Dear all,

I just purchased a second hand Hameg HM203-4 Oscilloscope on Ebay.
When I received it I connected it to my function generator to test it measuring different wave types.

It seems to measure correctly most of the wave forms but something doesn't seem right, see the following images:



Please note that it's like 'dots' forming the signal instead of a continuous line, you can especially appreciate this on the sine and saw waves. On the other hand, on the square wave, the vertical lines are faded - they aren't as bright as the horizontal lines.

So is this normal for an analog oscilloscope? If not, why could this be happening?

All serious answers are very much appreciated.
Thanks a lot!

 

What is the make and model of your function generator?

 

Hi MrChips,

Thank you for your reply.

The function generator I'm using is a small digital oscilloscope that has an integrated function generator, to be honest, it's a cheap device I got online, I attached some pictures:



Do you think the issue could be the function generator?

 

Hi MrChips,

Thank you for your reply.

The function generator I'm using is a small digital oscilloscope that has an integrated function generator, to be honest, it's a cheap device I got online, I attached some pictures:

View attachment 148246 View attachment 148247

Do you think the issue could be the function generator?

Yes. The function generator is synthesizing the sine and triangular waveforms digitally.

 

What about the square sine wave? Is it normal for the scope to show the vertical lines more faded than the horizontal ones?

Will all digital function generators produce this effect on analog scopes?

 

Starting with your square wave display at a glance it looks like your probe compensation needs adjusted. Where the probe connects to the vertical inputs there should be a small hole and screwdriver adjustment on the probe cable. Compensation adjustment should be covered in your manual. Here are a few examples:

Over Compensated:


Under Compensated:


Normal Compensated:


As to the dots the last time I saw dots like that was an old sampling scope and you do not have a sampling scope. You may want to look at your vertical trace mode as to alternate and chopped mode. When running faster sweep speeds you want Alt and slower sweep speeds you want Chop. When running a fast sweep if chop is inadvertently selected you will see some of the chop action. The dots are unusual if you only have one vertical channel selected. Some scopes do allow external Z axis blanking but unless the Z axis intensity is maladjusted that should not be a problem.

I would start by getting your square wave square with probe compensation adjust and then move from there.

<EDIT> I see while I typed you had some replies. </EDIT>

Ron

 

#1 problem

1. Use the 0.2 V cal pin on the scope.
2. For this step the probe must be other than x1; x1 won't work.
2. Adjust probe compensation (small pot on probe or connector) until display is square.

 

What about the square sine wave? Is it normal for the scope to show the vertical lines more faded than the horizontal ones?

What you are observing is related to the persistence of the phosphor of the oscilloscope screen. The brightness of the trace is directly related to the length of time the dot stays on one spot on the screen.

The electron beam moves across the screen in the horizontal direction at a constant speed. Hence if you look at the square wave trace, the length of time the beam spends drawing the vertical traces is a tiny fraction of the time that it spends moving across one screen division. Hence you can hardly see the vertical traces. If you were to turn up the beam intensity you will see the trace.

In normal usage, you do not want the trace intensity to be too high to avoid burning the phosphor on the screen. For the same reason, turn off the power on the scope when you are finished using it.

Will all digital function generators produce this effect on analog scopes?

It depends on the make, design, and quality of the function generator. You can reduce the appearance of the dots by filtering the signal with a low-pass filter. If you decrease the Horizontal Sweep Time (TIME / DIV) you will be able to measure the spacing of the dots. From this you can determine the clock frequency of the internal digital to analog converter (DAC). You want a low-pass filter with a cut-off frequency about 2 to 10 times below this frequency.

 

Hi guys,

Thank you for your help and you're valuable experience! I will try the adjustments you've suggested and come back with the result!

Have a good day/afternoon

 

For the dots, make sure all the buttons under the input attenuators are out. This will select channel 1 with no chop or altrnate. The dots could be caused by the chop function.

 

I got a nice scope the probes where not set right but If you had a 5 vote ac wall wart have a look at that.

But with mine I found the book and set it with instructions it has a pin you hook the ground to and probe.

It works great after tuning it up. I had to set the cap mine is on the plug end of the probe.

Nice looking scope.

 

First of all, sorry for taking so long to reply, I've been rather busy.

So I just adjusted the little pot on the probe and now my square signal is looking pretty good:



On the other hand, regarding the continuity of the sine and triangular waves, it definitely was the function generator...

I measured the main AC voltage transformed down to 30V AC and I got a continuous sine wave (please don't pay attention to the noise, it was just for testing purposes ):



So the good thing is I know the scope is decent and working fine!

Thank you all for your help!!