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The difference, application scope and usage of passive crystal and active crystal oscillator
2018-08-23 09:18:45
1. Passive Crystals - Passive Crystals require on-chip oscillators in DSP, and there is a suggested connection method on the datasheet. Passive crystals have no voltage problem, the signal level is variable, that is, according to the starting circuit to determine, the same crystal can be applied to a variety of voltage, can be used for a variety of different clock signal voltage requirements of DSP, and the price is usually low, so for general applications if conditions permit crystal is recommended. This is especially suitable for producers with large number of products and large quantities. The defect of passive crystal relative to crystal oscillator is that the signal quality is poor, usually need to match the precise peripheral circuit (for signal matching capacitance, inductance, resistance, etc. to replace different frequency crystal peripheral configuration circuit needs to be adjusted accordingly. It is suggested that quartz crystals with high accuracy should be used as far as possible so as to avoid using low precision ceramic vigilance.
2. Active crystal oscillator-Active crystal oscillator does not need DSP internal oscillator, signal quality is good, more stable, and the connection is relatively simple (mainly to do a good power filter, usually using a capacitor and inductance of the PI filter network, output with a small resistance filter signal can), do not need complex The configuration circuit. The usual usage of active crystal oscillator is: one foot is suspended, two feet are grounded, three feet are connected, and four feet are connected. Compared with passive crystal, the defect of active crystal oscillator is that the signal level is fixed, and the appropriate output level needs to be ed. The flexibility is poor, and the price is high. For time-sensitive applications, I think it is better to have an active crystal oscillator, because you can choose a more sophisticated crystal oscillator, or even high-grade temperature compensation crystal oscillator. Some DSP have no vibration circuit inside, and can only use active crystal oscillator, such as TI's 6000 series. Active crystal oscillator is usually larger than passive crystal, but now many active crystal oscillator is surface-mounted, the volume and crystal equivalent, and some even smaller than many crystals.
Several points for attention:
1, the frequency doubling DSP needs to be configured with PLL peripheral configuration circuit, mainly isolation and filtering.
Crystal oscillators below 2 and 20 MHz are basically fundamental devices with good stability. Most of the oscillators above 20 MHz are harmonic (such as third harmonic, fifth harmonic, etc.) and have poor stability. Therefore, it is strongly recommended to use low-frequency devices. After all, the peripheral configurations of PLL circuits for frequency doubling are mainly capacitors, resistors and inductors, and their stability is poor. Calibration and price are much better than crystal oscillator devices.
3. Clock signal path length as short as possible, line width as large as possible, and other printed lines as far as possible, close to the device ** wiring, if necessary, can go through the inner layer, and surrounded by ground wire;
4. There are special design requirements when the clock signal is introduced from the outside through the backplane, and the relevant information needs to be referred in detail. ?
In addition, we should make some explanations.
Generally speaking, the stability of crystal oscillator is better than that of crystal, especially in the field of precision measurement. Most of them use high-grade crystal oscillator, which can integrate various compensation techniques and reduce the complexity of design. Imagine the complexity of the design if you use a crystal and then design your own waveform shaping, anti-jamming, and temperature compensation. We here design RF circuit and other high clock requirements occasions, is the use of high-precision temperature compensation crystal oscillator, industrial-grade several hundred yuan a.
If you can't find a suitable crystal oscillator for special applications, that is, if the design complexity exceeds the level of the finished crystal oscillator on the market, you have to design your own. In this case, you have to choose crystals, but these crystals are certainly not ordinary crystals on the market, but special high-end crystals, such as ruby crystals. Wait.
1. The more demanding areas are more special. The clocks we use here are even atomic clocks, rubidium clocks and other equipment used in high-precision testing. They are connected by dedicated RF connectors. It is a large-scale equipment, quite bulky.
Crystal oscillator: the so-called "quartz crystal resonator" and "quartz crystal clock oscillator". But since resonators are used more in consumer electronics, the general concept equates crystal oscillator with resonator theory.
It was solved. The latter is usually referred to as Zhong Zhen. ?
2, classification. First, let's talk about resonators.
Resonators are generally divided into plug-ins (Dip) and patches (SMD). The plug-ins are also divided into HC-49U, HC-49U/S and tuning fork type (Yuan Zhu). HC-49U is commonly known as 49U, some purchases are commonly known as "high", while HC-49U/S is commonly known as 49S, commonly known as "short type". Tuning fork type can be divided into 3*8, 2*6, 1*5, 1*4 and so on. Patch type is classified according to size and foot position. For example, 7*5 (0705), 6*3.5 (0603), 5*3.2 (5032) and so on. The feet are divided into 4Pin and 2pin.
The oscillator can also be divided into plug-ins and patches. Plugins can be divided into size and foot. For example, the so-called full-size, also known as rectangular or 14pin, half size is also known as a square or 8pin. Note, however, that both 14pin and 8pin refer to the foot number of the core IC in the oscillator, and the oscillator itself is 4pin. From different application levels, it can be divided into OSC (ordinary clock oscillator), TCXO (temperature compensation), VCXO (voltage control), OCXO (constant temperature) and so on. ?
3. Basic terms. I think this is also a lot of vague points for purchasing students. Here I have chosen some commonly used terms for resonator to give an explanation.
Frequency Tolerance: The allowable deviation from the nominal frequency at the reference temperature (25 + 2) under specified conditions. It is usually expressed in PPm (parts per million).
Frequency Stability (Temperature Frequency Difference): refers to the allowable deviation from the nominal frequency within the specified operating temperature range. It is expressed in PPm.
Aging: Permissible relative change of crystal operating frequency over time under specified conditions. The annual rate of aging is measured in terms of annual unit time.
Shunt Capacitance: Capacitance in an equivalent circuit in parallel with a series arm, also known as a shunt capacitor, usually expressed as C0.
Load Capacitance: Effective external capacitance, usually expressed in CL, that together with the crystal determines the load resonant frequency fL.
Generally, there are 2 parameters that concern most, that is, adjusting frequency difference and load capacitance. Some of them are required for temperature difference. If the working temperature range is relatively wide, the working temperature range will be required, that is, the wide temperature range.
4, choose. Mainly about resonator. Theoretically, as long as the parameters are determined, any type of model can be used normally. For example, 49U and 49S replace, 49S and cylinder and patch replacement are no problem. But in actual ion, we will consider and choose according to the characteristics, cost and convenience of the circuit. Generally speaking, simple applications are mainly based on cost considerations. But some products or circuits will require the equivalent resistance of crystal oscillator, excitation power and so on, so they will be ed in different models. In addition, patches are mainly designed to adapt to the increasingly miniaturization of products and improve the efficiency of production. After hearing some purchases, friends say that they can only choose 49S instead of 49U or vice versa. This is a minor mistake.
The ion of the clock oscillator mainly determines the characteristics of the product circuit requirements, generally speaking, the clock oscillator in precision and need to meet the requirements of related applications will be better. For example, mobile phone communication station, satellite and so on.
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