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PCBA test steps and precautions.

  • October 22, 2020
  • 81

1. Test purpose


Evaluate the driving capability of the crystal oscillator drive circuit to ensure stable oscillation of the crystal oscillator.


2. Required equipment


Adjustable regulated power supply, oscilloscope, adjustable resistance, multimeter


3. Test procedure description


(1) Parameter calculation


a) To ensure stable oscillation of the crystal oscillator, the negative impedance (-R) of the crystal oscillator is at least 5 times the impedance of the crystal oscillator, that is |-R|》 5RL


b) Calculation of RL:


RL=R1*(1+C0/CL) 2


Among them, R1: the effective series resistance of the crystal oscillator (Effective Series Resistance), which can be queried in the datasheet of the crystal oscillator;


C0: Shunt Capacitance of the crystal oscillator, which can be queried in the datasheet of the crystal oscillator;


CL: load capacitance of the crystal oscillator,


CL=CT+CS


CT=(Cg*Cd)/(Cg+Cd)


CS: Stray capacitance distributed on the line, the typical value is 5pf;


Cg and Cd are shown in Figure 1, and generally take the same value in circuit design.

Figure 1 RL calculation


(2) Test of negative impedance


The test circuit is as follows:

Figure 2 Test of negative impedance


a) Connect the adjustable resistor R in series to the output terminal of the crystal oscillator (EXTAL of the MCU chip);


b) Use an adjustable regulated power supply to supply power to the circuit board according to the power supply requirements of the tested circuit board, and connect the oscilloscope probe to the adjustable resistor 2 pin (as shown in Figure 2, near the EXTAL end of the MCU chip) to test the oscillation waveform of the crystal oscillator , Adjust the adjustable resistance R to make the crystal oscillator from start to stop;


c) When the circuit starts to oscillate to stop oscillating, turn off the output of the adjustable regulated power supply and measure the R value with a multimeter;


d) Calculate the negative impedance of the crystal oscillator |-R| = R+RL, where RL is the load impedance value of the crystal oscillator.


4. Test qualification criteria


The negative impedance (-R) of the crystal oscillator is at least 5 times the impedance of the crystal oscillator, that is |-R|》 5RL


5. Matters needing attention


(1) In this test, you can first calculate the load impedance RL of the crystal oscillator, and then select an adjustable resistor with a suitable resistance according to the value of 5 times RL;


(2) During the test, it may appear that the adjustable resistance R has been adjusted to the maximum, and at this time R》》5RL, but the crystal oscillator still does not stop oscillating. At this time, it is considered that the crystal oscillator can oscillate stably. If the value of R is about n times of RL at this time, you can write |-R| "(n+1) RL in the test record.

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