Electronics > Beginners

testing square waves rise time with Rigol scope

(1/5) > >>

festus:
Hi, I have a few question about testing square waves rise time with new Rigol scope.

First question, why does the rise time measurement change as I turn the horizontal scale clockwise?  I understand if the time base is too slow for measurement, but is what I am seeing-

(above, it reads 240ns to 250ns)


(this one reads 208ns)


(this one reads 192ns)

A similar question, why does the Freq readout start reading (****) just because I turn the timebase clockwise?

saturation:
You have more resolution with the faster timebase, the scope can provide more accurate measures.

The Rigol also measures what is on the screen, so what it can see, it can measure.

You don't have the entire cycle so frequency cannot be calculated, i.e., the fall time is not visible.

festus:
OK, I understand, sort of like using a 6000 count meter instead of a 2000 counts meter.  More resolution.  Can I assume, then, that the highest resolution number is the most accurate?  In my example, the 192ns is closer to the true rise time than the others?

For the freq measurement, I just assumed that the scope would continue to display the previous freq unless it detected a change.

scrat:
Just because yo umention it... I find the frequency measurement on the Rigol a bit tricky, since it seems to measure based on the points displayed. This is what happens: if I try to measure a certain square wave's frequency, the thing that seems best to me is to average on many periods, so to use a large time base. Over a certain base, the rising/falling edges on screen confuse and graphically "overlap", and at that point the measure doesn't work any more. It sounds strange to me, since the memory contains far more points than what is on screen, and I expected the measure was calculated based on all the memory acquired.

Is this normal? IIRC this is not the behaviour of higher end scopes...

However, maybe just to overcome this issue, there is a counter, which I expect is a sort of added hardware, which works fine, no matter what the time base is.

festus:
Hi scrat,

Can you expand upon that -- "However, maybe just to overcome this issue, there is a counter...."
What counter you referring to?

In my 1-day experimentation with the scope, I know what you mean about freq measurements.  As you reduce the number of periods on screen, you can definitely get a bogus freq displayed on the screen.

Navigation

[0] Message Index

[#] Next page