A deep dive into the triggering models of modern day oscilloscopes

James Lewis returns to HDDG22 to talk about scopes that are “Top of the P(acq)”

Supplyframe’s mission is to create more access to information about electronics design and manufacturing. As such, we do a meetup in San Francisco called, Hardware Developers Didactic Galactic. These events include talks by industry experts in hardware and software. The speakers are often building hardware for recreation or as part of their employment. The common thread is that they want to give a view “under the hood”.

HDDG22 was held June 29th, 2017 at the Supplyframe San Francisco office. We welcomed James Lewis (@baldengineer) back for his second talk; his first talk continues to be our most popular to date, “They’re JUST capacitors”. This talk covered how a scope does sampling by diving into the different elements of modern oscilloscopes. To follow along, here are the slides on James’ personal site:

Oscilloscope Triggering: HDDG #22 Presentation

Tradeoffs

Scope users need to trade off sampling speed, accuracy, repeatability and total depth of signal when doing their measurements. This ties back to the hardware involved.

Much of the struggle comes from the fact that the scopes are triggering, sampling, interpolating and then displaying the data. Sometimes a measurement is also applied. Each of these steps starts from knowing nothing about the true nature of the signal, so certain assumptions are made. One of these is the “90/10 threshold”, which assumes certain voltages and then interpolates and figures out the 50% point based on those initial data. As James says, “If you’re really worried about the precision of the measurement…tell the scope what voltage to measure at”.

Other struggles when measuring signals comes down to which data is actually being used to evaluate a particular parameter. James shows a Rigol scope which only measures the data that is shown on screen, regardless that there is more data in the memory. This could impact the final measurement.

Triggers don’t need to be complicated

The circuits used to detect the trigger voltage are often quite simple. These involve simple arrangements of delay lines and digital gates (NAND) to detect when a transition occurs in the positive or negative direction. But while the trigger detection is initially simple, the various modes of triggering quickly changes how the trigger circuits get layered on top of one another. While most people would consider taking this problem into the digital domain (using raw data plus DSPs for detection in lieu of individual circuits), only Rhode and Schwarz are doing this currently.

Though the triggering models of most scopes replicates past models’, it’s because the high speed of detection and simplicity of current solutions is sufficient for many users’ needs. The triggering portion of scopes continues to have an effect on the capabilities of the device, especially as the speeds of devices increases. Engineers and others troubleshooting circuits should understand how triggering works so they are better able to diagnose the signals they are viewing on screen.

For more details about scopes, check out the rest of James’ talk embedded above or on the YouTube page for this video.

https://upscri.be/8d2837/

This article was originally posted on Medium.

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