“Sometimes crystals are called passive crystals, and crystal oscillators are called active crystals. Although this statement is not accurate enough, it is also enough to vividly reflect the difference between crystals and crystal oscillators: crystals do not need power supply, and crystal oscillators need power supply.
The difference between crystal and crystal oscillator
This problem plagues most hardware engineers who have just started work.
Crystal is crystal, Crystal. Abbreviated as XTAL
Crystal oscillator is a crystal oscillator, Crystal Oscillator. Abbreviated as XO
One is not powered, one is powered
The crystal cannot oscillate by itself, and an external circuit is needed to output the clock signal (the chip that needs the crystal has an internal oscillation circuit)
The crystal oscillator can oscillate as long as it is energized and output a clock signal. Its built-in oscillator circuit.
Sometimes crystals are called passive crystals, and crystal oscillators are called active crystals. Although this statement is not accurate enough, it is also enough to vividly reflect the difference between crystals and crystal oscillators: crystals do not need power supply, and crystal oscillators need power supply.
One double-ended output, one single-ended output
The difference in hardware circuit design: The crystal has no power supply, but has two pins for input and output. The crystal oscillator is powered and has only one output.
One without direction, one with direction
The crystal has no orientation, and it can be welded on both sides. The crystal oscillator has a direction, because there is only one output pin, the chip will not be output if it is soldered upside down.
Although XTAL_IN and XTAL_OUT are marked on the chip that uses the crystal, they are only marked for the internal oscillator circuit of the chip, and have nothing to do with the external crystal. The crystal itself is a thin sheet, and the piezoelectric effect is the same on both sides.
Chips using crystal oscillators are generally marked with XTAL_IN or only XTALCLOCK and the like, with only one pin. In short, just connect the output pin of the crystal oscillator.
Two feet must be crystals, and four feet are both possible
For crystals, only a thin piece of crystal can be oscillated. So the two pins of a crystal are the few pins of the crystal. The small size has 2 ground pins for grounding and shielding the shell of the crystal.
The crystal oscillator needs at least power, ground, clock output, and at least three pins. There is another pressure control foot, which is 4 feet.
Look at the following circuit diagrams, you can clearly see the difference.
The 37.4MHz crystal used by the dual-band WIFI module, with an external load capacitor. (Some chips have built-in load capacitors, so there is no need to add external ones) (Different crystals have different requirements for load capacitors!)
The Snapdragon 625 platform also uses a crystal. Although it is better to use a temperature compensated crystal oscillator (TCXO) on the mobile phone platform, Qualcomm still uses a crystal with temperature feedback, combined with the internal oscillator circuit and algorithm, to build it by itself. A temperature-compensated crystal oscillator is installed. That’s right, Qualcomm likes to be different from others so much.
Broadcom GPS chip, active 26M crystal oscillator. With temperature compensation function. GPS has high requirements for frequency accuracy, and temperature compensation is required to ensure the signal.
Lianxin 1860C is a pure domestic 4G communication platform, and LTE communication also requires high radio frequency signal accuracy, so it is also a temperature-compensated crystal oscillator. The output frequency of the crystal can be controlled through the RF_AFC pin, which can correct the frequency difference with the base station.
The reason why the GPS in the picture above does not require voltage control (VC) is because the frequency band of GPS is narrower than that of 4G, so it is not necessary.
What is the difference between a crystal and a crystal oscillator?
In fact, there is not much difference, the main chip design. If it is designed with its own oscillator circuit, just use a crystal. If it is designed without internal oscillating circuit, it is necessary to use a crystal oscillator.
Where the frequency needs to be controlled, a voltage-controlled oscillator (VCXO) can be used to adjust the oscillation frequency of the crystal. (As shown in the picture above, the part of 4G signal transmission)
If a wide temperature range is required, or for high requirements, a temperature-compensated crystal oscillator (TCXO) should be used.