The question was asked on the TI forum.
Dear Community Members,
I have a big problem about the 26MHz crystal oscillator. I am a electronic hardware design enginner in a water meter factory. On our AMR (868MHz RF) water meter’s PCB we are using CC1101 transceiver. In the first 10K pieces of production we used 26MHz crystal with 20pF internal load capacitance (27pF capacitors were on the each pin of the crystal). Our handheld terminals have this crystal too. We are using 26MHz crystal with 16pF load capacitance since the first production. For getting the true matching between crystal’s load capacitance and capacitors on each pin of crystal we changed the capacitors from 27pF to 20pF. They are working well too. But the problem is the handheld terminals do not communicate with the new PCBs(with 16pF load capacitance crystal). I think there is a channel shift. The PCB have same specifications, same layout, same schematic. The embedded software(in MSP430F2121) is same. And the register settings of the CC1101 is same too. Everything is same. The only difference is crystal (crystal’s load capacitance) and the capacitors connected to the pins of crystal. Does anybody help me? Thank you very much.
Chris Pinter replied: ” The issue you are having is directly related to the load on the crystal. The capacitors you have on your board were tuned for a 20 pf crystal. You will need to re tune the loading capacitors. Please take a look at the datasheet on page 22. http://focus.ti.com/lit/ds/swrs061f/swrs061f.pdf
I see you are using the same value of load capacitors as the crystal load. If this design passed quality assurance during design you might be able to replace your load capacitors with 16 pf and see if that helps. Again I should mention that you need to read the datasheet on the subject and retest for quality for whatever change you make.
I hope this helps,