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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