In this article, I will be sharing with you the way the ESP8266EX is made from PCB level.
Schematic
This was done using the PCB software known as Kicad. I used the ESP8266 datasheet to understand the connection between components. The schematic is as follows:
Now looking at each section:
It creates a voltage divider network with the parasitic capacitance of the trace and the pin. This voltage divider network reduces the voltage swing on the SD_CLK pin and consequently reduces the cross-talk.
Crystal Oscillator
The crystal oscillator is used to provide stable and precise clock signal that is then used for the proper operation of digital devices. It is connected to the ESP8266EX microcontroller through pin 27 and 28. They have capacitors which are grounded. The capacitors are grounded so as to provide a stable reference voltage and filter out unwanted noise.To add on, it provides a path of low-impedance to the ground for high freq noise and interference.This reduces EMI. The values of the two capacitors can be flexible, ranging from 6pF to 22pF, however, the specific capacitive values of C1 and C2 depend on further testing and adjustment on the overall performance of the whole circuit. Normally, the capacitive values of C1 and C2 are within 10pF if the crystal oscillator frequency is 26MHz, while the values of C1 and C2 are 10pF<22pF if the crystal oscillator frequency is 40MHz.
RF section
According to my RF classes, matching is making the input resistance equal to the load resistance. If the antenna is approaching 50 ohms then no matching is required since the output impedance of pin 2 is 50 ohms. However cheap antennas do not have 50 ohms impedance, so the N – type matching is there to help with the matching.
The resistance measured at the terminals of the antenna accounts for the power that is radiated from the antenna which is primarily the “radiation resistance”.
Antenna exhibits two types of noise:
i) Thermal noise generated from ohmic resistance
ii) The noise received from external sources (anybody with Temperature greater then 0K radiates noise energy). This is known as noise temperature.
where Ta = noise temeperature
V = noise voltage
R = resistance of the antenna
B = Bandwidth
Routing
This is a four layer board. The layers details are as follows:
1. First layer: signal lines and components are placed here
2. Second layer: GND layer and no signal lines are placed here. Vias are used to connect GND signals in other layers.
3. Third layer: Pwr layer. Vias also used to connect to power signals
4. Bottom layer: Used for signal lines only.
Power Supply
Power lines placed on the third layer. When the power lines reached the pins of the chipset, VIAs are needed so that the power lines can go through the layers to connect the pins of the chipset on the TOP layer
Crystal Oscillator
Crystal oscillator should be placed adjacent to the XTAL Pins, the connection lines shouldn’t be too long, and should be wrapped up for shelter. The input and output lines cannot be punched, cannot cross the layers or be crossed. The input and output bypass capacitor should be located near the chip; never set it on the lines. No high frequency digital signal lines shall be placed under the four layers of the crystal oscillators. The best choice is that no signal lines is placed under the crystal oscillator. The TOP layer where is crystal oscillator is placed should be as large as possible.
RF section
RF lines connecting the chip and antenna should not cover drills. RF antenna should be set away from high frequency transmitting devices, such as crystal oscillators.RF lines should not be set at a 45 degree angle (any sudden change in direction can create impedance mismatches and cause signal reflection). No signal lines should high frequency should be set near RF lines. (incl. clock lines, crystal oscillator operates at 26MHz, clk signal lines etc )
3D View
