Differential crystal oscillator output waveform analysis: three types to know

YXC扬兴晶振

Differential crystal oscillator output waveform analysis: three types to know [Copy link]

At present, the output waveforms of differential crystal oscillators are mainly divided into three categories: sine wave, square wave, and quasi-sine wave. Commonly used differential crystal oscillators output square waves, with high output power and strong driving ability, but with many harmonic components.

The two output modes are the output logic of the differential crystal oscillator, with opposite phases, which can form a higher performance system while eliminating common mode noise. Outputs such as PECL, LVDS, and HCSL can meet the requirements of high-speed data transmission and are used in high-speed computers, digital communication systems, radars, measuring instruments, frequency synthesizers, etc.

Category 1: Sine Wave

Regarding the crystal oscillator output waveform, I believe some people may think that there are only two waveforms, that is, the passive crystal oscillator output waveform is a sine wave, the active crystal oscillator output waveform is a square wave, and a small part is a sine wave. Since the active crystal oscillator adds a shaping circuit, the output is a square wave, and the sine wave is rarely used. Generally, square wave output is used (most of the time, what you see on the oscilloscope is a sine wave with a very poor waveform, which is due to the insufficient bandwidth of the oscilloscope.

For example: If the active crystal oscillator is 20MHz and is measured with a 40MHz or 60MHz oscilloscope, a sine wave will be displayed. This is because the Fourier decomposition of a square wave is the superposition of the fundamental frequency and odd harmonics. If the bandwidth is not enough, only the fundamental frequency 20MHz and the harmonics of 60MHz will be left, so a sine wave will be displayed. Perfect reproduction of a square wave requires at least 10 times the bandwidth, and 5 times the bandwidth is barely acceptable, so an oscilloscope of at least 100M is required.

Category 2: Square wave

Square waves are mainly used for clocks in digital communication systems to drive pure counting circuits or gate circuits. Square waves mainly have several index requirements such as output level, duty cycle, rise/fall time, and driving capability. Sine waves are mainly used for circuits with special requirements for EMI and frequency interference. Such circuits require that the output high-order harmonic components are very small; choosing sine waves for the subsequent analog circuits is also a good choice. Usually, indicators such as harmonics, noise, and output power need to be provided. Square waves have large output power and strong driving capabilities, but rich harmonic components; the output power of sine waves is not as good as that of square waves, but its harmonic components are much smaller. The frequency output of an active crystal oscillator must have a certain waveform as the output carrier, and the output of the waveform must be accompanied by a certain load value. In actual use, waveform load is also a very important parameter indicator of the crystal oscillator. Improper selection may cause abnormal operation of the crystal oscillator or other modules, failure of functions, damage to the module or even the entire machine.

The third category: quasi-sine wave

There are also several types of quasi-sine waves, from square waves that are almost the same as square waves to rounded trapezoidal waves that are closer to sine waves. PECL is a very important logic circuit in the high-speed field. It has the advantages of fast circuit speed, low driving capability, low noise, and high frequency. However, it has high power consumption and cannot be driven at different levels. If a low-voltage 3.3V/2.5V power supply is used, it is called LVPECL, that is, low-voltage PECL. LVDS can solve these problems with high speed, low noise, long distance, and accurate transmission. The maximum output frequency of LVDS is 2.1GHz, and the voltage is 1.83.3VHCSL has minimal output jitter and higher power consumption.