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What are the advanced options to configure a differential pair?

This tutorial requires:

DesignSpark PCB V11.0.0

The requirements of how to set up and route a differential pair were covered in the article “How do I setup a simple Differential Pair in DSPCB?”. This covered the settings and process of routing the differential pair with a defined spacing between two components.

Note: Differential pairs is only available with a paid DesignSpark Engineer subscription.

Here we cover the advanced tools available to display the skew (track length difference), the track lengths and the spacing between the differential pair and other components.

The following video illustrates in depth the steps discussed below.

Headup display

Here we illustrate the settings required to display at the cursor a “head-up display” showing live routing data while editing.

headup display - showing live routing data while editing

Key points are the display of the length of current net being placed and the ‘other’ net of the pair. 

The Skew is displayed and compared with the maximum skew that has been defined. This is also colour coded green for a quick visual indication of when within the set limit.

Nets – Dockable Bar

The real-time net length and skew can also be displayed in the Nets dockable bar.

Nets dockable bar

This is a versatile tool and useful for other net applications as well, the video illustrates how to configure the “Options” and “Columns” for the differential pair.

The advantage of the Nets bar is it keeps the PCB editing window clear for routing the tracks and the display is available when the differential pair has been routed.

With the above tools, we can visualise real-time the length and skew of the differential pair. We next illustrate how to show the clearance of the differential pair from other components and tracks. This is particularly important for clearance from other high speed signals.

Guard Spacing – additional spacing option

In the Design Technology dialog, there is a useful tool to provide special spacing requirements for a particular net as is required around high voltage nets, or to avoid noise coupling as in the case of a differential pair. 

This provides a guard band around the net to exclude poured copper pour and can be used with the “Show Clearances” feature in the Colours display. This then provides real-time coloured clearance areas around the current selected object to show if you are within the allowed spacing settings.

The guard band is set in the Design Technology:

1. Enter the required guard band around the required nets. The default of zero will apply the global spacing values.

global spacing values

2. Now in “Colours” check the “Show Clearances” check box.

Show Clearances

3. Now as soon as you encroach within the appropriate spacing limits, the colours will change to show the “Warning Colour” or “Error Colour”.

This image shows the differential pair being successfully routed through the highlighted the warning areas that should not be encroached.

differential pair being successfully routed

The video illustrates options on how to successfully route the differential pair, particularly in respect to starting and finishing the route where you ‘must’ break the guard band rules due to the proximity of other pads. A clue in the above image is the start and finish differential pair track stubs.

Conclusion

This tutorial illustrates DesignSpark PCB Engineer tools to route a differential pair with the required track to track spacing for a particular impedance, how to display the track length and skew and also how to maintain the required distances from other nets and components.

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