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             PV Power : RF circuit design FAQ (frequent asked questions)

1#  The Interference Between the Digital Circuit Module and The Analog Circuit Module

If the analog circuits (RF) and digital circuits work alone, they may work well. However, once the two work on the same circuit board, using the same power supply to work together, the whole system is likely to be unstable. This is the reason that the digital signal frequently oscillates between ground and positive power (> 3 V), and the period is particularly short, often nanosecond. Due to the larger amplitude and shorter switching times to make the digital signals contain a large number of high frequency components independent of the switching frequency. In the analog part, the signal transmitted from the radio tuned loop to the receiving part of the wireless device is less than 1μV. So the difference between the digital signal and the RF signal will reach 120 dB. Obviously. If the digital signal can not be separated from the RF signal. Weak RF signals may be destroyed, so that the performance of wireless devices will deteriorate, or even completely unable to work.

2# Power Supply Noise Interference

The RF circuit is very sensitive to power supply noise, especially for burr voltage and other high frequency harmonics. Microcontrollers will suddenly inhale part current during a short period of time in each internal clock cycle, this is the fact that modern microcontrollers are fabricated using CMOS processes. therefore. Assuming that a microcontroller operates at an internal clock frequency lMHz, it will draw current from the power supply at this frequency. If you do not take the appropriate power decoupling. Will inevitably cause voltage on the power line glitches. If these voltage burrs reach the RF pin of the RF section of the circuit, it may cause malfunctions.

3# Unreasonable Ground Line

If the RF circuit ground processing properly, may causing some strange phenomenon. For digital circuit design, even without the ground layer, most digital circuit function also performed well. But in the RF band, even a very short ground will act like an inductor. Roughly calculated, the inductance per mm length is about lnH, and the resistance of the 10 toni PCB line at 433 MHz is about 27Ω. If you do not use the ground layer, most of the ground will be longer, the circuit will not be able to design features.

4# The antenna to other analog circuit part of the radiation interference

In the PCB circuit design, the board usually have other analog circuits. For example, many circuits have analog, digital conversion or digital-to-analog converters. The high frequency signal from the antenna of the RF transmitter may reach the analog input of the ADC. Because any circuit line may send or receive RF signals like an antenna. If the ADC input is unreasonable, the RF signal may be self-excited within the ESD diodes of the ADC input. Resulting in ADC bias.