Electronics > Beginners
Two Oscilloscopes Connected to the Same Input Source Showing Differing Traces
rman:
Hello. My first posting. I'm very much a novice and I don't know how to interpret some results. I recently purchased a Tektronix 2215 60MHZ 2 channel scope and a Tektronix 2246 100MHZ 4 channel scope. I don't know when either scope was calibrated last, but both seem to function reasonably well. I'm only going to keep one of the scopes. I connected each scope to the same function generator set to input a sine wave at the same frequency and amplitude to compare the resulting trace on each scope. The Vertical and Horizontal parameters were set the same so I expected to see the same looking trace on each scope. However, the 2215 was showing 4 complete sine waves while the 2246 showed only 3 complete waves. The peak to peak voltage was the same. So, don't know if I have a problem with one of the scopes. The peak voltage was a bit over 1V, but I didn't check what the frequency was set to, but seems to me both scopes should be showing the same trace. Right?
Circlotron:
Most scopes have a knob to adjust the horizontal sweep rate to any desired in-between rate besides the 1-2-5 stepped selection knob. Maybe this first knob is not right at the "cal" position?
ArthurDent:
Have you checked to see that the timebase on both scopes are set to the same sweep rate and both are in the calibrated position?
Brumby:
SNAP!
The two posters above beat me to saying the same thing.
Check the variable time base control is in the "CAL" position - on both scopes.
DDunfield:
Make sure both scopes:
- Have exactly the same number of graph divisions across the screen.
- Are set to exactly the same horizontal sweep rate
- If there is a variable sweep control that it is set to "Calibrate" which means exactly the rate on the horizontal sweep control.
- Are set to trigger in exactly the same position on the waveform.
Any of the above can make the traces look different.
If the distance between the peaks is the same on both scopes (as measured on the graph) then they are both showing the same horizontal timebase and most likely the problem is that the scope is being triggered at a different point in the waveform.
If the peaks are NOT the same distance apart, then the horizontal sweep rate is different.
If you know the precise frequency of the signal, then you know the expected time between peaks and you can work out which scope is "wrong" ... then it's just a matter of seeing how it's set differently (or possibly if there is an actual fault).
Dave
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