I am using a CD4040 to divide a oscillator frequency and reading its period using PCA. The problem is I was using 3 oscillators, 120kHz, 70kHz and 30kHz and dividing it by 32. The problem is, when I shake my board, the measuring get really screwed. I tried my oscillator, I can't see any change on it using the oscilloscope, but the Capture value shows that it changed. What could the problem be? I'm starting to think that I could have harmonics between CD4040 and 8051 input.


I can't send my setup pics unfortunately but about the mechanical problem, if I hold my board and try to hit it softly at random places, the only place my measurement becomes different than it should be, is when it hits near the 8051. I can hit my LC tank, I can hit my 1:1 transformer, I can hit my transistors and etc. I tried to resolder all my SMD components again and even got a new 8051, but the problem continues. Later today I'll change my CD4040 exit to a lower divider so my frequency gets a bit higher, because I'm afraid my lower frequency can be causing this sensitivity.

About the PCB, it's a real PCB, we designed it, made it at a PCB house and soldered the components.

My major problem is: My uC can detect the period variation, but my oscilloscope can not. I'm using a Tektronix TDS-1012.

It's a "Differential Amplifier" configuration, but one of my NPN collectors has the LC Tank. My L is a 1:1 transformer inside one of these and my Inductor is a loop for metal detection. So for values I don't know them very well, because I didn't measure the loop inductance nor the transformer's mutual inductance. The Tank's capacitors are polyester, mostly bought from Digikey.

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    \$\begingroup\$ "when I shake my board, the measuring get really screwed" - suggests a mechanical problem (dry joint, bad contact, broken track?). Show us a photo of your setup. \$\endgroup\$ – Bruce Abbott Jun 14 '18 at 22:22
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    \$\begingroup\$ Did you build your circuit on one of those pluggable proto boards? \$\endgroup\$ – Michael Karas Jun 15 '18 at 1:36
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    \$\begingroup\$ What type of oscillator(s) are you using? What are the component values? If the oscillators include capacitors, what composition are they? \$\endgroup\$ – duskwuff Jun 15 '18 at 3:37
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    \$\begingroup\$ Crystal oscillators don't like getting hit or dropped to the floor. However I think they stay broken once they break(?), some expert would have to answer that. \$\endgroup\$ – Lundin Jun 15 '18 at 12:01
  • \$\begingroup\$ (a) "I can't send my setup pics" What's stopping you? How are you planning to resolve that, so that you are able to add those photos to your question? (b) Can you add a schematic diagram? (c) Your description has changed from "when I shake my board" to "I hold my board and try to hit it softly". Why did you change your test? Shaking does not cause the same G-forces as hitting it. (d) Even though you don't see a change in the signal, it would be interesting to add "good" and "bad" scope images. (e) Please give some examples of the size and duration of the change in measured signal period. \$\endgroup\$ – SamGibson Jun 15 '18 at 12:42

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