Hacker News Reader

Comment by nativeit | Hacker News Reader

> Random equivalent-time sampling takes advantage of the nature of a repetitive signal by using samples from several trigger events to digitally reconstruct the waveform. Since sampling occurs on both sides of the trigger point, pretrigger capability is very flexible. Because repetitive signals are being sampled, the bandwidth of an equivalent-time scope can far exceed its sample rate.

https://www.tek.com/en/documents/application-note/real-time-...

View on HN

But you can't use equivalent-time sampling for something non-repetitive like network packets.

For measurements like this the SDK will usually include some utilities to send the same data over and over on a port.

Even if it's the same data, the bit stream will be a variety of 0 and 1s. The period of that waveform will then be 1 frame length / data transfer rate (or rather 1/4 frame length / data transfer rate as this is a QSGMII link). I wonder how the scope triggers on that. Trigger criterium would be a bit pattern, say the Ethernet frame preamble of 7 octects (* 10/8) spread across four streams ...

Otoh, at 5Gbps, a sample rate of "just" 10GS/s would be sufficient (barely).

I rather suspect the oscilloscope is capable of 1TS/s equivalent time sampling, but that mode wasn't used.

Here's a more specific example: PicoScope 9400 series supports just 500Msps per channel, however it's advertising "70ps transition time and 1TS/s (1ps resolution) random equivalent-time sampling", this sort of "equivalent sampling" is presumably where that seemingly crazy spec comes from.

Reply on news.ycombinator.com