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The USB bottleneck myth debunked

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The USB bottleneck myth debunked

Postby alan » Sun Apr 13, 2014 5:50 am

PicoScope oscilloscopes are well known for offering far deeper memories than competing benchtop or PC oscilloscopes. In fact for most of the last 20 years different generations of PicoScopes have offered the deepest memories available on any oscilloscope at any price.

Due to the fact our oscilloscopes offer deep memory and connect to the PC via USB we get quite a few questions about whether the USB link is a bottleneck restricting performance. Typically the questions are similar to the ones below:

1. I am trying to choose between a PicoScope and a traditional benchtop scope. Whilst the PicoScope has much more memory than the benchtop, the sales person for the benchtop scope manufacturer told me that you can not really use the deep memory because the USB connection to the PC causes a bottleneck that slows the screen update rate down. Is this true?

2. I have been researching which oscilloscope to buy and had decided on a PicoScope 3000. Someone knowledgeable on an electronics forum however told me that as soon as you enable deep memory on a PC oscilloscope it grounds to a halt and the controls become unresponsive. Do I need to reconsider my choice as I am keen to use deep memory for serial decoding?

Rather than keep answering such questions individually this post is an attempt at a generic answer.

Some oscilloscopes (both benchtop and PC based) DO have poor waveform update rates and unresponsive controls making their deep memory of limited use / something only to enable when essential. This however is not the case with PicoScope products - you can enable deep memory all the time without penalty.

One way of showing that the USB link is not a bottleneck is to benchmark a PicoScope against a traditional benchtop scope.

Benchtop oscilloscope. Tektronix DPO5054B
4 channels
500 MHz bandwidth
5 GS/s sampling rate
25 M point memory
Price $15,300

PC oscilloscope. PicoScope 6404C
4 channels
500 MHz bandwidth
5 GS/s sampling rate
1 G point memory
Price $6,600

At 500us/div both oscilloscopes sample at 5GS/s and collect 25 million samples per screen update. The measured screen update rates are as follows:

Tektronix DPO5054B 9 waveforms in 10 seconds
PicoScope 6404C 180 waveforms in 10 seconds

The Tek scope is collecting and displaying 22.5 million samples per second whilst the PicoScope is collecting and displaying 450 million samples per second. Of course the PicoScope can capture far more than 25 million samples. For example the PicoScope 6404D can sample at 5GS/s for timebases as long as 20ms/div. It is collecting 1 BILLION samples per waveform / screen update. Under these conditions it can still update the screen 2.5 times per second (bear in mind 20ms/div is 200ms across the screen so the scope is collecting data for 50% of the time and processing /displaying it for 50% of the time).

In the above example the PicoScope 6404D is collecting and displaying 2.5 BILLION samples per second. To put it another way it is collecting and displaying a DVDs worth of data every 2 seconds. We believe this capture and display is faster than any other oscilloscope on the market today.

Compare the 2.5 GB/s data rate with the theoretical maximum available USB bandwidth.

USB2.0 60 MB/s (480 Mbit/s)
USB3.0 625 MB/s (5 Gbit/s)

In other words the oscilloscope is collecting and displaying data far faster than its USB 3.0 port can transfer data. The reason behind this is simple - all PicoScopes with over 1 MS of memory use dedicated hardware inside the oscilloscope to intelligently form the image to be displayed on the PC screen without any loss of high frequency data. By only transferring the image to be displayed across the USB link any concerns about either a USB or PC CPU bottleneck are eliminated.

With each new generation of PicoScope we have improved our hardware acceleration engines to the point where they can process samples at least as fast as the oscilloscope can collect the data (5 billion samples per second in the case of the PicoScope 6000s). This is approx 2 orders of magnitude faster than the samples can be processed in a PC CPU.

As well as accelerating deep memory captures the hardware acceleration engine is also responsible for the following abilities:

To collect a burst of waveforms at speeds of up to 1 million waveforms per second
To continuously collect and display 10000s of waveforms per second to help find intermittent glitches. (For more on finding intermittent glitches see also topic13657.html )
Segment the memory into a circular buffer to automatically store 100s or 1000s of previous waveforms.

In short the USB bottleneck is a myth, PicoScope oscilloscopes cope better with deep memory captures than competing oscilloscopes be they PC based or benchtop.
alan
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