一、The difference between passive crystal and active crystal oscillator
1. Passive crystal -- Passive crystal needs an oscillator in DSP chip.Passive crystal without the problem of voltage, signal level is variable, that is, according to the circuit of vibration to decide, the same crystal can be applied to a variety of voltage, can be used for a variety of different clock signal voltage requirement of DSP, and the price is usually lower, so if conditions permit suggestion for general application with crystal, it is especially suited for product line rich batch big producers.Compared with crystal oscillator, passive crystal has the disadvantage of poor signal quality, which usually requires accurate matching of peripheral circuits (capacitors, inductors, resistors, etc.). When replacing crystals with different frequencies, peripheral configuration circuits need to be adjusted accordingly.It is recommended to use high precision quartz crystal, as far as possible do not use low precision ceramic alarm.
2. active crystals, crystal active vibration does not require DSP internal oscillator, signal quality is good, stable, and the connection method is relatively simple, mainly completes the power filtering, usually using a capacitance and inductance type PI filter network, consisting of the output resistance filter with a small resistance signals), does not need complex configuration circuit.Common use of active crystal oscillator: one leg suspended, two legs ground, three legs connected to output, four legs connected to voltage.Compared with passive crystal, the defect of active crystal oscillator is that its signal level is fixed, and it is necessary to choose the appropriate output level, which is less flexible, and the price is high.For sensitive applications with timing requirements, I think active crystal oscillator is better, because you can choose more precise crystal oscillator, or even high-grade temperature compensation crystal oscillator.Some DSPs do not have internal vibration circuit, can only use active crystal oscillator, such as TI 6000 series.Active crystal
Oscillators are usually larger in volume than passive crystals, but many active crystal oscillators are now superficially sized, and some are even smaller than many crystals.
二、 a few points to note:
1. DSP requiring frequency doubling needs to be well configured with PLL peripheral configuration circuit, mainly isolation and filtering;
2. The crystal oscillator below 20MHz is basically a fundamental frequency device with good stability. Most of the oscillator above 20MHz is harmonic (such as 3 harmonics, 5 harmonics, etc.), and its stability is poor
The peripheral configuration of PLL circuit is mainly capacitance, resistance and inductance, and its stability and price are far better than crystal oscillator devices.
3. the clock signal line length as short as possible, line width as large as possible, and other printed line spacing as large as possible, close to the device layout and wiring, if necessary, can go to the inner layer, and the land line surrounded;
4. There are special design requirements when the clock signal is introduced from the outside through the back plate, so it is necessary to refer to the relevant information in detail.
三、 in addition to do some explanations:
Generally speaking, the stability of crystal oscillator is better than that of crystal, especially in the field of precision measurement. Most of the high-grade crystal oscillator is used, so that various compensation technologies can be integrated together and the complexity of design is reduced.Imagine, if you use crystals, and then design their own waveform shaping, anti-jamming, temperature compensation, then what will be the complexity of the design?We design radio frequency circuit and other occasions that require high clock, is the use of high precision temperature-complementary crystal oscillator, industrial grade to several hundred yuan a.
Application in the field of special if no suitable crystals, meaning that the complexity of the design is beyond the finished product on the market crystal vibration level, must be own design, this kind of circumstance will choose crystal, but these crystals is certainly not the ordinary crystals on the market, but a special high-end crystal, such as ruby crystal, etc.
The more demanding areas are more special. The clocks we use here for high-precision testing are even provided by atomic clocks, rubidium clocks and other equipment, which are connected by special radio frequency connectors. It's a large device, rather heavy and cumbersome.