# Frequency/amplitude instablity

Hi there

I had a lab session today in which a square wave was synthesised using an oscillating crystal...this signal was fed into an oscilliscope and so various features of the square wave could be observed:

Gibb's phenomenon, the rising edge having a 'rounded' corner, the top part of the square wave (which should theoretically be perfectly flat) had an oscillating pattern that resembled a decaying sinusoid etc.

My teacher sai that the crystal was generating a stable fraquency (at least I think that's what he said!) and if that wasn't the case it would be called frequency instability and the effect that this would have on the signal being displayed by the osc. scope is that the signal would keep drifting across the screen...

He also mentioned amplitude instability but I did not follow what he said there...

I would be very grateful if somebody could explain these two terms to me (hopefully the above account is coherent enough!).

berkeman
Mentor
Hi there

I had a lab session today in which a square wave was synthesised using an oscillating crystal...this signal was fed into an oscilliscope and so various features of the square wave could be observed:

Gibb's phenomenon, the rising edge having a 'rounded' corner, the top part of the square wave (which should theoretically be perfectly flat) had an oscillating pattern that resembled a decaying sinusoid etc.

My teacher sai that the crystal was generating a stable fraquency (at least I think that's what he said!) and if that wasn't the case it would be called frequency instability and the effect that this would have on the signal being displayed by the osc. scope is that the signal would keep drifting across the screen...

He also mentioned amplitude instability but I did not follow what he said there...

I would be very grateful if somebody could explain these two terms to me (hopefully the above account is coherent enough!).

A crystal oscillator should not directly have a square wave at the crysta. You can square up the output of the oscillator with a buffer, but the waveforms at the crystal should be fairly rounded.

What kind of probe were you using to see the waveforms? Were you using Z-lead probes, or some other probe that has low inductance (no ground wire lead and clip)? If not, some of the ripples you saw could just be from the probe inductance ringing (artifact).

Frequency instability will not generally be visible as the waveform rolling by -- you were using Normal trigger on the 'scope, right?

Amplitude instability could be visible as small amplitude variations as you watch the waveform. If the frequency and amplitude instabilities are large enough, you could see them using "infinite persistance" mode on the 'scope (the integrated waveforms blur some).

the probe I used had ground clip, I think it had a $$10\Ohm$$ resistance in it too, why is this resistance necessary?

Also the ringing effect seen on the upper part of the square wave, that is caused by the act of measuring the signal? Can you expand a little more on that please?

meant to say:

10 OHM resistance in the probe