Electronics > Beginners

Crystal Oscillator Waveform

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I realized a proto with this oscillator circuit (not designed by me, copied somewhere) which is intended to drive an AVR chip at 16-24 MHz (here 16).

Actually -despite a routing error in this first proto- it works, as can be seen in attached pictures the oscillator is switched to external and the flag reports that the frequency is stable.

Nevertheless I'm not really happy with the waveform, my naivety led me think to something more square-y. The source of this behavior is not the wire bridge as the waveform is the same when I disconnect it.
I found this application report but here, in fig 20 the waveform is non-square-y the other way around while in other pictures the non-square-iness is symmetric at difference from mine.

I tried to (randomly, well, quite) replace resistors and capacitors with the following caveats: I don't have capacitors less than 10p so I tried only to increase and I don't have resistors larger than 1M so I only tried to decrease.

all my attempts gave almost negligible modifications in the waveform  (for worst)

I even tried to replace U1A with a 74hc14 to no avail.

I could even be happy with that but, as I'm correcting PCB errors I'd really like to understand what's going on.

To see 16MHz square properly your oscilloscope has to be setup for a higher BW than 20MHz, etc.
Also you may go lower with the 470ohm resistor, up to say 180ohm.

So simple!


replacing the scope GND alligator with a short spring now I get this, I can probably be happy with that


--- Quote from: uliano on September 28, 2022, 07:44:54 am ---So simple!


--- End quote ---
Yes and also be aware every measurement process has the potential to actually change the measurement and circuit behaviour.
Ideally any probe is perfect and won’t change circuit behaviour but the truth can be somewhat different especially when measuring high impedance circuits.
Active probes that load the circuit with just a pF or 2 are the preferred solution but also can be costly and have limitations of the voltage they can be used for.

Another solution is a 100:1 probe that can just 1/3 the tip capacitance of a 10:1 probe and have the added versatility of providing added protection and safety at elevated low impedance voltages.


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