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手机RF开发心得总结-走线分析篇


5.3 RF走线详细分析

在硬件设计篇中虽然写了些走线规则,但是总觉得有必要还是好好写写走线,结合出现的问题详细分析一下,希望能对你有所帮助。虽然下面原理图中给出的是SKY74117SKY77318,但是走线和走线原则适用于所有手机RF电路。


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5.3.1 RF的关键走线

·    Vbat lines from connector to PA and VCC1,2,3 of transceiver.

·    RF Traces: Transceiver to PA, PA to ANT and ANT to LNA.

·    APC control: PAC_Out.

·    TX_I/Q and RX_I/Q.

·    AFC control trace (Pin19 of sky74117)

·    Crystal to Pin18 of sky74117

·    13/26MHz reference clock to baseband

这些关键走线中的微带线,已经在硬件设计篇中介绍过了,而且都很熟悉了,下面只详细分析最关键的走线,VBATT走线的方法以及容易出现的问题。

5.3.2 VBAT走线原则

 


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·    Separate the VBAT traces from PA to transceiver, other device, and so on.

·    Keep the PA supply trace and return path far away from the sensitive circuits.

·    All power supply filters should be put near the relevant components. And the rule is the smallest capacitance is the nearest.

5.3.3 VBATTPA走线的建议

 

image025 
 

·    The value of 33uF cap depends on the noise level on the phone boar,可根据实际情况修改电容值。

·    Depending on phone board noise level, not all 0402 and 0.01uF caps may be needed.

·    Ensure sufficient numbers of vias connect VBATT pin to battery trace.

·    VBATT trace should be >= 0.25mm.

·    Ground terminals of all bypass caps are connected to ground plane with vias.

·    The PAC_OUT trace from baseband should be surrounded by ground.

·    DC supply trace to Vcc1 must not be routed via Vbatt pin.

下图为走线是非对比。

 


image026 

DC supply trace to VCC1 must not be routed via VBATT pin.

Pin No

Designators

Description

2

Vcc1A

DC supplies to 1st stages of GSM and DCS/PCS PAs. Routing from Vcc1A to battery should not via PIN VBATT.

6

Vcc1B

DC supplies to 2nd stages of GSM and DCS/PCS PAs and BiCMOS control circuitry. Routing from Vcc1B to battery should not via PIN VBATT.

17

VBATT

DC supplies to final stages of PAs. Current drawn on this pin is due to PA output stages only. A large bypass capacitor is required on this pin due to high current draw.

5.3.4 走线问题展示

1)问题133uF的缓冲电容被旁路。


image028 

 

从电池来的VBATT走线没有先经过33uF的大电容,而直接走到PAVBATT管脚,33uF电容被bypass后,它的缓冲作用没有了,导致在PA大功率发射时,电压降很大,会导致手机直接掉线,更有可能直接导致关机或重启(基带ADC会采样电池电压,当PA大功率发射时,如果没有缓冲电容,电池电压会瞬时被拉低,这时如果ADC采样电压,读取的电压值到达关机门限要求,会导致系统关机或者重启,这个由系统电源管理特性决定。)

2)问题2,其他设备的VBATT走线直接从PAVBATT管脚取走线

 

image030 

 

VCC1或者其他使用电池电压设备的电源走线不能直接从PAVBAT管脚取走线,这样PA发射时对VBATT上的电压波动会影响VCC1或其他设备的电源供给,或者互相影响。VBATT电源走线最好以树状走线方式,VBATT为树干,走线最粗,其他使用VBATT电压的器件为枝干,直接从树干取走线,走线宽度根据实际电流需要做修改。这样可以避免各种使用VBATT电源的走线的相互影响。

3VBATT走线和电容旁路对PVT造成的影响


image032 

 

5.3.5 其他RF相关PCB设计原则

·    Differential I and Q signal pairs should be parallel and symmetry and should be surrounded by ground on both sides, plus ground above and below if routed on internal layers.

·    Routed the RF traces as microstrips or striplines with 50 ohm impedance, and should be as short as possible.

·    Clear internal layer of metal immediately below RF microstrips to allow wider traces as well as reduce parasitic capacitance.

·    Use enough ground vias to connect RF ground fill area to the internal ground plane.

·    The line XTUNE from baseband to SKY74117 pin 19 should be surrounded by ground on both sides, plus ground above and below if routed on inner layers.

·    XBUF (26MHz clock) from SKY74117 pin 18 to baseband should be surrounded by ground on both sides, plus ground above and below if routed on inner layers.

·    Trace direction in adjacent layers: Vertical, not parallel.

·    The RF part and Baseband part should be shielded respectively.

 待续。。。。。。

手机RF开发心得总结-前言篇

手机RF开发心得总结-硬件设计篇


 
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相关评论

评论人:RF GONGZHU2010-10-22
总的来时很不错。但,“从电池来的VBATT走线没有先经过33uF的大电容,而直接走到PA的VBATT管脚,33uF电容被bypass后,它的缓冲作用没有了”值得商榷。33UF电容的缓冲是由它的充放电实现的,它的作用主要跟他到电源脚的电阻有关,跟电源走线是否过它不太有关。

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