Incorrect frequency from crystal oscillator

Posted on Feb 5, 2014

Acquired an old Eistar SJ-1 Digital pulser. However the frequency it generates is always 66% of what its supposed to produce. Turns out that the frequency coming out of the frequency stage(at Pin 11)is 66% of the 16MHz crystal or about 10. 6MHz. (The 74LS38 is an open collector quad nand gate) I opened it up and traced out the layout. The frequency stage looks like

Incorrect frequency from crystal oscillator
Click here to download the full size of the above Circuit.

to that I have now learned as a Pierce Type nand gate oscillator. I have drawn an abbreviated schematic of the digital pulser showing the frequency generation stage. In the Pierce type oscillators I have researched, there is a capacitor in between the nand gates where resistor R1 is. However in this design there is a 2. 2k resistor(R1). What could be causing the wrong frequency to be generated How I proceed to troubleshoot and correct the pulser Should I: 1. Replace the crystal with another and check 2. Replace resister R1 with a capacitor as I have seen in a Pierce type oscillator. I am guessing since the design would have worked at one point in time, R1 is not an issue here. In any case, this circuit looks like it will drive a lot of current through the crystal and the crystal may have been damaged over time as a result. R2 in particular should probably be much larger. Dave Tweed™ Jan 12 `13 at 21:43 66% sounds suspiciously like 2/3. Are you sure there aren`t some multipliers and a PLL in there, and they aren`t set as you think Olin Lathrop Jan 12 `13 at 22:47 2/3 is what I thought too, but I(google) has been unable to find a harmonic for a crystal at 2/3. Could it be a crystal failure mode For the whole device, the 16MHz from the pierce oscillator feeds into a 74LS93 for a divide by 8, then into three successive CD4518BE for divide by 10 each. All the separate inputs go into a mux (74LS151) which gets selected by the...

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