Crystal Oscillator output is not sinusoidal

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
22
I am using the part ASV-12.000MHZ-EJ-T for generating a 12MHz Clock signal on my custom PCB. The following screenshot is the circuit. (Please ignore the part number in the screenshot, I have replaced that part with the part mentioned before).
1624704786919.png

When I try to observe the output of the crystal using oscilloscope at XIN, I see the following waveform.

1624706146968.png

Its doesn't appear like a sinusoidal waveform, instead it looks like a capacitor charging and discharging waveform.

I have tried the following to see if the waveforms becomes sine
1. replaced R189 with 0 Ohm Jumper - No change
2. Varied the load capacitances between 7.5 pF tthrough 45 pF - No change

Can anyone please have a look at this and let me know what could be the issue.
 

DickCappels

Joined Aug 21, 2008
7,985
I cannot see your waveform.
EDIT: Can see it now.

Depends on the oscillator. Many crystal oscillators don't produce nice sine waves on their outputs. Check the oscillator's specification and see what it says about the waveform.
 
Last edited:

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
22
What leads you to believe and/or expect the output to be a sinewave? Is the datasheet misleading on this subject?
Thank you for the reply.
Here is the link to the datasheet. https://abracon.com/Oscillators/ASV.pdf

Datasheet doesn't specify anything about the output being sine wave. I have seen few crystals earlier which give sine wave as an output and hence I assumed that this output should also look like the same.

Please let me know if I am wrong.
 

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
22
Why would that surprise you when there is a capacitor across the output? Without the capacitor, the output is likely close to a square wave.

Bob
Thank you for the reply.

I was expecting a smooth sinusoidal waveform instead of the one which I got. Can you please let me know if the waveform which I am seeing is normal ?
 

MrChips

Joined Oct 2, 2009
24,627
Remove the capacitor at the output.

btw, when taking a photo of the oscilloscope screen don't tilt the camera. Take the photo with the camera level and square with the oscilloscope.
 

Papabravo

Joined Feb 24, 2006
17,303
Thank you for the reply.
Here is the link to the datasheet. https://abracon.com/Oscillators/ASV.pdf

Datasheet doesn't specify anything about the output being sine wave. I have seen few crystals earlier which give sine wave as an output and hence I assumed that this output should also look like the same.

Please let me know if I am wrong.
There is nothing magic about sinewaves. Perhaps the shape is designed for faster and more consistent rise and fall times. In a digital application the shape of the waveform is hardly relevant. There are other factors that are more important. If you are that concerned, call the company @ 949-546-8000 and ask one of the FAE's. I'm sure they will be happy to educate you.
 

BobTPH

Joined Jun 5, 2013
4,051
The waveform at a crystal might be a sine wave, but this is a crystal oscillator whose output is likely buffered through a schmitt trigger.

Bob
 

Deleted member 115935

Joined Dec 31, 1969
0
This is a scope measurement problem

12 MHz is to to fast by modern standards, but it still easy to miss see it on a scope.

Check your scope is fast enough to see the clock, and that your scope probe has the required bandwidth,
If you are not using a scope probe, and the output is directly connected to the scope, you are suffering loading problems,

Check the scope probe is compensated correctly , that little screw on the probe,

Then do away with that long bit of wire on the scope probe, use the small clip that came with the probe that attaches to the end of the scope probe itself,

then probe and earth the scope directly at the output of the oscillator,
 
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