Does the feedback resistor for a crystal oscillator need to have a very accurate value? If a 16Mhz crystal requires 1Mohm, should one expect a problem when 200kohm or 2Mohm is used instead?
Electrical – feedback resistor value for a crystal oscillator
crystal
Related Solutions
With the details provided, it does seem like the crystal may be damaged - it's the most sensitive component on the board, and there's a good chance the pulser has been dropped a few times during it's long life.
The circuit seems reasonable and if it was originally designed with the resistor (rather than the capacitor), then I'd leave it like that. There are similar circuits out there that do use a capacitor as you say:
I'd go for replacing the crystal - choose a crystal designed for series operation such as the FOXLF160 (as opposed to the FOXLF160-20) While you're grabbing the crystal, getting a couple of 74LS38s wouldn't be a bad idea either, given they are only around 50 pence/cents each.
Also, given the price of decent CMOS oscillators nowadays, the other option is just to buy a 16MHz CMOS oscillator (5V supply tolerant) and replace the first stage with that.
10 seconds / year is 0.3 parts per million error (or as the time-nuts would usually put it, an error of 3 * 10^-7). A decent watch crystal is usually specified as +/- 20 ppm (2 * 10^-5), which is far worse than what you're asking for. The higher-frequency crystals used for clocking CPUs are usually worse than that.
Temperature-compensated oscillators (TCXOs) are better, in the 10^-6 range, but still worse than your spec.
Ovenized oscillators (OCXOs) can be better than 10^-7 but require watts of power, likely more than you can afford running on battery.
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Best Answer
If you are using a standard crystal oscillator configuration like this: -
Then you have to be a little careful about lowering "R" because it can reduce the overall phase shift from output to input from reaching 180 degrees. The phase shift needs to reach 180 degrees in order to have a total phase shift around the loop of 360 degrees in order for the circuit to oscillate.
If you look at the diagram below I've simulated a pierce oscillator crystal circuit and used approximate equivalent values for the crystal for it to resonate at about 9.2 MHz (for convenience). I am particularly interested in the phase shift hence I'm doing an open loop AC simulation: -
With the feedback resistor at 1 Mohm (R3), the crystal and associated external components attains a phase shift of 180 degrees at 9.189 MHz. This is the oscillation frequency. As you can also see there is scope to reach a phase shift that is a few degrees more and so we would be assured that with typical component tolerances, the circuit would oscillate very close to 9.189 MHz.
If I now lowered R3 to 100 kohm I would be more uncertain of the oscillator working (due to component tolerances) because the phase angle exceeds 180 degrees to a lesser extent: -
So, the answer is "it depends" on what you equivalent circuit of your crystal is and what other external components are needed around that crystal such as R2, C3 and C4.