Product Info

Smart LTE Module Series
SC20 Hardware Design
SC20_Hardware_Design 68 / 130
interference when the module is transmitting at EGSM900MHz. Without placing this capacitor, TDD noise
could be heard. Moreover, the 10pF capacitor here is used for filtering out 1800MHz RF interference.
Please note that the resonant frequency point of a capacitor largely depends on the material and
production technique. Therefore, customers would have to discuss with their capacitor vendors to choose
the most suitable capacitor for filtering out high-frequency noises.
The severity degree of the RF interference in the voice channel during GSM transmitting largely depends
on the application design. In some cases, EGSM900 TDD noise is more severe; while in other cases,
DCS1800 TDD noise is more obvious. Therefore, a suitable capacitor can be selected based on the test
results. Sometimes, even no RF filtering capacitor is required.
The capacitor which is used for filtering out RF noise should be close to the audio device or audio
interface. The trace should be as short as possible, and it is recommended to route the trace for
capacitors first and then for other points.
In order to decrease radio or other signal interference, RF antennas should be placed away from audio
interfaces and audio traces. Power traces cannot be parallel with and also should be far away from the
audio traces.
The differential audio traces must be routed according to the differential signal layout rule.
3.23. Emergency Download Interface
USB_BOOT is an emergency download interface. Pull up to LDO5_1V8 during power-up will force the
module enter into emergency download mode. This is an emergency option when there are failures such
as abnormal startup or running. For convenient firmware upgrade and debugging in the future, please
reverse this pin. The reference circuit design is shown as below.
LDO5_1V8
S1
Module
USB_BOOT
R1
10K
Figure 30: Reference Circuit Design for Emergency Download Interface