1. Passive crystal-Passive crystal needs the oscillator on the DSP chip, and the connection method is suggested on the datasheet. Passive crystal has no voltage problem, and the signal level is variable, that is to say, it is determined according to the starting circuit. The same crystal can be applied to various voltages and DSPs with different clock signal voltage requirements, and the price is usually low. Therefore, for general applications, if conditions permit, it is recommended to use crystal, which is especially suitable for producers with rich product lines and large quantities. Compared with crystal oscillator, the defect of passive crystal is that its signal quality is poor. Usually, it is necessary to accurately match the peripheral circuits (capacitors, inductors, resistors, etc. for signal matching). When replacing crystals with different frequencies, the peripheral configuration circuits need to be adjusted accordingly. It is suggested that Shi Ying crystal with high precision should be used, and ceramic with low precision should not be used as much as possible.
2. Active crystal oscillator-Active crystal oscillator does not need the internal oscillator of DSP, and its signal quality is good, relatively stable, and its connection mode is relatively simple (mainly to filter the power supply, usually using a PI filter network composed of capacitor and inductor, and filtering the signal with a small resistance at the output end), so it does not need complicated configuration circuit. Common usage of active crystal oscillator: one foot is suspended, two feet are grounded, three feet are connected to output, and four feet are connected to voltage. Compared with passive crystal, the defect of active crystal oscillator is that its signal level is fixed, so it is necessary to select the appropriate output level, which is less flexible and expensive. For applications with sensitive timing requirements, I personally think it is better to use an active crystal oscillator, because a more precise crystal oscillator or even a high-grade temperature compensated crystal oscillator can be selected. Some DSPs have no starting circuit inside, so they can only use active crystal oscillator, such as TI’s 6000 series. Compared with passive crystals, active crystal oscillators are usually larger in volume, but now many active crystal oscillators are surface mounted, and their volume is equivalent to that of crystals, and some of them are even smaller than many crystals.
A few points for attention:
1. The DSP that needs frequency doubling needs to be equipped with PLL peripheral configuration circuits, mainly for isolation and filtering;
2. Crystal oscillators below 20MHz are basically fundamental frequency devices with good stability, while those above 20MHz are mostly harmonic (such as 3rd harmonic, 5th harmonic, etc.) with poor stability. Therefore, it is strongly recommended to use low frequency devices. After all, the peripheral configuration of PLL circuit for frequency doubling is mainly capacitors, resistors and inductors, and its stability and price are far better than crystal oscillator devices;
3. The clock signal wiring length is as short as possible, the line width is as large as possible, and the distance from other printed lines is as large as possible. It is close to the device layout and wiring, and if necessary, it can go to the inner layer and be surrounded by ground lines;
4. There are special design requirements when the clock signal is introduced from the outside through the backplane. Please refer to relevant information in detail.
In addition, some explanations should be made:
Generally speaking, the stability of crystal oscillator is better than that of crystal, especially in precision measurement and other fields. Most of them are high-grade crystal oscillators, which can integrate various compensation technologies and reduce the complexity of circuit board design. Imagine, if a crystal is used, and then the waveform shaping, anti-interference and temperature compensation are designed by yourself, what will be the complexity of the design? Here, we design RF circuits and other occasions with high clock requirements, that is, we use high-precision temperature compensated crystal oscillators, and industrial-grade ones cost several hundred yuan each.
If you can’t find a suitable crystal oscillator for special applications, that is to say, the complexity of the design is beyond the level of the finished crystal oscillator on the market, you must design it yourself. In this case, you have to choose crystals. However, these crystals are definitely not ordinary crystals on the market, but special high-end crystals, such as ruby crystals and so on.
The situation in the field with higher requirements is even more special. The clocks we use here for high-precision testing are even provided by atomic clocks, rubidium clock and other equipment, which are connected by special RF connectors. It is a large equipment and rather bulky.
