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Sine wave blues.

Question asked by ScopeDope on Jul 2, 2018
Latest reply on Jul 4, 2018 by ScopeDope

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Hello Forum,

I have come across an issue while attempting to view two sine wave outputs simultaneously, sort of. The key players are as follows:

Output 1

CH1 - Generic fleaBay GPSDO 10Mhz oscillator.

CH2 - MSOX-2024A WaveGen

I have adjusted the Vpp level for CH2 so that it is near / at the same Vpp level as the GPSDO source. Both inputs are running at HighZ. No matter what combination of triggering, delay, tongue angle or moon phase I am unable to get the built in WaveGen to display a nice sinusoidal wave form (see attachments). It gets interesting from here if I remove the source CH1. Even terminating both inputs at 50Ω does not solve the issue.

Output 2

CH1 - MSOX-2024A WaveGen

CH2 - Rigol AWG (DG1022)

In this setup I am able to view both sine waves without issue and visually see the difference as the phase shift marches back and forth. This has me stumped, but not defeated. Am I missing something key here? I would really like to utilize the GPSDO as a reference wave form due to the stability and accuracy of the output.

Any assistance is most appreciated.

Regards,

Chris

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algoss

Jul 3, 2018 6:44 AM

I don't see a problem with either picture. The difference has to do with the ability of a scope to capture and display data quickly. 'Output 1' shows thescope that can capturing and displaying a lot more screen updates quickly. It is showing about fifty fast acquisitions on the screen at once, and the green trace is moving across the screen, due to phase shift. The built-in persistence, even set at 'off' or minimum, will show multiple waveforms overlaid. If you switch to 'Single' mode and hit the Single button as fast as you can, will see the relative locations of the waveforms changing, too.

I don't know why one waveform looks more stable than the other, but they are both accurate.

BTW, you should almost never use HighZ inputs or outputs. Most outputs are 50 Ohm, the cables are 50 Ohm, so the input should also be 50 Ohm. HighZ -> 50Ohm will give you bad results. 50 Ohms -> HighZ will give you bad results. Be consistent. That said, there are times when you might want to use HighZ inputs. You might have two inputs that you want to connect to a 50 Ohm output. In that case, you might set one of the inputs to HighZ and the other to 50 Ohm, so the output sees a 50 Ohm load.

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ScopeDope @ algoss on Jul 3, 2018 8:54 PM

Thanks for the response Al. The 50 Ohm / HighZ info has been filed for future reference as I had pondered this as well.

Have a safe 4th Hoilday.

Chris

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Fungible Jul 3, 2018 7:07 AM

Two things to keep in mind here. First, the scope can only trigger on one signal at a time. Second, while the scope is running, two un-synchronized signals will not display cleanly unless they happen to be (very) close in frequency. Only the one you are triggering on will display cleanly.

In output 1 you are triggered on chan 1 and it is displayed cleanly. Chan2 is not. But look closely at the pattern in chan2. There are sine waves in there. They are just not on top of each other so it looks smeared.

In output 2 you are also triggered on chan1 and it is displayed cleanly. But so is chan2. However, I bet chan 2 is walking across the screen. This is because the source in output 2 just happens to be closer in frequency to chan1 than was the source in output 1. Remember, the sources are not perfect. Just because you programmed it to 10KHz does not mean you got 10.000000000MHz.

Try pressing single. That will take one acquisition of the two signals and they will both be cleanly displayed. That will remove the “smear” in output 1.

Try adjusting the frequency of one of the sources. If the resolution of the source is good enough, you should be able to control the smearing. The un-trigger signal will be smeared, then slowly walk across the screen when close to the triggered signals frequency and then smear again as you continue to change the frequency.

Do a web search on frequency beating.

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ScopeDope @ Fungible on Jul 3, 2018 9:03 PM

Clear, concise and well said. I just acquired a Rubidium standard today (Stanford Research) and I will pair it up with the GPSDO on the scope to see how well the two sing in harmony.

Having acquired this oscilloscope just keeps peeking my interests in electronics. It is a real benefit to have this forum for the questions that come up from time to time about the Keysight scopes.

Have a safe 4th Holiday.

Chris

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ScopeDope @ ScopeDope on Jul 4, 2018 6:21 PM

As a follow up I was able to bring my new Rubidium standard online and compare it with the GPSDO. Wow! Having set the Rubidium as the trigger source the GPSDO seemed content to drift ever so slightly. For me it was quite satisfying to see the Delay measurement step down, slowly, from ns, ps and then 0 for a short lived Phase alignment.

Thanks again for the assistance on the original forum inquiry.

Regards,

Chris

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5 Replies

algoss Jul 3, 2018 6:44 AM
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I don't see a problem with either picture. The difference has to do with the ability of a scope to capture and display data quickly. 'Output 1' shows thescope that can capturing and displaying a lot more screen updates quickly. It is showing about fifty fast acquisitions on the screen at once, and the green trace is moving across the screen, due to phase shift. The built-in persistence, even set at 'off' or minimum, will show multiple waveforms overlaid. If you switch to 'Single' mode and hit the Single button as fast as you can, will see the relative locations of the waveforms changing, too.

I don't know why one waveform looks more stable than the other, but they are both accurate.

BTW, you should almost never use HighZ inputs or outputs. Most outputs are 50 Ohm, the cables are 50 Ohm, so the input should also be 50 Ohm. HighZ -> 50Ohm will give you bad results. 50 Ohms -> HighZ will give you bad results. Be consistent. That said, there are times when you might want to use HighZ inputs. You might have two inputs that you want to connect to a 50 Ohm output. In that case, you might set one of the inputs to HighZ and the other to 50 Ohm, so the output sees a 50 Ohm load.
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- ScopeDope @ algoss on Jul 3, 2018 8:54 PM
  Mark Correct
  Correct Answer
  Thanks for the response Al. The 50 Ohm / HighZ info has been filed for future reference as I had pondered this as well.
  
  Have a safe 4th Hoilday.
  
  Chris
  Like • Show 0 Likes0
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Fungible Jul 3, 2018 7:07 AM
Mark Correct
Correct Answer
Two things to keep in mind here. First, the scope can only trigger on one signal at a time. Second, while the scope is running, two un-synchronized signals will not display cleanly unless they happen to be (very) close in frequency. Only the one you are triggering on will display cleanly.

In output 1 you are triggered on chan 1 and it is displayed cleanly. Chan2 is not. But look closely at the pattern in chan2. There are sine waves in there. They are just not on top of each other so it looks smeared.

In output 2 you are also triggered on chan1 and it is displayed cleanly. But so is chan2. However, I bet chan 2 is walking across the screen. This is because the source in output 2 just happens to be closer in frequency to chan1 than was the source in output 1. Remember, the sources are not perfect. Just because you programmed it to 10KHz does not mean you got 10.000000000MHz.

Try pressing single. That will take one acquisition of the two signals and they will both be cleanly displayed. That will remove the “smear” in output 1.

Try adjusting the frequency of one of the sources. If the resolution of the source is good enough, you should be able to control the smearing. The un-trigger signal will be smeared, then slowly walk across the screen when close to the triggered signals frequency and then smear again as you continue to change the frequency.

Do a web search on frequency beating.
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- ScopeDope @ Fungible on Jul 3, 2018 9:03 PM
  Mark Correct
  Correct Answer
  Clear, concise and well said. I just acquired a Rubidium standard today (Stanford Research) and I will pair it up with the GPSDO on the scope to see how well the two sing in harmony.
  
  Having acquired this oscilloscope just keeps peeking my interests in electronics. It is a real benefit to have this forum for the questions that come up from time to time about the Keysight scopes.
  
  Have a safe 4th Holiday.
  
  Chris
  Like • Show 0 Likes0
  Actions
  - ScopeDope @ ScopeDope on Jul 4, 2018 6:21 PM
    Mark Correct
    Correct Answer
    As a follow up I was able to bring my new Rubidium standard online and compare it with the GPSDO. Wow! Having set the Rubidium as the trigger source the GPSDO seemed content to drift ever so slightly. For me it was quite satisfying to see the Delay measurement step down, slowly, from ns, ps and then 0 for a short lived Phase alignment.
    
    Thanks again for the assistance on the original forum inquiry.
    
    Regards,
    Chris
    Like • Show 0 Likes0
    Actions

Sine wave blues.

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