crystal oscillator circuit; if the product performance is considered, it is recommended to choose an active crystal oscillator circuit, which saves time and convenience. Can guarantee product performance.

3.3 accuracy

The highest accuracy of passive crystal oscillator is 5ppm, while the accuracy of active crystal oscillator can reach 0.1ppm. The higher the accuracy, the better the frequency stability. Active crystal oscillator is better than passive crystal oscillator in terms of stability, but it also has its own small defects. The signal level of active crystal oscillator is fixed, so it is necessary to choose a suitable output level, which is less flexible.

3.4 Timing requirements

For applications with sensitive timing requirements, I personally think that an active crystal is better, because you can choose a more precise crystal, or even a high-end temperature compensation crystal. Some DSPs have no internal oscillator circuit and can only use active crystal oscillators, such as TI's 6000 series. Compared with passive crystals, active crystals are usually larger in volume, but now many active crystals are surface-mounted, and the volume is equivalent to crystals, and some are even smaller than many crystals.

3.5 Shape

Active crystal oscillators generally have 4 pins, one power supply, one grounding, one signal output terminal, and one NC (empty pin). The one marked with a dot is pin 1, and counterclockwise (pin down) is 2, 3, and 4 respectively. Passive crystal oscillator has 2 pins, and it needs the help of an external clock circuit (connected to the oscillation circuit inside the main IC) to generate an oscillating signal, and it cannot oscillate by itself.

3.6 Voltage and load

The active crystal oscillator has voltage without load, and the passive crystal oscillator has load without voltage.    (Summarized by Easybom)

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