Layout Review Techniques Exercise Solution Sheet

Layout Review Techniques Exercise Solution Sheet

Exercise – Review a Bad Design

Suyash Jain FAE Training Oslo January 2011

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Exercise – Review a Bad Design

LAYER DEFINITIONS

Top_SilkScreen.GTO Top_Layer.GTL Power_Plane.GP1 Ground_Plane.GP2 Bottom_Layer.GBL = TOP SILKSCREEN GERBER FILE = LAYER 1 TOP SIGNAL GERBER FILE = LAYER 2 POWER PLANE (NEGATIVE) GERBER FILE = LAYER 3 GROUND PLANE (NEGATIVE) GERBER FILE = LAYER 4 BOTTOM SIGNAL GERBER FILE CC2520_CC2591FAE.DRR Drill_Guide_Gerber_File.GG1 = DRILL SIZE REPORT = 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 2

Solution Sheet

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Stack - up

Customer Design TI Reference Design

15mils 28mils 15mils Fig: Customer’s Design – Stack-up Top Signal Layer Power Plane Ground Plane Bottom Signal Layer 7 mils 45 mils 7 mils Fig: TI Reference Design Stack-up Top Signal Layer Ground Plane Power Plane Bottom Signal Layer 4

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. Fig: Customer Design (Above) TI Reference Design (Below) 6

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. Fig: Customer Design (Above) TI Reference Design (Below) 7

T-Line

T-Line on Pin 13 is missing Fig: Customer Design (Left) TI Reference Design (Right) 8

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 9

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. Fig: Customer Design (Above) TI Reference Design (Below) 11

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 Fig: Customer Design (Above) TI Reference Design (Below) 12

RF Path Between CC2520 and CC2591

Mismatched RF Traces in the differential section.

Fig: Customer Design (Above) TI Reference Design (Below) 13

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. Fig: Customer Design (Above) TI Reference Design (Below) 14

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. Fig: Customer Design (Above) TI Reference Design (Below) 15

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. Fig: Customer Design (Above) TI Reference Design (Below) 16

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 removed. the antenna Battery below the antenna acts as a ground plane. It will be recommended to remove the battery from below the antenna. Fig: Customer Design with Ground Plane Below the antenna (Left), Customer Design with Bottom Silkscreen showing battery below the antenna 17

Ground Plane - Divided

The Ground plane is divided below the RF path. Recommendation will be to have solid ground plane below the RF path. Fig: Customer Design with ground plane divided below the RF Path 18

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. Fig: Customer Design (Above) TI Reference Design (Below) 19

Silk-Screen Below the Chip

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 20

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. Fig: Customer Design (Above) TI Reference Design (Below) 21

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. Fig: Power Pad Layout for CC2591 as shown in CC2591 datasheet Fig: Customer Design (Above) TI Reference Design (Below) 22

Thanks!

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