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Exercise – Review a Bad Design
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Suyash Jain
FAE Training Oslo January 2011
Exercise – Review a Bad Design
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LAYER DEFINITIONS
Top_SilkScreen.GTO = TOP SILKSCREEN GERBER FILE
Top_Layer.GTL = LAYER 1 TOP SIGNAL GERBER FILE
Power_Plane.GP1 = LAYER 2 POWER PLANE (NEGATIVE) GERBER FILE
Ground_Plane.GP2 = LAYER 3 GROUND PLANE (NEGATIVE) GERBER FILE
Bottom_Layer.GBL = LAYER 4 BOTTOM SIGNAL GERBER FILE
CC2520_CC2591FAE.DRR = DRILL SIZE REPORT
Drill_Guide_Gerber_File.GG1 = DRILL GUIDE GERBER FILE
Drill_Drawing.GD1 = DRILL DRAWIING GERBER FILE
CC2520_CC2591FAE BOM Rev A.XLS = BILL OF MATERIALS
CC2520_CC2591FAE Schematic.pdf = SCHEMATIC PDF
Solution Sheet
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Stack - up
TI Reference
Design
Customer Design
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Top Signal Layer
Bottom Signal Layer
Power Plane
Ground Plane
15mils
28mils
15mils
Top Signal Layer
Bottom Signal Layer
Power Plane
Ground Plane
7 mils
45 mils
7 mils
Fig: Customer’s Design – Stack-up Fig: TI Reference Design Stack-up
Schematic Review
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Component Values
1. RF Cap Values on the
RFP and RFN pins are
reversed.
2. Loading Capacitor
Value on the crystal
Oscillator needs to be
checked in datasheet.
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Fig: Customer Design (Above) TI Reference Design (Below)
Power Supply
Power Supply on
the reference
design to the
CC2520 and
CC2591 have been
isolated for better
decoupling.
Recommendation
will be to copy the
power supply
routing as in the
reference design.
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Fig: Customer Design (Above) TI Reference Design (Below)
T-Line
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T-Line on Pin 13 is missing
Fig: Customer Design (Left) TI Reference Design (Right)
BOM - Review
Component Vendor and Component Type
1. Inductor L9 and Capacitor C16 are of 0603 type. 0603 components have
higher parasitic and lower SRF’s. It is recommended to use 0402 components
as in the reference design
2. Crystal Oscillator is the same as TI Reference Design so the Loading Cap
Values must be 27pF instead of 18pF on the schematic
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Layout Review
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Output RF Path
Trace width and lengths are not as recommended in
the reference design. It is recommended to follow the
trace width and lengths for optimum performance.
50 ohm trace from the end of the matching network to
the inverted – F antenna and the SMA is not of correct
width. It will be recommended to follow the reference
design.
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Fig: Customer Design (Above) TI Reference Design (Below)
Output RF Path
Grounded Pads have a small length of
trace going to the ground plane and
then connected to the ground plane
using vias.
Recommended structure will be as
shown in TI reference design figure
below
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Fig: Customer Design (Above) TI Reference Design (Below)
RF Path Between CC2520 and CC2591
Mismatched RF Traces in the
differential section.
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Fig: Customer Design (Above) TI Reference Design (Below)
Decoupling on CC2520
Decoupling should be done
as Pin<>Capacitor<>Via to
power plane
As observed in the design
the structure is not correct.
It is recommended to follow
the correct structure for
better performance.
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Fig: Customer Design (Above) TI Reference Design (Below)
Decoupling on CC2591
Decoupling should be done as Pin<>Capacitor<>Via to
power plane
As observed in the design the structure is not correct. It
is recommended to follow the correct structure for better
performance.
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Fig: Customer Design (Above) TI Reference Design (Below)
Crystal Oscillator Loading Caps
Crystal Oscillator Loading Caps on other side
of the board this adds additional reactance to
the load oscillator sees. It is recommended to
have the loading caps on the same layer as
the oscillator.
Distance of oscillator from the active pins
must be minimized.
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Fig: Customer Design (Above) TI Reference Design (Below)
Ground Plane and Battery Beneath the Antenna
PCB monopole antenna
do not require ground
plane beneath them.
It is recommended to
have the ground plane
beneath the antenna
removed.
Battery below the
antenna acts as a ground
plane. It will be
recommended to remove
the battery from below
the antenna.
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Fig: Customer Design with Ground Plane Below the antenna (Left), Customer
Design with Bottom Silkscreen showing battery below the antenna
Ground Plane - Divided
The Ground plane is divided below
the RF path.
Recommendation will be to have
solid ground plane below the RF
path.
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Fig: Customer Design with ground plane divided below the RF Path
Vias
Recommendation will be to add lot of vias
as in the reference design . The spacing
between the vias can be kept around
wavelength/10.
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Fig: Customer Design (Above) TI Reference Design (Below)
Silk-Screen Below the Chip
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Silkscreen markings below the area where
the chip is to be placed provides
obstructions for the chip to be soldered
properly to the PCB pads, leading to
undesired performance
Fig: Customer Design with Top Silkscreen
T-Lines
T-Lines on Pin 1, 10, 13 are critical for
optimum performance. It is recommended to
follow the trace length and width of these
traces.
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Fig: Customer Design (Above) TI Reference Design (Below)
Power Pad
It is recommended to follow the via structure
on the power pad in the design. Have the
recommended number of vias and spacing
between them. Have the vias tented. This
allows to have good RF ground connection
and thermal relief.
Data sheet for
CC2591 provides
Detailed information
about the power pad
layout as shown in
the figure.
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Fig: Customer Design (Above) TI
Reference Design (Below)
Fig: Power Pad Layout for CC2591
as shown in CC2591 datasheet
Thanks!
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