electronic_eel has quit [Ping timeout: 256 seconds]
electronic_eel has joined #scopehal
<azonenberg>
So playing around in the lecroy SDA software, it looks like i can undo the pcie de-emphasis with an 8-tap delay line filter, one UI per tap, using some magic coefficients
<azonenberg>
the question is how to calculate those coefficients
<azonenberg>
the filter is a little slow right now because there's no AVX optimizations yet. That's on my to-do list
<azonenberg>
But it does work
<sorear>
why is the right eye wider, isn't the pre-emphasis supposed to improve things
<azonenberg>
sorear: i'm undoing the de-emphasis
<azonenberg>
The emphasis is intended to make things better *at the far side of the link*
<azonenberg>
it will actually look worse up front
<azonenberg>
basically you're applying the inverse of the estimated channel transfer function up front
<azonenberg>
i'm measuring right next to the TX
<azonenberg>
Very often you will want to remove emphasis from a signal in order to measure the true amount of jitter present
<azonenberg>
as you can see in the left plot, the histogram has multiple peaks indicating there's data dependent jitter being added by the emphasis filter
<azonenberg>
which makes sense, emphasis is essentially ISI by definition
<azonenberg>
It's just calculated to be the exact inverse of the channel ISI
<azonenberg>
(or at least close enough)
<azonenberg>
So if you want to measure jitter at the tx, you have to either turn off the emphasis (if there's register settings to do that)
<azonenberg>
or just remove it in post
nelgau has quit []
<azonenberg>
also i think i figured out my audio noise problems, it seems the cheap 2.4 GHz wireless mic i was using was one of the main contributors
<azonenberg>
but the pc sound card was also not goo
<azonenberg>
good*
<azonenberg>
a usb sound card and the same headset i was using appears to have a massively better snr
juli966 has joined #scopehal
<lain>
:3
<azonenberg>
lain: i'm gonna be putting together a new glscopeclient demo video shortly, focusing on PCIe as the test subject
<azonenberg>
i'm gonna be showing off both some new features as well as old stuff all in one video
<azonenberg>
trying to go for slightly higher production value than i have in the past so i'm actually writing up a high level outline of the demo
<azonenberg>
not like a full script with everything i say, but the list of things i'm demonstrating in order etc
<_whitenotifier-f>
[scopehal] azonenberg pushed 1 commit to master [+0/-0/±2] https://git.io/JkxwG
<_whitenotifier-f>
[scopehal] azonenberg 550bbf2 - TIE measurement: allow skipping a short time at the start. Persist min/max autorange better.
<lain>
nice
<azonenberg>
right now my plan is to do spread spectrum clock analysis, looking at modulation frequency/depth
<azonenberg>
then PCIe protocol decoding up to TLPs
<azonenberg>
eye pattern of the original signal, then remove the 6 dB of de-emphasis and compare the difference in eyes
<azonenberg>
then emulate a lossy channel and show how the emphasis helps
<azonenberg>
also probably playing with removing some of the emphasis to see how much you need for different channels
<azonenberg>
then jitter analysis of the signal as is, with emphasis removed, and after a lossy channel
<azonenberg>
Anything else you can think of that would be cool to demo on the same signal?
<lain>
not off the top of my head
<azonenberg>
oh, i'll throw a fft on there too
<azonenberg>
so you can see how there's no peaks because of the spread spectrm clocking
<monochroma>
oh that will be cool
<azonenberg>
and a waterfall
<azonenberg>
that will be a cool lead-in to the spread spectrum analysis
<lain>
:D
<_whitenotifier-f>
[scopehal-apps] azonenberg pushed 1 commit to master [+0/-0/±1] https://git.io/JkxSF
<_whitenotifier-f>
[scopehal-apps] azonenberg 433b252 - Updated to latest scopehal
<azonenberg>
Well that was annoying. i just recorded a whole glscopeclient demo
<azonenberg>
and the video is totally hosed
<azonenberg>
Choppy audio and dropped frames. it looks like the recording software couldnt keep up or something
<azonenberg>
i told it 30fps and it's like 2fps
<lain>
oop
<monochroma>
:<
<_whitenotifier-f>
[scopehal] mjgerm forked the repository - https://git.io/Jk0MD
<_whitenotifier-f>
[scopehal-apps] azonenberg opened issue #291: Crash when history depth is set to zero - https://git.io/Jkxdg
<_whitenotifier-f>
[scopehal-apps] azonenberg labeled issue #291: Crash when history depth is set to zero - https://git.io/Jkxdg