What is the waveform of the crystal oscillator? Sine wave or square wave?
2023-07-31
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. However, the square wave has the choice of non-differential and differential output; the sine wave has the choice of quasi-sine wave and peak-clipping sine wave.
The output waveform of the passive crystal oscillator is a sine wave; the output waveform of the active crystal oscillator is a sine wave or square wave. The active crystal oscillator itself has a sine wave output waveform, and a shaping circuit is added inside, and the output waveform is a square wave.
The square wave output power is large and the driving ability is strong, but the harmonic components are rich; the sine wave output power is not as good as the square wave, but its harmonic components are much smaller. Square waves are mainly used in digital communication system clocks to drive pure counting circuits or gate circuits.
Square wave output mode
(1) TTL output
TTL is a transistor-transistor logic (Transistor-Transistor Logic) circuit with fast transmission delay time and high power consumption. It is a current control device.
(2) CMOS output
CMOS output is a common type, which is a voltage control form and is used to drive logic level inputs. CMOS output has slow transmission delay time and low power consumption. It has a larger noise margin than TTL, and its input impedance is much larger than TTL input impedance.
(3) LVPECL output
LVPECL is developed from ECL and PECL. Its typical output is a pair of differential signals, and the emitter is grounded through an AC source. When using LVPECL, please note: different levels cannot be driven directly, and AC coupling, resistor network or special chip can be used for conversion in the middle.
(4) LVDS output
LVDS is a low-voltage differential signal (Low-Voltage Differential Signaling). It is a differential pair of input and output. There is an internal 3.5-4mA constant current source that changes the direction and level on the differential line to represent "1" and "0".
It is converted into a differential level of ±350mV through an external 100 ohm matching resistor (connected in parallel on the differential line close to the receiving end). Note on the use of LVDS: it can reach more than 600MHz, PCB requirements are high, and the differential lines are required to be strictly equal in length, with the difference not exceeding 10mil (0.25mm); the distance between the 100 ohm resistor and the receiving end cannot exceed 500mil, and should be controlled within 300mil.
Sine wave output mode
Sine waves are mainly used in circuits that have special requirements for EMI and frequency interference. Such circuits require that the high-order harmonic components of the output are very small. It is also a better choice to use sine waves for later analog circuits. Indicators such as harmonics, noise and output power are usually required.
There is also a special clipped sine wave (Clipped Sine Wave), which has much less harmonic components than the square wave, but has weaker driving ability. Vp-p is 0.8Vmin when the load is 10K//10PF. Surface mount temperature compensated crystal oscillators in SMD 7050, SMD5032, SMD3225 and other packages usually use this form of output waveform.
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