I have not abandoned you. It is just that work has been sucking up my time.
Mr. Stone, Thanks for jumping in with some clarifications.
I will try to help with some of the issues here. There are multiple things going on.
mauifan wrote:I can get the watch crystal to oscillate, but the output is clipped -- it looks like a sine wave that has been fed through a diode.
Yes, this is what you should expect with the circuit you have. With enough gain to begin oscillation, the amplitude of the oscillation will increase until the peak voltage approaches the supply rails. The supply voltage is the maximum voltage the oscillation can reach (the flat spot).
mauifan wrote:My original goal was just to try to get the crystal to vibrate at its indicated frequency
The crystal is
vibrating at it's frequency. You can cause the opamp to oscillate at the crystal frequency (the fundamental) or multiples of the fundamental (harmonics).
Now we are talking about the opamp issues.
mauifan wrote:the TS922 is a more "forgiving" op amp that has "rail to rail input and output
The TS922 is a nice opamp for many reasons. But, let's talk about why the LM324 didn't work as you expected. Where as the LM324 can also go rail-to- rail, it does not do it symmetrically. The LM324 uses bi-polar transistors in a totem-pole arrangement that simply means it can pull the output towards the (-) supply much stronger than it can pull the output up towards the (+) supply. The LM324 is only linear in the mid range and at moderate output current. The LM324 also has potential problems when the inputs are over-driven. The TS922 uses a CMOS process and the output transistors are very similar in gain and on-resistance. In addition, the input circuitry is immune to the problems of the LM324. So, as Adafruit said, the TS922 is more forgiving because it may still operate 'normally' even if not in the middle of it's operational range.
mauifan wrote:What do I need to adjust to make the circuit work at the new frequency?
The crystal sets the frequency of operation, not the resistors. The resistors are used to set the gain and DC bias the opamp so that it is within it proper operating range. The capacitors are used to stabilize the crystal and the AC signal component.
mauifan wrote:There is also a fixed 10k resistor between op amp (-) and ground
Are you using split supplies? In other words, do you have 2 power sources?
mauifan wrote:but I am not certain what you mean by provide some DC coupling.
A quartz crystal is a good electrical insulator. There will not be DC current through it.
The opamp (especially the LM324) needs an average voltage on it's inputs at about ½ of the total supply voltage.
Without some DC voltage source to keep the inputs near the center of the supply voltage (usually called ground), the inputs will drift up to the (+) supply voltage or down to the (-) supply voltage.
mauifan wrote:Sigh... I have read several tutorials
Mr Stone referenced some good material. They are all worth checking out.
And, good luck, John