Strange phenomenon or my concept is wrong?

[powersys]

Let's say we hv two signals, P and Q.
The frequencies of P and Q are 1kHz and 2kHz respectively.
I captured both signals using Channel-A and Channel-B, respectively, of an oscilloscope. And, Channel-A is set as the trigger source. Input couplings for both channels are 'DC'.

Questions
[1] Do you think both signals can be displayed STATICALLY on the oscilloscope screen?
[2] If one signal static (i.e. the one captured by the channel which is set to be the trigger source), and another is moving/running, is that normal? If NO, may I know why?

Thanks

 

[hsingarajah]

I think it should capture both because the two signals are harmonics.

 

[rudane]

You should see them staticly, unless some weird phase thing is going on. In fact this is a technique used a lot. If you have a signal that is hard to trigger to, you set up another signal of a similar frequency and lock to it so you can see the first.

 

[powersys]

I think it should capture both because the two signals are harmonics.

Do you mean, if both are SINEWAVE, but one is 1kHz and another is 2kHz, both signals should be captured and displayed STATICALLY on oscilloscope screen?

If signal P is a 1kHz squarewave, and Q is a 2kHz sinewave, will both of them displayed STATICALLY?

Thanks

Added after 11 minutes:

You should see them staticly, unless some weird phase thing is going on. In fact this is a technique used a lot. If you have a signal that is hard to trigger to, you set up another signal of a similar frequency and lock to it so you can see the first.

Kindly note that both signals P and Q have different frequencies. Do you think both signals will be displayed STATICALLY?
Thanks.

 

[FvM]

The non-triggering signal will move, if it's not exactly of the double frequency. The waveform doesn't matter in this respect.

You can measure the frequency error from the beat frequency.

 

[XNOX_Rambo]

[1] Yes
[2] Yes

 

[Siniša]

If ratio between two signals is exactly 2 then yes, both signals should be static on the screen if one of them is properly used to trigger oscilloscope.

On the other hand, if those two sources do not have same source clock, e.g. you are using two signal generators, they will always be small difference in this ratio and you will see signal that is not used for triggering slowly scrolling across the screen. You might be able to get frequency manually for signal to stay on screen for a short time only.
Only if you synchronize both signals in some way, they will be static on the screen.

 

[echo47]

Borrow a scope and two signal generators, and try it!
You'll learn quickly by playing with the instruments, trying various experiments, and thinking about the results you see.
For extra fun, configure the scope so channels A and B go to the X and Y axes.

 

[powersys]

All the while, I thought if two signals with different frequencies will be displayed STATICALLY. I did try measuring both signals using oscilloscope before asking the question here. And, yes, the signal measured by non-triggered source was moving. I thought there was something wrong with the scope. Thanks for all of your comments. I learn new knowledge today, both signals will be displayed STATICALLY if:
1) They have same frequencies
2) The frequency of P is multiple integer of Q, or vice versa.

 

[echo47]

Some scopes have a special sweep mode that triggers each channel individually. If you use that mode, then all channels will appear stable even if the signal frequencies are unrelated. For most measurements, that's not a useful mode.

You can also press the single-sweep button to display one sweep of all channels. That will freeze all the waveforms, assuming it's a storage scope or digital scope.

 

[RF-OM]

This is right for today’s high quality oscilloscopes with good triggering and low frequency signals. If you use old scope with switching channels there maybe another picture. Also when you work with high frequency signals and stroboscopic instruments you may see some alias effect.