Scope does not trigger

[koolnaal]

Hello,

I am trying to trigger a scope at a certain point. I have a test signal coming after a regular sinusoid. I want to measure the output when the test signal arrives. What happens is that when my test signal comes after 1 second the trigger works perfectly fine but when my test signal comes after 3 seconds the scope does not trigger at the point I want and keeps on triggering at unwanted points. This is a conceptual question. Could anyone please help me in that.

Thanks,
Krunal

[Jesse_O]

Hello Krunal,

I am not exactly sure what your external signal looks like, however I can suggest a couple things for you to look at.  First, if the absolute value of your sine wave amplitude is less than the test point you want to measure, the scope can simply level trigger at that point.  It should not matter if the signal comes in after 1 seconds or 3 seconds.  The only problem you might be seeing with this is a timeout error.  This is caused by the read function not being able to sample enough data within the default 5 seconds limit.  Adjusting the timeout would help if you are getting this error.

If your test signal value you need to trigger off of is within the absolute value of your sine wave signal it becomes more difficult because if you specify to trigger off of a rising edge 0.5V signal this point will occur many times before the ideal trigger point.  Perhaps a combination of using the trigger hold off and trigger delay might help you time the start of your acquisition a little better.  If this does not help, can you please post more detail about the sine wave, your test point you want to trigger off and the behavior you are currently seeing.

Regards,

Jesse O.
Applications Engineering
National Instruments

[koolnaal]

Hey Jesse,

I am generate signals required for the line drop out test and I feed the generated arbitrary waveform through Labview into the waveform generator 33120A. I have two cases

(1) Drop out occurs every 1 second

(2) Drop out occurs every 3 second

In order to trigger the scope to start the output waveform acquisition I have used the sync pulse  which is generated  when the first data point of the arbitrary waveform is outputted.

The 1 second case works perfect and I can even see the sync pulse but for the 3 second I get multiple triggers even at unwanted places which does not allow me to see a triggered same waveform.

Can you tell me what might be the reason for this.

Thanks,

Krunal

[Jesse_O]

Hello Krunal,

Thank you for the extra information.  The things I would want to look at next are the sync pulse to see if it is behaving as I would expect.  Are you using an NI Scope, or do you have a scope that you are controlling through GPIB like I believe you are with the 33120A?  What is the model number of your scope?

Also are you able to verify the sync output by running it into an analog input channel on the scope?  If you are using an NI-Scope the Soft Front Panel, it has measurements that would be able to determine the frequency of the input.  If the input is just a square pulse that occurs every 3 seconds than you should be able to measure an input frequency of 1/3 Hz. 

Also, what input is the sync pulse going into to?  Is it a TTL pulse, and if so is it going into a EXT Trig line or PFI line on the scope (if it is an NI Scope)?

Regards,

Jesse O.
Applications Engineering
National Instruments