The output resistance of the crystal oscillator comes from minimizing the design. For the most important clock source part of the digital circuit, special attention should be paid to ensuring signal integrity. In addition, the crystal oscillator peripheral circuit must have some other components besides the resistor in the design.
Passive crystal oscillator output waveform is sine wave, active crystal oscillator output waveform is sine wave (sin) or square wave. Active crystal oscillator itself output is a sine wave, in its internal plus a shaping circuit, so the output is a square wave, sine wave is usually used rarely, throughout the use of square wave output (many times on the oscilloscope or waveform Not a good sine wave, this is due to the bandwidth of the oscilloscope can not be.For example: active crystal 20MHz, if measured with a 40MHz or 60MHz oscilloscope, the sine wave appears, this is due to the Fourier decomposition of the square wave to the fundamental frequency With odd harmonics superimposed, if the bandwidth is not enough, only the harmonics of the fundamental frequencies of 20MHz and 60MHz are left, so the sine wave appears. The perfect reproduction square wave needs at least 10 times the bandwidth, 5 times the bandwidth can only be considered far-fetched. , so the minimum required 100M oscilloscope).
Passive crystal has 2 pins, need to use the external clock circuit (connected to the internal oscillating circuit of the main IC) to generate the oscillation signal, it can not oscillate itself.
The active crystal oscillator has four pins and is a complete oscillator. In addition to the quartz crystal, there are transistors and RC components. Only a power supply is needed to output a good waveform. The general crystal oscillation circuit is in a The inverting amplifier (note that the amplifier is not an inverter) is connected to the crystal at both ends. Two capacitors are connected to the two ends of the crystal respectively. The other end of each capacitor is connected to ground. The capacity of these two capacitors is connected in series. The value should be equal to the load capacitance. Please note that the pins of the general IC have equivalent input capacitance. This cannot be ignored.
A crystal oscillator is an abbreviation for a crystal oscillator, and it can be electrically equivalent to a two-terminal network in which a capacitor and a resistor are connected in parallel and a capacitor is connected in series. Electrotechnics has two resonance points in this network, with the frequency being high and low, where the lower frequency is the series resonance; the higher frequency is the parallel resonance. Due to the crystal's own characteristics, the distances between the two frequencies are quite close. In this extremely narrow frequency range, the crystal oscillator is equivalent to an inductor. Therefore, as long as the crystal capacitor is connected in parallel with suitable capacitance, it will constitute a parallel resonant circuit. .
The active crystal oscillator's frequency output must have a certain waveform as the output carrier, and the waveform output must also be accompanied by a certain load value. In practical use, the waveform load is also a very important parameter target of the crystal oscillator. If the selection is inappropriate, the operation of the crystal oscillator or other modules may be abnormal, and the function may not be completed, while the damage to the module or the entire unit may be caused.
The output waveform of the crystal oscillator has three major categories: sine wave, square wave, and quasi-sine wave.
The crystal oscillator load is mainly the following:
1, sine wave: load 50 ohms or 1k ohms;
2. Square wave: N TTL loads or N PF capacitors;
3, quasi-sine wave: 10K ohm parallel 10PF capacitor.
In addition, differential output PECL, LVDS and other high frequency (above 100MHz) are commonly used. In actual use, the crystal output is usually used to drive the following circuit types:
1. Long-line output of coaxial cable
2. The output of the filter circuit.
The above two circuits are generally suitable for 50 ohm loads. This is due to the fact that the above two types of circuits usually require a 50 Ohm load for matching. There are 75 Ohm, 300 Ohm, and other characteristic impedances in the RF domain, which should be clarified when required. The output waveform of this type is most suitable for a sine wave. After the sine wave is transmitted through the long line, the waveform attenuates only, and the waveform is input.
In some cases, the user uses a triode and a high-speed op amp to process the crystal oscillator waveform for the purpose of shaping and expanding. In this case, the load impedance is usually not too heavy, and the sine wave waveform is most suitable. Demand supply load, waveform fluctuations and other parameters.
Circuits that have special requests for EMI and frequency disturbances require a high harmonic component of the circuit output. Therefore, sine waves are the best regardless of the circuit being driven.
Square wave output is divided into: TTL level slow CMOS level at TTL level input low level <= 0.8v, high level> = 2.0V; output low level <0.4v, high level> 2.4V, maximum low Between the minimum level and the minimum level is an invalid voltage; CMOS level input low level <0.3vcc, input high level> 0.7Vcc, output low level <0.1vcc (close to 0), output high level > 0.9 Vcc (close to the supply voltage). To make the oscillator's "overall performance" balanced and reasonable, it is necessary to weigh many factors such as stability, operating temperature range, crystal aging effect, phase noise, and cost. The cost here does not only include the price of the device. And it includes the cost of use of the product's full life
The application scope of the crystal oscillator is very wide, and its application in different product requirements is different. In the past few years, the crystal industry has also changed not only with the emergence of various intelligent products to meet the market demand of the electronics industry. The large-volume plug-in transforms into today's ultra-small and ultra-thin patch crystals with less and less precision, making the product more stable.
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