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Anyone who can tell me how Rise Time affect signal sampling

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Anyone who can tell me how Rise Time affect signal sampling

Postby layfolk » Wed May 18, 2016 8:00 am

Hi everyone,

Is there anyone who can tell me the effect of Rise Time on the acquired signal. I noticed that the Rise Time is different among Picoscope product. Would it happen that the bandwidth matches the requirement but the rise time need extra performance?

And what does "Calculated 10% to 90%" mean as in the datasheet.

Thanks. :D
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Re: Anyone who can tell me how Rise Time affect signal sampl

Postby Gerry » Thu May 19, 2016 2:09 pm

Hi Layfolk,

To answer your questions:

1) The smaller the rise time is for a PicoScope, the more vertical the edges and sharper the corners will be on a captured high frequency square wave. Bandwidth is not independent of Rise Time, and their relationship is expressed as:
Bandwidth = 0.35/Rise Time
or
Rise Time = 0.35/Bandwidth
where Rise Time is expressed in Seconds and Bandwidth in Hertz, or the equivalent multiple/derivative of the units (e.g. mS and kHz, uS and MHz, nS and GHz, etc).
There is a good article showing why this is true with a simple mathematical model here: http://www.interferencetechnology.com/r ... lications/

So, theoretically, the rise time and bandwidth are inversely proportional to each other. However, in practice they tend to relate, independently, to the criteria used when looking for a PicoScope for very specific purposes/applications. Bandwidth, i.e. the maximum signal frequency that can be captured tends to be important for considering the limits of analogue based signals (e.g. RF signals), while Rise Time is a criteria that is more useful when considering the limits of digital signals (e.g. clock signals and pulses).

2) The 10% and 90% references are the proportions of maximum height of the signal that represent the start and end of the rising edge measurement. The rising edge can also be specified as starting at 20% and finishing at 80%.

Regards,

Gerry
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Re: Anyone who can tell me how Rise Time affect signal sampl

Postby layfolk » Fri May 20, 2016 3:06 pm

Hi Gerry,

Thanks a lot for your detail reply. I was asking the previous question, because I faced some problem with Picoscope.

Currently we are using Picoscope 5444B. We use its all 4 channel to send and collect signal. The current setting are: Div 100us, 500mV, 12bits hard resolution and 14bits enhanced resolution.

To check the effect of the sampling rate on the signal, I put the sampling rate to be 31.25M, 62.5M, and 125M Hz. My finding was:
1. If the sampling rate is 31.25M, both the 2M and 4M Hz components are missed in the signal.
2. if the sampling rate is 62.5M, the 2M Hz compoents can be captured, with amplitude 0.16
3. If the sampling rate is 125M, both the 2M and 4Hz components will appear in the signal. But the 2M amplitude increases to 0.24.

According to the datasheet, 4M component shall be obtained without any loss even with 31.25 M sampling rate. Can you help me to fix this problem or provide any explanation for this phenomenon.

Thanks again. :)
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Re: Anyone who can tell me how Rise Time affect signal sampl

Postby Martyn » Fri May 20, 2016 9:01 pm

If you can post some psdata files showing what you are doing we will be able to replicate your settings here.
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Re: Anyone who can tell me how Rise Time affect signal sampl

Postby layfolk » Mon May 23, 2016 2:01 am

Martyn wrote:If you can post some psdata files showing what you are doing we will be able to replicate your settings here.


In the previous setting, I take 12bits hard resolution, and 14bits enhancement resolution. Now, I remove the enhancement resolution, that's 14bits hard reslution and 14bits enhancement resolution. I can see clearly the 4MHz signal even with 18Mhz. I think the smoothing effect was the cause the problem.

Thanks.
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Re: Anyone who can tell me how Rise Time affect signal sampl

Postby Gerry » Tue May 24, 2016 8:46 am

Hi Layfolk,

The enhanced resolution mode feature performs a kind of moving average filter on the waveform, which will filter out high frequency noise, but also high frequency signal. If you are going to use enhanced resolution mode you need to be aware of exactly what effect the filtering will have on your data. There is a good explanation of that on our web page about Enhanced Resolution here: https://www.picotech.com/library/oscill ... nhancement

Regards,

Gerry
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