4 Functional Block Diagram and PCB

Figure 4-1 shows the block diagram for the conducted emissions test board designed using the CC1120 and four different buck converters with individual jumper enables. This board was designed to test features such as spread spectrum, adjustable switching frequency, low-cost and low-switching frequency options. The buck converters chosen to test these parameters and determine how some of our devices impact emissions are the LM5166, LMR38010, LMR36510, LMR36506.

The LM1566 was implemented in the original design of the system and was therefore used as the control circuit for testing. The LM38010 was chosen for its spread spectrum feature and forced PWM mode at light loads part option. The spread spectrum feature minimizes EMI by spreading ripple energy over a wider bandwidth (for example, noise shaping). The forced PWM is provided to achieve low output voltage ripple, tight output voltage regulation, and constant switching frequency at light load. It also has design options to change its switching frequency to frequencies in range of 200kHz to 2.2MHz with changes to component values. The LMR36510 is a cost optimized buck converter with a fixed 400-kHz frequency. Lastly, the LMR36506 has an ultra-low operating quiescent current. It also has a low EMI architecture (without spread spectrum), with PFM at light load version of the device on the test board. It supports an adjustable switching frequency that can be modified with changes to component values and can vary between 200kHz and 2.2MHz.

Figure 4-2 shows the PCB design of the conducted emissions test board. Solder pads were placed for radio power supply selection to minimize emissions and extra board space was left for future redesign to include PCB antenna. This board allows for emissions testing to be conducted on four separate buck converters using the same board, which allows for a comparison of buck converter parameters and their impact on the emissions test for the radio.