Dual Trace Oscilloscope and Time/ Div. question

Thread Starter

PeteHL

Joined Dec 17, 2014
409
You can connect for example the output of a sine wave generator to both CH1 and CH2 input jacks of the oscilloscope and see both traces simultaneously on the screen of the CRO over a wide range of settings of Time per division of the scope.

Vertical mode and normal trigger of the scope are set at dual and CH1.

However, when I connect the output of a tone burst generator to both CH1 and CH2 input jacks of the CRO, if Time/ Div. is set at less than 1 millisecond, then what happens is firstly only the CH1 trace appears on the screen, then secondly only the CH2 trace appears on the screen, and then a repeat of one and then the other trace separately on the screen.

The burst generator is (Siegfried) Linkwitz's cosine burst generator, which is ten half-waves in a burst at whatever audio frequency a sine wave generator at input to the burst generator is set.

It would be more convenient if both traces would appear simultaneously on the screen as I want to use the scope to measure any time delay effected by circuitry inserted between the burst generator and the CH2 input of the scope.

Time/ Div set at less than 1 millisecond is better as I anticipate a delay of about 0.5 ms and possibly less.

The two traces not appearing simultaneously on the screen would be acceptable if both traces start at exactly the same position on the screen. But even so, I am curious as to why the "split" occurs when Time/ Div. is made less than 1 ms.

Thanks for your help with this puzzle where possible,
Pete
 

crutschow

Joined Mar 14, 2008
30,806
Sounds like your oscilloscope displays each channel on alternate sweeps.
Is this an analog 'scope?
The may may be an option switch that controls that (such as Alternate or Chopped).
 

MrChips

Joined Oct 2, 2009
27,141
That is the way a CRO works.

A dual trace oscilloscope has only one electron beam. You do not get two traces simultaneously. Only a true dual beam oscilloscope can do that (or a digital oscilloscope).
There are two modes a CRO can be used to give you the appearance of dual traces, Alternate mode and Chopped mode.

In Alternate mode the single electron beam sweeps across for CH1 and then sweeps across for CH2 on the next trigger event. This is what you are seeing.

In Chopped mode, the electron beam is time-shared back and forth between CH1 and CH2 at a faster rate. This way you will obtain simultaneous display of CH1 and CH2.

Try setting your scope to CHOPPED mode.

What is the make and model number of your oscilloscope?
 

Thread Starter

PeteHL

Joined Dec 17, 2014
409
Sounds like your oscilloscope displays each channel on alternate sweeps.
Is this an analog 'scope?
The may may be an option switch that controls that (such as Alternate or Chopped).
This is a cathode ray oscilloscope, so yes, an analog 'scope.

Reading the manual for it, I had previously underlined that the 'scope by default is in ALT triggering for Time/Div. set at less than 1 mS/ div. At longer intervals it switches to Chopped.

Thanks,
Pete
 

Thread Starter

PeteHL

Joined Dec 17, 2014
409
That is the way a CRO works.

A dual trace oscilloscope has only one electron beam. You do not get two traces simultaneously. Only a true dual beam oscilloscope can do that (or a digital oscilloscope).
There are two modes a CRO can be used to give you the appearance of dual traces, Alternate mode and Chopped mode.

In Alternate mode the single electron beam sweeps across for CH1 and then sweeps across for CH2 on the next trigger event. This is what you are seeing.

In Chopped mode, the electron beam is time-shared back and forth between CH1 and CH2 at a faster rate. This way you will obtain simultaneous display of CH1 and CH2.

Try setting your scope to CHOPPED mode.

What is the make and model number of your oscilloscope?
Thanks for that explanation. My CRO is an Instek OS-622B (about 20 years old now). Reading the manual it does have the controls allowing for selecting ALT or Chopped mode with respect to any Time/ Div. setting.

In Chopped Mode, how quickly is the beam switching between CH1 and CH2 traces?

Thanks, Regards,
Pete
 

MrChips

Joined Oct 2, 2009
27,141
Thanks for that explanation. My CRO is an Instek OS-622B (about 20 years old now). Reading the manual it does have the controls allowing for selecting ALT or Chopped mode with respect to any Time/ Div. setting.

In Chopped Mode, how quickly is the beam switching between CH1 and CH2 traces?

Thanks, Regards,
Pete
I don't have an answer for that. Try Chopped Mode and see if that resolves your problem.
 

crutschow

Joined Mar 14, 2008
30,806
If you increase the sweep speed in the chopped mode, at some point you will start to see the chopping between the two traces.
From that, and knowing the sweep speed, you can calculate the chopping frequency
 
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