I am using this 26MHz crystal from NDK. The specified load capacitance is 10pF, I am using two 18pF capacitors in series accordingly. The circuit is shown in figure below. The problem is that the crystal is not oscillating. I observe a constant 2.4V at OSCP and GND at OSCN [VDD for U1, which is an RF transceiver, is 3.3V].
I have an evaluation board for U1, where the same crystal (and 18pF caps) is oscillating OK - the average voltage at OSCP is 0.5V in this case. I tried replacing the crystal and the capacitors (C1,C2) on my PCB to make sure the problem is not due to defective parts - but with the same result.
Finally, the observation that is currently puzzling me is that out of desperation, I tried checking for short on C1, C2 using a digital multimeter - when the part was powered up. While C2 did not report a short, C1 did - I also noticed that the reported resistance was (negative) -600 ohm when the red probe was at point B and black probe at point A (points A, B shown in figure below). I understand that this might be due to the capacitor discharging in to the multimeter - but the problem is I do not see the same behaviour with C1 on the evaluation board where crystal is oscillating OK.
- What might explain the difference in behaviour for C1 b/w the evaluation board and my PCB - i.e., why don't I see a negative resistance between points B and A on the evaluation board?
- [For people who have experience debugging crystals] What can I try next to debug why my crystal is not oscillating?