It is just a 5KHZ PWM wave form fro a Pic.

The scope is a Rigol DS1102E. What is causing these spikes? Is it actually something from the pic or an artifact of the digital scope? The spikes appear to pulsate a bit.

 

X1 or X10 probe? If X10 probe is it correctly compensated?
The flat top of the waveform is only about 3.9V. Why is it so much the (assumed?) 5V PIC supply. Is anything else connected to the PWM output (other than the scope probe)?

 

Stray wire/trace inductance?

 

X1 or X10 probe? If X10 probe is it correctly compensated?
The flat top of the waveform is only about 3.9V. Why is it so much the (assumed?) 5V PIC supply. Is anything else connected to the PWM output (other than the scope probe)?

It is from a 3V pic. I didn't have the bottom of the waveform positioned at 0.

This is from my calibration port. The difference is the calibration port is at 1K.

1X




And 10x

 

Stray wire/trace inductance?

Well it is it is in a zif socket/ breadboard type deal right now so maybe that is it?

 

A little strange, you would expect rings on every edge like here from simple reactance.

Is there another pin on the PIC toggling at a rate that matches the timing of the PWM spikes?

 

A little strange, you would expect rings on every edge like here from simple reactance.

Is there another pin on the PIC toggling at a rate that matches the timing of the PWM spikes?

The only pin active.

Here is what it looks like close up.

 

It would appear to be resonance from stray circuit capacitance and inductance.
It may seem to pulsate at slower sweep speeds due to the undersampling of the high frequency transient at that sweep setting.

 

@spinnaker
Add a 1k resistor from the output pin to ground and measure again.

 

It would appear to be resonance from stray circuit capacitance and inductance.
It may seem to pulsate at slower sweep speeds due to the undersampling of the high frequency transient at that sweep setting.

That would be my thinking. The leading edge of a pulse waveform contains an infinite number of odd harmonics (think high frequencies here). You are working with a circuit on a breadboard type affair which due to its nature will induce all sorts of capacitance, inductance and assorted parasitic coupling effects. Should you wish to try something interesting while observing the ringing try and touch a few exposed terminals of any components, see what the ringing does with just a finger touch.

You can try as Gopher suggested terminating the signal into a few resistive loads which may smooth the ringing a little but I doubt it will go away.

Ron

 

@spinnaker
Add a 1k resistor from the output pin to ground and measure again.

Still there.. Like I mentioned it is a zif socket and breadboard.

 

The 1 K does decrease it a couple of milli volts. Touching has no effect with or without resistor.

 

Try a 10:1 probe with a very short ground connection to the probe (such as a short wire wrapped around the probe ground tip to the ground nearest the signal output).

 

With 10X scope set to 10X probe. I am not too worried about this. I do not think it will have any effect even if it still shows up on the PCB version. I don't think it will have any ill effects for my purpose. Just wanted to be sure that it was not my scope. And curious as to what is causing it and perhaps how to fix it if possible.

 

If you try the short ground lead on the 10:1 probe you may find that the ringing is significantly reduced.
If that is the case, then the ringing is due to to ground wire impedance.

 

The only pin active.

Here is what it looks like close up.

View attachment 137712

Looks like a normal 'transmission line' high frequency reflection (ringing).
That why it's useful to have a good analog scope as a reality check when you see something strange, maybe the missing signal at some edges is an artifact of the digital scope.

 

That why it's useful to have a good analog scope as a reality check when you see something strange, maybe the missing signal at some edges is an artifact of the digital scope.

You don't have to keep an analog boat anchor around for that, just stay within the specs of your digital scope and know its limits.

Artifacts on a digital scope means the user didn't pay attention to the latter.

 

You don't have to keep an analog boat anchor around for that, just stay within the specs of your digital scope and know its limits.

Artifacts on a digital scope means the user didn't pay attention to the latter.

My boat anchor analog (TEK2465DM 400MHz BW) has much better spec's than most of the affordable digital scopes on the market, I'll keep it.

 

You don't have to keep an analog boat anchor around for that, just stay within the specs of your digital scope and know its limits.

Artifacts on a digital scope means the user didn't pay attention to the latter.

Ha, boat-anchor. I’d call most of the digital scopes around here, boat anchors.

 

I ended up selling my analog scope. Takes up too much space. And mostly when I realized I would need to replace the filter caps someday that was just too much for me. On a previous repair where I was lucky to get it working, I noticed how compact everything was. The PSU especially. Scared the heck out of me!