手机RF开发——HW development

HW development包括schematic和PCB layout design and debug。circuit debug在整个开发过程中是不断重复的。因为，不同产品的结构不同，layout不同，最后的测试结果就不同。根据测试结果，最终来确定需要改变或增加schematics中的哪些components，需要如何修改layout，进行最终的优化。所以，HW development就是一个根据实验结果不断进行性能优化的过程。另外，由于RF是高频layout，这就更需要注意RF的走线要求，严格按照transceiver，PA和switch的layout guide对要求网络严格走线。

注：XCV是射频收发器transceiver的简写。

       下面是SKY74117＋SKY77318的schematics要点。其实其他类型的射频收发器，PA和Switch也一样遵循下面的设计原理，在设计时，可以参考设计。

（1）Power supply

 

 

Figure4. SKY77318

 

 

Figure5. SKY74117

一般，有源器件会干扰RF performance，无源器件不会干扰，只会衰减。所以，Power supply有源部分的设计对整个RF设计比较重要，会影响整个RF performance。要点如下：

（A）Power filter caps的排列，按照电容值大小，分别从远到近排列到power管脚附近。

（B）SKY77318的VBAT大电容C8特别重要，会影响switch spectrum和正常打电话；因为打电话时，PA发射对电源电压的下拉严重，所以大电容的放电作用可以补偿一部分下拉电压。若没有或者电容不够大，无法补偿发射时的电压降，导致手机无法正常工作。

（C）SKY74117的VBAT电容会对modulation spectrum产生影响；使用uF及pF电容并联，在主VCC1管脚还需要10uF大电容，并且XCV_VBAT走线必须在到PA前分支出来。防止PA对电源产生的影响波及到XCV。详细请见layout guide。

（2）RAMP RC filter

       RAMP RC filter起到滤波作用，用于滤除噪声，加大R或C都会使低通截至频率降低，降低干扰。在频谱的表现上，主要是改善TX switch spectrum。但是除了调整RC外，必要时，还需要调整driver的TX enable，ramp up timing和ramping table。见Figure6的RAMP部分。

 

 

 

Figure6. RAMP filter

（3）Matching between SW and XCV

 

 

Figure7. Matching Circuit between XCV and SW

       这部分matching对接收影响很大，会影响RX level和RX sensitivity。当有线测试模式，发现RX level不平坦，RX sensitivity不好时（layout OK），可以通过调整match circuit来改善。虽然skyworks给了demo，但是最终还是通过试验结果来决定问题的。调整的时候，可以先从调整XCV端的匹配开始，从调整caps（C63/C67）开始，从右向左方向进行。调整后，有线模式下，RX level波动范围±2，灵敏度为－107dBM以上时为准。

（4）Matching between PA and SW and ANT

 

 

Figure8. Matching between PA and SW and ANT

PA到SW的TX链路上的电容都是用来调节VSWR。PA到SW的电容C57/C58，会衰减发射。若发射功率不够，可以通过调整caps来改善。SW到ANT的电容C69，同时会影响接收sensitivity和发射功率。一般，Switch datasheet都有推荐值，可以先按照推荐值，然后通过试验，来最终确定最佳的components value。

（5）Attenuation from XCV to PA

 

 

Figure9. Attenuation from XCV to PA

SKY74117的TX输出功率为8－12dBm，SKY77318的输入功率要求是0－6dBm，所以117的输出超过了318的输入，需要对输出功率进行适当的衰减才可以满足PA输入的要求。这个是设计schematics的一个重要要求，输入功率衰减不够，会损坏PA。输入衰减过大，则会降低正常发射功率。可以使用π型电阻衰减网络，阻值可以用衰减工具计算获得。

（6）AFC RC filter and the crystal selection

 

 

Figure10. AFC RC filter

       这部分是和frequency相关的。AFC RC filter是BP AFC滤波，它会影响frequency tuning的相应速度。一般，按照demo来即可，如果datasheet说可以不用，就直接接到BP。Crystal的选择会影响clock setup time。因为BP需要RF提供clock，称其为RF_CLK。RF_CLK需要注意两个地方，一个是power on的system run，system必须等RF_CLK稳定时，才能run。另外一个是paging FINT，这个是system wakeup，也必须等RF_CLK稳定时才可以，详细的timing解释见后面的driver development。但是，确定的是，datasheet DC characteristic中，会有详细的clock setup time。除了阅读datasheet外，还需要亲自在示波器上查看clock setup time。

       以上是SKY74117＋SKY77318的schematics的设计详细描述。

5.2 Layout design

Cell phone RF layout直接关系到RF performance。Layout中比较重要的trace是：RF trace，Power trace，clock trace，ramp trace。详细如下（都是英文就不翻译了）：

1.    PCB trace design sequence of the whole system

1.    Place all the ground VIAS on the components relevant pads.

2.    Route the VBAT trace to PA. The VBAT trace to PA is the most important power trace. Pay attention of the VBAT branch to XCV and PA. Refer to the layout guide.

3.    Route the VUHF net.

4.    Route the VRF net.

5.    Route the VIO net.

6.    Route the RF strip-lines and micro strip-lines.

7.    Route the PLL loop filters.

8.    Route the PAC circuit.

9.    Route the other RF traces.

10.  Route the audio traces.

11.  Route the digital traces.

       If you layout only the RF part, please skip some steps useless for RF.

2.    The detailed traces layout description

2-1. TX line

·   TX_I+, TX_I-, TX_Q+, TX_Q- should be parallel and symmetry. The I/Q signal traces should be separated by ground.

·  The TXHB and TXLB should be short and surrounded by ground. And this line should be routed as RF line strictly. It should be the 50Ohm matching.

·    VAPC should be short and surrounded by ground.

2-2. RX line

·    All RX lines from T/R switch should be short and routed as RF line strictly. It should be 50Oom matching.

2-3. Power

·    All the power filter caps should be put near the relevant components and pins. And the order is the smallest caps are put the nearest.

·    The power supply trace, such as V_BAT and V_IO should be wide and short. Here set V_BAT trace 80mils on dev board.

·    The voltage of V_BAT is often changing from time to time. So it is better to keep the RF power supply lines far away from V_BAT (it means that place RF far from charging), such as V_RF, V_IO……

·    The XCV V_BAT line should be branched before the PA V_BAT line. And the other PA VCC lines should not be routed via the main PA V_BAT line. Refer to the SKY77318 recommendation.

2-4. GND

·    The GND should not be separated into several parts. It is better one.

·    There are enough ground pins and the corresponding place should not be solder-resist.

·    If possible, the ground layer should not be divided. Keep at least one ground layer completed.

·    The APC circuit is very sensitive. Make the layer bellow ACP circuit a completed GND area.

2-5. Clock line

·    SYSCLK line should be short and protected by GND.

·    The layer under the crystal should be cut off a part which is as big as crystal.

2-6. Antenna

·    Cut off all of the GND plane/layer beneath the antenna pad on every layer. The Antenna line should match the 50Ohm.

2-7. Others

·    The RF traces should be calculated carefully.

·    Components of Microphone input circuit should be put as symmetry as possible.