Differential pair nets are routing patterns with a certain parallel clearance apart
on which reversed signals flow at the same time to send information.
Generally, two signals are represented as +/- or POS/NEG and keep sending stable
signals. Differential pair nets are employed to resolve SI noise, return path, PI,
EMI issues that appear often in high speed signal nets. Differential pair nets take
care of return path and impedance issues by themselves by being side-by-side with
uniform distance apart. For common signal trace, designers consider impedance at
design stage. However, for differential pair nets, designers must consider coupled
odd impedances. In other words, a differential pair net needs to be considered as
one coupled signal pair. Generally, two signals are represented as +/- or POS/NEG
and keep sending stable signals. Therefore, two signals must keep coupling status at
design stage. This item checks the coupling status of paired signals. Near the
component pins, due to the pin pitch, the distance may not be able to satisfy the
differential pair distance criteria. The Pin Escape option is available to exclude a
region near pin locations.
Item: Input item name.
Net: Select Differential Pair Net Group.
Filter: Enter a filter to choose differential pair net from selected net
group. After entering a base net and pair net add them into the list by
clicking Add Filter.
Separation: Define parallel segment separation criteria. If you do not enter
any specific value, the DFE uses the longest net’s separation by default.
Check all layer with same separation value: If selected, DFE checks
the spacing between Nets with same spacing distances for all
layer.
Separation Value: The spacing that applies equally to all
layers.
Separation Value(Composite): The spacing to be applied when
using Composite Net.
Use Width Value(W): If selected, use Value * Width as the
spacing.
Use the separation of longest pairs: If selected, test using
the spacing between the longest parallel sections in the
entire layer.
Check each layer with different separation values: If selected, DFE
checks the spacing between Nets with different spacing distances for
each layer.
Layer No: Define layer number. If you do not enter any
specific value, the DFE gets layer number from PCB property
by default.
Separation (Each Layer): Define different separation values for
different layers.
Separation: Required separation for each layer.
Use the separation of longest segment pairs: The DFE uses the
longest net’s separation by default. (per layer).
Check the layer based on net structure: Option to apply different
spacing depending on the location of the net. (Microstrip or
Stripline)
Separation value on Strip Layer: Spacing to be used in
Stripline structures.
Separation value on Microstrip Layer: Spacing to be used in
Microstrip structures.
Use Width Value(W): If selected, use Value * Width as the
spacing.
Use the seperation of the longest segment pairs: The DFE
uses the longest net’s separation by default. (per
structure)
Separation Tolerance (%): Tolerance (%) to measure an allowable distance
between nets in a differential pair.
Coupling Ratio (%): Ratio (%) of parallel length that satisfies the
differential pair separation condition with total net length. Lower ratio
will be a fail in the report.
Ratio Fail: If result ratio is lower than this value, violating
regions are displayed in yellow and acceptable regions are displayed
in cyan.
Component Keep IN/OUT: Define test region. For example, you can test inside
or outside the breakout region of CPU with different test
values.
Component Group: Select required component group which is used for
defining the test region.
Region: Define the required test region.
IN: DFE tests the inside of the breakout region of target
component.
OUT: DFE tests the outside of the breakout region of target
component.
Range (COC+distance): Enter the distance value to define breakout
range. The COC (Component Overlap Check) plus this distance value is
considered the test range.
Target Layer: Select the required test layer.
All Layer: Test all layers.
Component Place Layer: Test only component placement
layer.
Different Rule Setup in Special Region: Assign additional test
region for breakout region testing.
Extended Region from COC: Enter the distance value to define
additional range. The COC (Component Overlap Check) plus
this distance value is considered the additional test
range.
Allowable Values: Assign additional allowable test value for
this additional region.
Value: Add additional allowable value.
Range: Add additional allowable range.
Deviation: Add additional allowable value with deviation
using this option.
Deviation (%): Add additional allowable value with deviation
(%).
The test range and test values will be:
Case1: Component Keep In region test
Figure 1.
Case2: Component Keep Out region test
Figure 2.
Case3: Component Keep Out with extended region test
Figure 3.
Check Options
Composite Net Separation Check: Checks if there is composite
net.
Composite Component: Select the composite component.
TP Separation Check
TP Component: Specify used test-point component group.
TP Separation: If test-points are used as pair, specify the distance
between test-components.
Nut Shape Anti-Pad: Checks whether the shape of anti-pad form a closed nut
shape.
Figure 4.
Except Layer: Select exception layers to exclude Nut Shape Check.
Minimum distance : Set minimum distance between via and
anti-pad.
Passive Component Fill-cut check: Checks if there is passive component
fill-check on PCB design.
In PCB design, "fill" and "cut" refer to the placement of copper
planes or traces in relation to the routing of a differential signal line.
Minimum fill-cut distance: Set minimum distance between
component's pad and fill-cut.
Fill-cut depth: Set the depth of fill-cut by number. DFE
checks up to "Fill-cut depth +1" layer. If there is no
fill-cut within the "Fill-cut depth", it is reported as a
failure. If a fill-cut exists on the "Fill-cut depth+1"
layer, it is also reported as a failure.
Measure Base: Select Pad base or Pad pair base of passive
component.
Do not check Ground Reference Layer: When checking for
fill-cut, the reference ground is ignored even if it
exists.
Additional passive component: This option to select
additional composite passive component.
Check Range: Set the range between start component and end
component.
Start Component: Select start component to measure
range.
End Component: Select end component to measure range.
Exception Options
Pin Escape: Enter a radius of circular region around pins to be
excluded for the rule check.
VIA Escape: Enter a radius of circular region around vias to be
excluded for the rule check.
Exclude Serpentine Area: DFE does not check serpentine section of
net.
Exclude Arc are: DFE does not check ARC section of net.
Exclude Short Segment Error: Exclude the short length trace shorter
than this value.
Display Separation & TP Distance Result: PollEx DFE report will show segment/TP separation
information.
Use Edge-to-Edge Separation: Checks edge-to-edge separation.
Default:
Center-to-Center
A Differential Pair Net usually keeps a minimum 80 percent of total net length
parallel. The pair nets may be allowed to have a distance tolerance in parallelism
when a net go around via or other objects on board. In this case, it is recommended
to make those distance tolerances less than 5 percent.
Figure 5.
To design a Differential Pair Net, you must pay attention to keep the continuity of
power and ground planes above and below of the differential net. Do not make branch
stub to make test point. In other words, impedance for overall routing must be kept
the same for the pair nets.
Not only for narrow side differential pairs but also for broad side differential
pairs, the rule explained above needs to be applied in design.
Figure 6.
Reference Material
Parallelism will cause impedance discontinuities that will directly affect signal
quality.
In this case it also contributes to the trace-length mismatch and will cause an
increase in signal skew.
