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Unexplained Phase Shift on CH2 of MSOX3024A Oscilloscope

Question asked by glowitz on Feb 10, 2014
Latest reply on Feb 10, 2014 by glowitz
I just upgraded to the latest v. 2.35 3000-series firmware for my MSOX3024A and noticed an unexpected phase shift issue on channel 2. It's possible this has nothing to do with the firmware, but I don't recall seeing this issue previously. Or, I may have something misconfigured, but it's not obvious what that could be and welcome your ideas. Also, if any of you have the MSOX3024A, it would be helpful to repeat this test setup to see if the issue can be duplicated.

*TEST SETUP*

1) Using an Agilent 33522A Dual Channel Arb/Function Generator set to TRACKING mode, I program a 2 MHz sinewave, 1V P-P on each output Channel. Phase offset is set to zero on both channels and I have independently verified this on a my handheld U1602A scope.

2) CH1 of the ARB is connected to CH1 input on the MSOX3024A, and CH2 of the ARB is connected to CH2 input on the MSOX3024A. I have verified that the input channel configuration for Ch1 and Ch2 is identical and that no bandwidth limited is set. Note that BW limiting would introduce a phase shift, as well as possible amplitude shift at certain frequencies.

3) The trigger output of the ARB (front panel) is connected to the EXT TRIG input of the MSOX3024, which is set for triggering in the center of the screen.

4) When comparing CH1 to CH2 of the scope, there is a major phase shift between the two waveforms, which I do not see on my other scope. This phase shift ONLY exists between CH1 and CH2 of the MSOX3024. As I vary the frequency of the test signal up or down from the 2MHz test, the phase shift moves rapidly. At 5MHz, for example, there is a 180 degree phase rotation on CH2 compared to CH1 and so forth. There is no change in amplitude, which remains consistent. Refer to the photo for the 2MHz test case showing phase shift.

5) To validate my findings, I move the CH2 signal to CH3 of the MSOX3024. The phase shift issue is gone. It's perfect (See Photo).

6) I then move it to CH4 of the MSOX3024 and the phase shift is also gone.

7) As a final test, I only use CH3 and CH4 of the scope to see if there is some interaction between adjacent channels. But no -- as expected -- both sine waves have zero phase delay relative to the trigger -- it's perfect.

8) In summary, I only see this phase shift issue between CH1 and CH2 of the scope. All other aspects of the signal seem perfect. Amplitude. Frequency. Noise levels. And the phase alignment is perfect across channels 1, 3 and 4. Something is going on with CH2 -- but I don't know what it is. It's as if it's not getting the trigger sync -- or there is a delay in the trigger sync on that one channel.

One thing I'll double check is whether there could be a trigger delay associated with CH2 that is set unwittingly. This issue seemed to occur following the firware update and I would have thought that any other internal settings would be reset to some baseline, particularly after hitting "AUTOSCALE" mode to acquire the waveforms. The more I think about my problem description, the symptom sounds like there is a trigger delay on CH2 that I din't expect. I'll check and report back if I've resolved my own question. Meanwhile, here are traces from the scope in the attached files.  

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