I want to create a CLK generator circuit that will work at 8Mhz.
I'm following this design (from http://www.electronics-tutorials.ws/oscillator/crystal.html):
My crystal is 8Mhz HC49/US, with 20pF capacitance as described in the website I bought it from.
I'm using R1 = 10Mohm , R2=1Kohm , and as for C the smallest I had was 150pF so I used 6 of them in series which should give 150/6 = 30pF .
My inverter is a 6 input buffer NOT gate from TI (cd40106b) , I'm using two of its gates for the circuit (AMP + buffer).
After connecting it all, and hooking up my (PC based) oscilloscope, I saw I'm only getting a 1.5Mhz signal , which was very far from a square wave I was expecting – more of a triangle kind of waveform.
Then I removed the crystal from the circuit, and the waveform stayed the same, so I'm obviously doing something wrong here as I'm just seeing the RC circuit oscillating.
I've replace the crystal with two others and got the same results.
The NOT gate should just fast enough for this circuit ,working at VDD=5V , transition time is typically 100nS and up to 200nS and I'm no way near this with my results.
I think I'm using the correct values for R2 and C but I'm not sure.
How do I calculate the correct R and C values to use for my circuit? Could it be the reason for this behavior?
I've also tried to assemble a circuit using CD4060 (counter) like the one from the comment.
to see if the problem is coming from the inverter but I still can't get the input frequency to be 8Mhz.
What can I do to further debug this?
Best Answer
The usual recommendation for the Pierce oscillator is to use un-buffered standard inverters, ie not schmitt trigger input inverters. The inverter needs to work in its linear region which is almost impossible with a schmitt trigger input. Try TI's LVC1GU04.