# Why does my crystal oscillator spike so much?

I just got a new oscilloscope (Rigol DS1102E) and I'm new to EE in general.

One of the first things I put on the scope was my 3.579Mhz "TKD" crystal oscillator.

What I didn't expect was the spike on each rising and falling edge (hope I said that correctly).

If you look at the attached picture, you will see what I mean.

So, what is that all about? I'm building an audio circuit that uses the NTSC frequency to drive an SN76489. The IC uses the frequency to generate square waves for audio so while my circuit sounds "good enough", seems like spikes like that would cause issues later on?

I don't have any resistors or capacitors on my circuit. Just the OSC out to the CLK pin of the IC.

Also, what's the deal with the scope reporting ~3.597Mhz? It seems to fluctuate in the 3.56 - 3.59 range. However, when I put a 1Mhz crystal on the scope, it shows a nice 1.000Mhz frequency.

• It's spiking because of the inductance of the ground-clip. – Connor Wolf Mar 15 '14 at 0:32
• If your scope probe came with a little metal spring thing that fits over the tip of the probe, try using that for grounding. The lower ground inductance should somewhat ameliorate the overshoot. Here is a question that covers the little ground thing: electronics.stackexchange.com/questions/40420/… . – Connor Wolf Mar 15 '14 at 0:34
• For various physical reasons (the primary one being that its impossible to achieve true "instantaneous" voltage jumping), all square wave generators will have some overshoot, the mathematical reason behind this is that a square wave is actually an infinite series of increasing frequency sine waves, since inductive/capacitative effects are frequency dependent, it affects different frequency components of the square wave differently, shunting of high-f components (no/well-damped fast ringing), and exaggerated overshoot usually indicates stray inductance, as Connor suggested. – crasic Mar 15 '14 at 4:00
• The frequency is jumping for two reasons, the excess ringing can mistrigger the frequency counter on the scope, so you have a measurement error. Otherwise all clocks have some amount of jitter (how accurately they keep their frequency), different crystals have different amount of jitter (like resistor tolerance, it depends on the component product line). Temperature fluctuations also cause the frequency to shift (clock drift). – crasic Mar 15 '14 at 4:06
• @crasic and ConnorWolf your comments look like good answers to me, go ahead and earn those rep points! – DuckTyped Mar 15 '14 at 4:19

## 1 Answer

Most important thing is to compensate your Oscilloscope probe. This may account for the overshoot. - - - Second, Conner Wolf is correct in the ground lead of the probe. I don't know how many times my technicians had problems, and I simply gave them a shorter ground clip lead and magically the ringing disappeared.

• Compensate the probe. Use a shorter ground lead. But also use a 10X probe to reduce the probe's capacitance. – m.Alin May 6 '14 at 9:55