Interactive Length Tuning

The Interactive Length Tuning dialog

Summary

The Interactive Length Tuning dialog allows you to optimize and control net lengths by allowing variable amplitude patterns to be inserted for existing lengths on the current PCB document.

This dialog has two sections: the Target Length and Pattern. The Target Length section allows you to control the net length from a user defined length, the current net length or one of the length or matched net length rules. The Pattern section allows you to control the amplitude pattern of the net on the PCB document by varying the amplitude and the gap.

The Target Length Tuning dialog allows users to see all applicable rules and select the one to be used. If no rule is selected by a user, the rule with the longer max length will be used by default. Alternately, if the target lengths for two rules are equal, then the rule with higher priority will be used.

The title bar of the dialog displays the current net name and the old length.

Access

The dialog is accessed by clicking Route » Interactive Length Tuning then pressing Tab while tuning a net's length. When length tuning Differential Pairs, click Route » Interactive Diff Pair Length Tuning. The dialog is the same for both options, except the From Net option is replaced with From Diff. Pairs when accessed through the second command.

Options/Controls

Target Length

The length tuning offers four approaches for specifying the target length. The length can be: manually defined, based on a net that has already been routed, based on a set of Differential Pairs, or defined by design rules. The grid area on the right side will update accordingly. Currently selected rules are highlighted in blue within the grid area.

Manual - enter the length in the Target Length field. The list on the right keeps track of the values you have typed in, so that you can use one again.

• List Recently Used Lengths - lists the recently used manual target lengths that you can use to define the Target Length value. The currently selected length value is shown in the Target Length field along with a description.

From Net - after selecting From Net you can choose a net from the list on the right. The length of this net will become the target, however, it will be overridden if there are more restrictive design rules defined.

• List Nets - lists the net names and their lengths on the current PCB according to their class. The currently selected net length value is shown in the Target Length field along with a description.

From Diff. Pairs - lists the differential pairs and their lengths on the current PCB. The currently selected differential pair length value is shown in the Target Length field, along with a description.

• List Diff. Pairs - select to choose an existing differential pair length from the Name - Averaged Length list that will be used to define the Target Length value. After choosing a differential pair, you can create a new max-min length rule if desired.

From Rules - you need to have one or both of the Length and Matched Length design rules defined to use this mode. Altium Designer will then obey the most stringent combination of these rules.

• List Rules - lists the length rules for the current PCB document. The currently selected rule maximum length value is shown in the Target Length field along with a description.

• Edit Length Rule - click to change the values used for the High Speed Length design rule. The From Rules option needs to be selected first. 

  You will need to set up a High Speed Length design rule first if such rules do not exist. Go to the PCB Rules and Constraints Editor dialog (Design » Rules) to set up a rule.

• Edit Matched Lengths Rule -  Click to change the values used for the High Speed Matched Lengths design rule. The From Rules option needs to be selected first.

  You will need to set up a Matched Net Lengths design rule first if such rules do not exist. Go to the PCB Rules and Constraints Editor dialog (Design » Rules) to set up a rule.

Target Length - displays the target length being defined by the rules. Note that the most stringent combination of the rules is used. The target length will be set to halfway between range values in rules.

Create Rules From Length - click to open the Edit PCB Rule - Max-Min Length Rule dialog, in which you can create length rules.

Clip to target length - enable to ensure that the final length does not exceed the target length. When enabled, the Amplitude and Gap values are automatically adjusted to achieve the target length.

Pattern

• Style - shows the current amplitude wave pattern. Click to view and select another pattern. There are three pattern styles: Mitered with Lines, Mitered with Arcs, and Rounded. The PCB Editor will attempt to match the target length by adding segments to the length according to the defined target length.

  The Rounded style is the most compact and Mitered with Lines is the least compact.

• Max Amplitude - shows the current maximum allowed amplitude of tuning segments. While routing, the PCB Editor will attempt to match the target length by adding segments according to the defined target length. The Amplitude value can also be adjusted automatically if the Clip to target length option is enabled. You can decrease or increase the amplitude interactively with the "," (comma) and "." (period or full stop) keys. Edit this field to change the maximum allowable amplitude, which can be defined in either mm or mil units. To specify the units when entering a number, add the mm or mil suffix to the value.
• Gap - shows the current distance between "legs" of the amplitude pattern. The Interactive Length Tuning tool will attempt to match the length by adding wave pattern segments to the existing track to reach the defined target length. Use the shortcut keys 3 or 4 to decrease or increase the gap while you are length tuning. You can also use the Gap Increment field to control the gap size steps. View relevant shortcut keys by pressing the "~" (tilde) key while routing and tune the length interactively. Edit this field to change the gap, which can be defined in either mm or mil units. To specify the units when entering a number, add the mm or mil suffix to the value.
• Amplitude Increment - shows the current step in amplitude when you increase or decrease it during interactive length tuning. Use the "," (comma) or "." (period or full stop) keys whilst tuning to change the amplitude by the step value. For example, if the maximum amplitude is 100mil and the amplitude increment is 20mil, then you pressed the"," key twice during routing and will get an amplitude of 60mil. Edit this field to change the amplitude step, which can be defined in either mm or mil units. To specify the units when entering a number, add the mm or mil suffix to the value. 
• Gap Increment - shows the current step in wave length when you increase or decrease it during interactive length tuning. Use the 3 or 4 keys while tuning to change the gap by the step value. For example, if the gap is 100mil and the amplitude increment is 20mil, then you pressed the 4 key twice during routing and will get an gap of 140mil. Edit this field to change the gap step, which can be defined in either mm or mil units. To specify the units when entering a number, add the mm or mil suffix to the value.

Tips

Once you have launched the length tuning command, length tuning segments are added by moving the cursor along the routed net.

The interactive length tuning cursor provides you with information during the tuning process including before and current track lengths, as well as a graphical representation to gauge how close you are to the ideal lengths. The yellow cursor bars indicate the possible minimum and maximum lengths. The green bar indicates the target length, as determined from the applicable Matched Length and Max Length design rules, or the settings in the Interactive Length Tuning dialog. The sliding indicator shows how close you are to achieving a match.

The length tuning can be created from straight or arc segments, with full control over the amplitude, pitch, and corner radius, or miter.