Crystal examiner

[audioguru2]

Hi Walid,
The 'scope pic of the oscillator's output shows severe distortion that is cleaned up with the 3rd-order lowpass filter.

 

[AN920]

It is difficult to comment without the whole picture. The missing parts are the crystal data. It is possible to use other ratios as long as you can get enough gain and enough negative resistance to satisfy oscillation. The article explains in detail most of the other circuit design values but none on the crystal oscillator? Many people get crystal oscillators to work by trial and error, not that I am suggesting that that was the case here.

By making C20 much larger the loop gain will drop off more rapidly with frequency which will attenuate harmonics. The designer may have experimented with values that gave the lowest harmonics with just enough gain to oscillate at 14.3MHz

Another crystal of the same value in series on the output will produce a very good sine wave output with low harmonics.

 

[walid1]

Hi
TO audioguru:

The 'scope pic of the oscillator's output shows severe distortion that is cleaned up with the 3rd-order lowpass filter.

(a) Clear from your talk that you agree with AN920 that this is not good design.
(b)

3rd-order lowpass filter

I thought that it is banpass filter. i think it is 3rd order because it is 3 elements filter. Let me look at the internet for further information on this filter.

TO AN920

Many people get crystal oscillators to work by trial and error, not that I am suggesting that that was the case here.

I have long believed that many of the designs are in a trial and error because of the different specifications of some electronic components from each other.

Another crystal of the same value in series on the output will produce a very good sine wave output with low harmonics.

(a) That is a very good and new idea, are you sure that Another crystal of the same value in series on the output work as lowpass filter?
(b) Can shed further light on the use of crystal as a LPF.

thanks GURU
thanks AN920

 

[AN920]

Using a series crystal on the output will act as a very narrow BPF at the crystal series frequency Fs.

 

[walid1]

I do not have the necessary equipment, but I submit this link to enrich the subject
http://www.giangrandi.ch/electronics/crystalfilters/xtaltest.html

 

[AN920]

Good find! Saves time doing calculations, but does not calculate Rs which is important.