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
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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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