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Picoscope 2205 post-trigger delay questions

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Picoscope 2205 post-trigger delay questions

Postby picopico » Tue May 04, 2010 9:15 am

Hi, the programmer's guide states that the post-trigger delay can be set to -100% to 100% of the requested number of data points. Does this mean that the post-trigger delay is dependent on the number of samples? Is it not possible to set the post-trigger delay to a specific amount of time, say 10us?

I have to sample a periodic "echo" signal that looks like the one on the attached picture. Since I need to sample at a high sampling rate, I need to use block mode. However, since the number of samples is limited, and I have to take a large number of sample points, I thought if I could try to do this: I would collect multiple blocks of data, the first block would start at the trigger event (the large echo), and then each block would be set to start where the block that preceded it ended by setting the post-trigger delay. Is it possible to do this?

I'd really appreciate any help/suggestions. Thanks!!!
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Re: Picoscope 2205 post-trigger delay questions

Postby Robin » Tue May 04, 2010 12:02 pm

Hi

You need to specify the trigger delay as a percentage. In your application, you know the sample rate and number of samples, so you can calculate the percentage delay that corresponds to a particular time, if that's what you want.

I'm not sure I follow the second point, but if you are using block mode with triggering, each block will contain a trigger event (or will be the result of auto trigger timeout). If you are using block mode there will always be gaps in between blocks. These are due to the time taken for data to be transferred from the scope's memory to the PC.

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Re: Picoscope 2205 post-trigger delay questions

Postby picopico » Wed May 05, 2010 2:59 pm

Hi Robin! It seems that I forgot to place my image file from before. The image is the waveform that I have to process. The waveform is periodic, with the first big "pulse" to serve as the trigger event for data collection in the picoscope 2205.

The waveform needs to be sampled at a high sampling rate, which is achievable only by block mode. However, I need to take a lot of samples, which would mean that I have to do some kind of software trick to make sure that I would be able to somehow "loop" multiple block transfers.

I can't use multiple consecutive block transfers due to the overhead which will cause gaps, right? That's why I thought instead to collect the first block of data right after the trigger event (the first big pulse), then collect the next block of data starting from where the first block ended (using a post-trigger delay), and so on. I would then just piece the blocks together to form the entire waveform.

My problem is that the trigger delay is set as a percentage of the samples, instead of a definite amount of time. Would it still be possible for me to process the waveform in the way that I have explained?
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Re: Picoscope 2205 post-trigger delay questions

Postby Robin » Thu May 06, 2010 4:15 pm

Hi

In block mode, the number of continuous samples is limited by the scope's memory.

There is no way to collect several blocks without having a small gap between them.

If you need to collect a block larger than 16 kS, then you need a scope with more memory.

Regards

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Re: Picoscope 2205 post-trigger delay questions

Postby picopico » Sun May 09, 2010 3:09 am

Hi again, Robin! I'm aware that collecting several continuous blocks would have a small gap in between. That's why I'm trying to use post-trigger delays, so that I can collect the parts of the waveform only whenever the scope is re-triggered again since the echo waveform is periodic (period of 300 Hz). For example, in period 1, I'll collect the first 16k samples of the waveform starting from the trigger event. On the 2nd period, I'll collect the next 16k samples by applying a post-trigger delay. For the 3rd period, I would start where the last sampling ended by applying a longer post-trigger delay. The process will continue until I get about, say 160k samples. Then I can just piece together the samples in my software to get the entire waveform.

My problem is how to be able to use the post-trigger delays properly so that I can achieve the above solution.
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Re: Picoscope 2205 post-trigger delay questions

Postby Robin » Mon May 10, 2010 12:21 pm

Hi

Sorry, I misunderstood.

I'm still not certain that I follow. Are you saying that you want the scope to trigger once and then collect several blocks depending on the delay that you specify? If so, this is not possible. If you have triggering enabled, each block will be the result of a different trigger event.

Whatever you are trying to do, the trigger delay will always be limited to the range -100% to 100% of the requested number of samples.

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