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I belive that every signal on the PCB has its return current. One layout hint for crystal oscillator is that "The ground connection for the load capacitors should be short and avoid the return currents from USB, RS232, LIN, PWM......and power lines" and I just wonder what the return current of crystal oscillator is. an image of crystal oscillator is shown below: enter image description here

I have to two ways to draw the return current:

the first one:

enter image description here

the second one: enter image description here

I just wonder which one is right or neither of them is right. can anybody give me some suggestions?

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I think you have mis-interpreted the idea of return currents. This does not refer to the currents flowing in, around and through the oscillator but to the other devices 'sharing' the ground track and using it to 'return current'.

enter image description here

C1 and C2, together with the input and output capacitances of the gate form the capacitive load of the crystal and so part of the total oscillator circuit. {see http://www.mpdigest.com/issue/Articles/2008/Mar/Crystek/ for a fuller treatment}

What the hint suggests is that when connecting the capacitors you keep the leads short (avoiding any parasitic inductance/capacitances - we are dealing with a few pF here) and avoid the return currents of ... The easiest way to avoid return currents is to connect the two ends of the capacitors directly. (see below)

enter image description here

As you can see there is no DC path through the crystal or to ground. Impedances are very high so even small currents can have a significant effect. By keeping the load capacitor leads short (and essentially connected at a single point) we avoid other currents flowing along the (same) ground return track between the capacitors and allowing the currents to mix. Hence the the hint 'avoiding the return currents from USB, RS232, LIN, PWM......and power lines". ( for further information check out ground loops)

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