My product have to pass SRRC certification,but the pre-test is fail. The certification lab need to test RF frequency at 2402MHz, 2441MHz and 2480MHz separately,and the results are as follows:
The frequence shift must under 20ppm,but the result shows that the frequence shift is over the limit.
My shematic and PCB layout is copy from NORDIC develop board.
We test frequence at the red arrow position.Now what should I do to pass the certification.
If the carrier frequency is off, it's load crystal load caps that have the wrong value. Look up the CL value of the crystal from it's datasheet and calculate the values (for each) as follows:
Ccap = CL * 2 - 4 pF
For a crystal with CL = 8 pF, the caps should be 8 pF * 2 - 4 pF = 12 pF each.
Round down to the closes value in case you need to.
Thank you for your reply.I have checked the CL value of the crystal from it's datasheet,and the value is 8pF,and I use 12pF load caps accordingly.
Now I guess the reason maybe the value of caps we use is not precise,the deviation may be ±5% or more.