I hooked up a crystal oscillator (4 pins) to Vcc + Gnd and observed the generated waveform using my scope. I think everyone will agree the waveform shown is very distorted and I am not sure why. Any ideas?
Electronic – Crystal oscillator wave shape
crystaloscillator
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.
If you are using a 100 MHz scope, then a 50 MHz square wave will look like a 50 MHz sine wave as all of the harmonics will be killed (150 MHz, 250 MHz, 350 MHz, etc).
Best Answer
This is a classic square wave as detected by a scope set up with an inadequate ground connection. A few tips (in order of importance):
1 - Connect the scope ground as close to the clock ground as possible.
2 - Make sure the clock chip is bypassed as close as humanly possible to the ground and power leads, with the capacitor leads as short as possible.
3 - If you have a choice of ground leads for the scope, use the shortest one available.
4 - Connect the scope probe as close to the clock output as possible.