I need a basic education on the scope

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I need a basic education on the scope

Postby gogetta » Sun Mar 23, 2008 11:24 am

please excuse my ignorance

where is the bottleneck in the system and why do you need a large buffer?

I would have thought that the scope is running at its maximum sample rate all the time (20MS/s)....the data must then move to the PC and be displayed or stored in PC RAM...I would have thought that USB 2.0 (480 Mbps) would be fast enough to handle the data output of the oscilloscope, because if the scope samples at 20MS/s and at a resolution of 12bits that equals 240Mbps which is well within USB 2.0 speed.

Now what happens if u are using USB 1.1 (12Mbps) obviously the PC cant receive data fast enough. I still cant see the point in having the buffer (its only 512K), if the scope samples at 20MS/s its going to fill the buffer in .025 of a second...how is this going to be of any use in an automotive situation like measuring ignition waveforms??





I have no idea of what the buffer does
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scope sampling

Postby Autonerdz » Sun Mar 23, 2008 5:24 pm

Not sure what you mean by bottleneck but a large buffer is needed to maintain robust sample rates at the longer time bases. This is very well suited to automotive.

In a nutshell:

The 20MHz ADC is shared so 20MHz at one channel, 10MHz for two and 5MHz for three and four. This is maximum speed. DSOs only run at maximum speed at the fastest time bases. You can't overfill the buffer, so the ADC must step down at the longer time bases. The true sample rate speed is a function of maximum ADC speed, Record Length (buffer), and capture time. This will be helpful:

http://www.autonerdz.com/java/SampleRateCalculator.html

The 512K buffer is also shared...512 for one channel, 256 for two, and 128 for four.

When Pico is running in block mode, all the samples are collected into the internal buffer before being displayed. Then the data block is moved to the PC and displayed all at once for a screen. Then it collects another block and so on.

Pico 6 has the ability to high speed stream. This changes the rules. Pico is no longer using the internal buffer and streams into the PC RAM for buffer. Now the trace draws on the screen and sample rates increase because a LOT more samples can be stored (over 2 million per channel). USB 2.0 is required to high speed stream with the maximum performance.

USB 1.1 is fine in any block mode and does not affect sample rate, but it will slow the screen update rate as each block takes longer to transfer.

Tom Roberts
(The Picotologist)
http://www.autonerdz.com
skype: autonerdz
THE PicoScope Automotive Authority
In North America
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