issues about a 32M crystal oscilltor

[ccjanee]

I used the EPson TSX-3225 crystal. The oscillator includes a NMOS amplifer with PMOS current source and a 900k feedback res.

Now the chips come back, some oscillors work, but some not. After I swaped the crystal, the good chip is still good, the bad is still bad, which means the crystal is not damaged.

So, what are the possible reasons for this? And how to troubleshoot?

 

[Humungus]

Your amplifier provides a gm which is at the limit of acceptable values. In fact it can be too low or too high.

Try, if you can to change the bias current by either increasing or decre&asing it and see what happens.

If too much transconductance is there, you can increase the limiting resistor connected between the driver output and the crystal.

In case of too low gm, use and external P-type source to provide more current to the NMOS.

This will give you a hint to correct your design.

 

[Hunter_BJUT]

output current don't meet crystal requirement.

 

[ccjanee]

The gm of the inverter was my first concern. But I tuned the bias current by +/-25%, it did not change anything. The good chip is still good, bad is still bad. The gm is 3.9/4.9/5.6m correspondingly(by simulation). Humungus, do you have an idea what the typical gm should be?

I also suspected the output current is too high. Because from the spec of the crystal, the drive leve is 100uW Max. Converting to current, it's 1.6mA. But the RMS of the output current by simulation is 2.7mA. But the crystal didn't fail, as I said, I once swapped the crystal to check, the good chip is still good, the bad is still bad. I am really confused.

The process variation shouldn't cause so big difference, should it?

Sorry for the late reply.