{"id":28946,"date":"2022-03-28T06:17:22","date_gmt":"2022-03-28T13:17:22","guid":{"rendered":"https:\/\/digilent.com\/blog\/?p=28946"},"modified":"2023-02-07T17:48:42","modified_gmt":"2023-02-08T01:48:42","slug":"creating-an-eye-diagram-in-waveforms","status":"publish","type":"post","link":"https:\/\/digilent.com\/blog\/creating-an-eye-diagram-in-waveforms\/","title":{"rendered":"Creating an Eye Diagram in WaveForms"},"content":{"rendered":"

Creating an Eye Diagram<\/h2>\n

An eye diagram, common in telecommunications, is a way of plotting the rising and falling edges of a digital signal in the same plot, creating an \u201ceye\u201d-like design. When combined with persistence, these plots are extremely useful for analyzing the properties of these most-important parts of these signals. Oscilloscopes are commonly used to create these plots, by using a clock signal synchronous to the signal of interest to trigger captures of that signal. Let\u2019s <\/span>take a look<\/span> at how to create one in\u202f<\/span><\/span>WaveForms<\/span><\/span><\/a>, our free test and measurement software.<\/span><\/span>\u00a0<\/span><\/p>\n

Why Would You Want or Need an Eye Diagram?<\/h2>\n

An eye diagram<\/span> using persistence<\/span> allows you to quickly identify how noisy a signal is.\u202f It does this by <\/span>repeatedly <\/span>tracing the measured signal path and layer<\/span>ing it onto the same <\/span>plot<\/span> over and over. Ideally<\/span>,<\/span> a clean signal<\/span> will have<\/span> very thin<\/span> lines<\/span>, meaning your system is very consistent. If you have wider paths, this indicates that there <\/span>were<\/span> some <\/span>inconsistencies<\/span>\/distortions in the data<\/span>, such as<\/span> jitter (rising and falling edges occurring slightly earlier or later than expected<\/span>)<\/span>,<\/span> variances in slew rate<\/span>, unexpected overshoots<\/span>, additional noise from sources like crosstalk, and <\/span>more<\/span>.<\/span> Many of these <\/span>issues have characteristic patterns that can be seen <\/span>at a glance<\/span> by an engineer with a trained eye (pun intended).<\/span><\/span>\u00a0<\/span><\/p>\n

Setting Up an Eye Diagram in WaveForms<\/h2>\n

Eye diagrams can be easily created for some signals using WaveForms<\/a>, by measuring both the signal of interest and the clock signal it\u2019s synchronous to and triggering repeated captures off of clock edges. The Scope instrument\u2019s Persistence view layers these repeated captures as previously described, and plots the frequency at which each data point occurs.<\/span>\u00a0<\/span><\/p>\n

Setting up the trigger for an eye pattern depends on the kind of protocol you are analyzing, if it\u2019s an encoded or separate clock, or if the data rate is single or double.\u202f For single data rate signals, you likely want to trigger captures centered on the clock edge (whether rising or falling) that the digital interface uses to sample the data. For double data rate signals, triggering on either edge is the way to go. Even though WaveForms currently doesn\u2019t support the recovery of encoded clocks, persistence is still handy for analyzing these signals, and either edge triggers are still likely to get you the consistency required, even if the resulting plot will be centered on the edges of the signal of interest rather than centering on the middle of the eye.<\/span>\u00a0<\/span><\/p>\n

The latest <\/span>WaveForms<\/span> Beta version, \u202f<\/span><\/span>WaveForms Beta 3.18.23<\/span><\/span><\/a>, adds a <\/span>\u201cT<\/span>op<\/span>\u201d<\/span> option <\/span>to the scope instrument\u2019s <\/span>P<\/span>ersistence<\/span> view<\/span>, <\/span>which helps to distinguish <\/span>the <\/span>data captured from multiple channels within the diagram<\/span>.<\/span>, <\/span>T<\/span>he system color<\/span>s<\/span> the <\/span>plotted <\/span>data points both by<\/span> their frequency, as normal<\/span> for the Persistence view<\/span>, and with the color representing the channel<\/span>.<\/span> Only data points occurring more frequently than the <\/span>chosen top setting percentage are colored by channel<\/span>;<\/span> to show the path <\/span>most commonly taken<\/span>.<\/span><\/span>\u00a0<\/span><\/p>\n

To simulate a communication system, we\u2019ve used a separate device (the\u202f<\/span>Digital Discovery<\/span><\/a>) to generate the signals at different drive strengths and slew rates (which affect the rise and fall times) and an Analog Discovery Pro to measure the signals. Two devices are used to ensure that the data signals aren\u2019t produced and consumed by instruments using the same clock \u2013 the system we\u2019re simulating would have multiple clock sources on either end of the transmission, so we should too.<\/span>\u00a0<\/span><\/p>\n

Here are a couple examples of signals generated by the Digital Discovery\u2019s pattern generator:<\/span>\u00a0<\/span><\/p>\n

\"\"
Generating the signals with a Digital Discovery<\/figcaption><\/figure>\n

\"\"<\/p>\n

\"\"
Here we generate double data rate signals, where both edges of the clock are used to capture data.<\/figcaption><\/figure>\n

 <\/p>\n

And now, lets look at some captures!<\/span>\u00a0<\/span><\/p>\n

\"\"
Here, we send the clock signal into an oscilloscope channel and trigger off of the (rising) edge that the receiving device is intended to sample on.<\/figcaption><\/figure>\n
\"\"
Using a<\/span>n <\/span>Analog Discovery Pro <\/span>(<\/span>ADP5250<\/span>)<\/span> to measure <\/span>a signal<\/span> with a <\/span>separate clock connected to the Trigger 1 input.<\/span> External triggers are perfect for performing this kind of <\/span>capture, since they don\u2019t take up an oscilloscope input channel, and are well suited to handle digital inputs.<\/span><\/figcaption><\/figure>\n
\"\"
If a signal doesn\u2019t have an associated clock for you to <\/span>trigger your captures <\/span>off of<\/span>, you can still produce eye-like diagrams using persistence by <\/span>triggering <\/span>off of<\/span> \u201cEither\u201d rising or falling edges of the signal of interest, as shown above.<\/span><\/span>\u00a0<\/span><\/figcaption><\/figure>\n

 <\/p>\n

0<\/span><\/a><\/div><\/div>
<\/div><\/div>
<\/div>","protected":false},"excerpt":{"rendered":"

Creating an Eye Diagram An eye diagram, common in telecommunications, is a way of plotting the rising and falling edges of a digital signal in the same plot, creating an … <\/p>\n","protected":false},"author":52,"featured_media":28961,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4323,4312,1563],"tags":[4352,4349,452],"ppma_author":[4462],"class_list":["post-28946","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-software","category-usb-scopes-analyzers-generators","category-guide","tag-adp3450","tag-adp5250","tag-waveforms"],"jetpack_featured_media_url":"https:\/\/digilent.com\/blog\/wp-content\/uploads\/2022\/03\/Screenshot-2022-03-24-141453.jpg","authors":[{"term_id":4462,"user_id":52,"is_guest":0,"slug":"abrown","display_name":"Arthur Brown","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/0157dde33e86ff2f253098657a5a774e?s=96&d=mm&r=g","author_category":"","user_url":"","last_name":"Brown","last_name_2":"","first_name":"Arthur","first_name_2":"","job_title":"","description":""}],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/posts\/28946","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/users\/52"}],"replies":[{"embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/comments?post=28946"}],"version-history":[{"count":1,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/posts\/28946\/revisions"}],"predecessor-version":[{"id":29639,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/posts\/28946\/revisions\/29639"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/media\/28961"}],"wp:attachment":[{"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/media?parent=28946"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/categories?post=28946"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/tags?post=28946"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/digilent.com\/blog\/wp-json\/wp\/v2\/ppma_author?post=28946"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}