一、dB，dB， dBm， dBi
dB should be the most basic and common concept in wireless communication. We often say "propagation loss is xx dB", "transmission power is xx dBm", "antenna gain is xx dBi"... Sometimes, these very similar dBx may be confused and even cause calculation errors. What is the difference between them?
This matter has to start with dB .
When it comes to dB, the most common is 3dB !
3dB often appears in power graphs or bit error rate graphs. In fact, there is nothing mysterious about it. A drop of 3dB means that the power is reduced by half, and the 3 dB point refers to the half power point.
+3dB means that the increase is doubled, and -3dB means that the decrease is 1/2. How did this come about?
In fact, it is very simple, let us look at the calculation formula of dB:
Here, please remember a formula. Remember this formula, you can basically walk sideways. 
This can greatly facilitate our calculation and description. Especially when drawing tables, you can make up your own mind. Before converting to dB, there are so many 0s, the coordinate axes have to be pulled into outer space. . .
If you understand dB, you can only walk sideways, and if you understand other members of the dB family, you can lie down and win.
Let's start with the most commonly used dBm and dBw.
dBm and dBw are to replace the reference power P0 in the dB formula with 1 mW and 1 W respectively: 
1 mW and 1 W are definite values, so both dBm and dBw can represent the absolute value of power.
Directly on the power conversion table for your reference. 
Here, we have to remember:
1 W = 30 dBm。
The simplified formula is "30 is the benchmark, which is equal to 1 W whole".
Remember this, combined with the previous "add 3 times 2, add 10 times 10; subtract 3 and divide 2, subtract 10 and divide 10", you can do a lot of mental calculations.
Here we need to note that except for 30 dBm on the right side of the equation, the rest of the split items must be expressed in dB. That is to say, when one dBx is subtracted from another dBx, the result obtained is expressed in dB.
[Example] If the power of A is 46 dBm and the power of B is 40 dBm, it can be said that A is 6 dB greater than B.
[Example] If antenna A is 12 dBd and antenna B is 14 dBd, it can be said that A is 2 dB smaller than B. 
For example, 46 dB means that P1 is 40,000 times larger than P0, and 46 dBm means that the value of P1 is 40 W. There is only one m missing in the symbol, and the meaning it represents can be completely different.
Common in the dB family are dBi, dBd, and dBc. Their calculation method is exactly the same as that of dB, and they still represent the relative value of power.
The difference is that their reference standards are different, that is, the meanings represented by the reference power P0 on the denominator are different. 
It is generally believed that for the same gain, expressed in dBi is 2.15 larger than expressed in dBd. This difference is caused by the different directivity of the two antennas, so we won’t go into details here.
In addition, the dB family can not only represent power gain and loss, but also voltage, current, audio, etc. You need to apply it in specific scenarios.
It should be noted that we use 10lg (Po/Pi) for power gain, and 20lg (Vo/Vi) and 20lg (Io/Ii) for voltage and current gain. 
How did this 2 times more come from?
This 2 comes from the square of the electric power conversion formula. The nth power in the logarithm corresponds to n times after calculation. 
Regarding the conversion relationship between power, voltage and current, you can review junior high school physics by yourself. . . 
Finally, the editor compiled some main dB family members for your reference.
Relative value: 
Absolute value: 
Pure counting unit
First, dB is a purely counting unit: for power, dB = 10*lg(A/B). For voltage or current, the meaning of dB = 20*lg(A/B).dB is actually very simple, which is to express a very large (followed by a long string of 0) or very small (preceded by a long string of 0) number relatively briefly. like:
X=1000000000000000 (15 zeros in total)
10lgX=150dB
X=0.000000000000001
10lgX=-150 dB
dBm defines miliwatt. 0dBm=10lg1mw;
dBw defines watts. 0 dBw = 10lg1W = 10lg1000mW = 30dBm.
dB always defines the power unit by default in 10lg. Of course, in some cases, the signal strength (Amplitude) can be used to describe work and power, and at this time, 20lg is used as the meter. This is true whether it is in the field of control or signal processing. For example, sometimes you can see the expression of dBmV.
Pay attention to basic concepts
In the calculation of dB and dBm, we should pay attention to the basic concepts. For example, 0dBw = 10lg1W = 10lg1000mw = 30dBm mentioned above; another example, when one dBm is subtracted from another dBm, the result is dB. For example: 30dBm - 0dBm = 30dB.
There is only addition and subtraction between dB and dB
Generally speaking, in engineering, there is only addition and subtraction between dB and dB, no multiplication and division. The most used is subtraction: dBm minus dBm is actually the division of two powers, and the division of signal power and noise power is the signal-to-noise ratio (SNR). dBm plus dBm is actually the multiplication of two powers, which is rare (I only know that there is such an application in the power spectrum convolution calculation). What is dBm times dBm, 1mW to the 1mW power? Except that my classmates keep writing me such expressions that can almost keep pace with Goldbach's conjecture, I have lived for so many years and have never seen any engineering field play this.
dB is the unit of power gain
dB, expressing a relative value. When calculating how many dBs the power of A is greater or less than B, it can be calculated according to the formula 10 lg A/B. For example: the power of A is double that of B, then 10 lg A/B = 10 lg 2 = 3dB. That is to say, the power of A is 3dB greater than that of B; if the power of A is 46dBm and the power of B is 40dBm, it can be said that A is 6dB greater than B; if the antenna of A is 12dBd and the antenna of B is 14dBd, it can be said that A is 2dB smaller than B.
dBm is a unit that expresses the absolute value of power, and the calculation formula is: 10lg power value/1mW. For example: if the transmit power is 1mW, the converted value in units of dBm should be: 10 lg 1mW/1mW = 0dBm; for 40W power, then 10 lg (40W/1mW) = 46dBm.
1、dBm
dBm is a value for examining the absolute value of power, and the calculation formula is: 10lg (power value/1mw).
[Example 1] If the transmit power P is 1mw, it will be 0dBm after converted to dBm.
[Example 2] For a power of 40W, the converted value in units of dBm should be:
10lg（40W/1mw)=10lg（40000）=10lg（4*10^4）=40+10*lg4=46dBm。
2. dBi and dBd
dBi and dBd are the value of the test gain (power gain), both of which are relative values, but the reference standards are different. The reference base of dBi is an omnidirectional antenna, and the reference base of dBd is a dipole, so the two are slightly different. It is generally believed that for the same gain, expressed in dBi is 2.15 larger than expressed in dBd.
[Example 3] For an antenna with a gain of 16dBd on one side, when its gain is converted into dBi, it is 18.15dBi (generally ignoring the decimal place, it is 18dBi).
[Example 4] 0dBd=2.15dBi.
[Example 5] GSM900 antenna gain can be 13dBd (15dBi), GSM1800 antenna gain can be 15dBd (17dBi).
3、dB
dB is a value representing a relative value. When considering how many dBs the power of A is larger or smaller than the power of B, the following calculation formula is used: 10lg (A power/B power)
[Example 6] The power of A is twice as high as that of B, then 10lg (power of A/power of B)=10lg2=3dB.
That is, the power of A is 3 dB greater than that of B.
[Example 7] The 100-meter transmission loss of a 7/8-inch GSM900 feeder is about 3.9dB.
[Example 8] If the power of A is 46dBm and the power of B is 40dBm, it can be said that A is 6 dB larger than B.
[Example 9] If antenna A is 12dBd and antenna B is 14dBd, it can be said that A is 2 dB smaller than B.
4、dBc
Sometimes you will also see dBc, which is also a unit that expresses the relative value of power, and it is calculated in exactly the same way as dB. Generally speaking, dBc is relative to the carrier (Carrier) power. In many cases, it is used to measure the relative value of the carrier power, such as used to measure the relative value of interference (co-channel interference, intermodulation interference, intermodulation interference, out-of-band interference, etc.) and coupling, spurious, etc. Where dBc is used, dB can also be used instead in principle.
dBm, dBi, dBd, dB, dBc that are often encountered in wireless and communication
Empirical algorithm:
There is a simple formula: 0dBm=0.001W plus 10 on the left=multiplied by 10 on the right
So 0+10dBm=0.001*10W ie 10dBm=0.01W
So 20dBm=0.1W 30dBm=1W 40dBm=10W
Also, add 3 on the left side = multiply by 2 on the right side, such as 40+3dBm=10*2W, that is, 43dBm=20W. These are empirical formulas, which are quite easy to use.
So -50dBm=0dBm-10-10-10-10-10=1mW/10/10/10/10/10=0.00001mW.
Calculation method of dBm: (dBm and mW)
Generally, there are specifications on the 802.11x wireless network APs sold in the market, and there will always be a description in it that the AP (or wireless network card) has a transmission power of 20dBm, or some products are based on mW (milliWatts).
What is going on with these units?
dBm is dB-milliWatt, that is, the reading is compared with a milliWatt. If 0dBm is displayed in the instrument, it means that the signal is no different from the 1mW signal, that is to say, the signal strength is 1mW. As for Watt (watt) is a unit of power, I think everyone knows it, so I won’t go into details.
So we must start with dB, what exactly is dB? The full letter of dB is decibel, and deci in English (actually Latin) means one-tenth. This unit was originally bel. However, because the energy difference required to achieve a bel value comparison is usually relatively large and is not commonly used in circuit science, it is more common to use one-tenth of bel, that is, the unit of decibel.
So what does decibel (or bel) refer to?
In fact, it means that when you encounter two energies (signals), dB is a unit of expression that we use to express the difference between the two energies. It itself is not an independent absolute unit (such as Volt, Ampere, etc.). The appearance of the unit dB means that it is a unit obtained by comparing two energies (or signals) of the same nature.
At this point, you may have doubts: "Since dB only expresses the energy difference between two signals, why not simply use "multiple" to express it? Is this unit created to pretend to be esoteric?"
Of course not! But it's a pretty good question. isn't it? Isn't it simple and easy to understand simply using "multiples" so that so many people get the concept wrong? To a certain extent, Instructor Lin quite agrees with this statement. For example, when you make a high-frequency linear amplifier (LINEAR Amp.), if its input power is 10Watts and its output can reach 40Watts, why not simply say that it has a four-fold gain instead of a 6dB gain? In this example, the term "quadruple" is actually used neatly, but try another example of the same kind...
Today we try to imagine a set of transmitting equipment gaining from the energy of the primary oscillation to the output power of the final stage...assuming that the power of the primary oscillation is 0.5mW (note that it is an assumption, of course it will be much lower than this number) and the output of the final LINEAR Amp. is 2kW. Now try to calculate the multiple difference between them... 2kW is 2000Watts, that is, 2,000,000mW. Divide 2,000,000mW by 0.5mW to get the multiple, which is 4,000,000 times. Just think about it, I have assumed that the oscillation level is as large as 0.5mW, and I have come up with such an astonishing number of 4 million times. Once the real number is used, the multiple is bound to be larger than 4 million. So far, everyone may have understood that in various electronic and radio circuits (especially in terms of reception), such multiple differences abound (that is, if the anti-interference ability of a factory-made transmitter is better than 1 million times, it is marked as better than 60dB). How inconvenient it would be to indicate numbers of millions to tens of millions or even billions of times in various levels (such as manuals, specification sheets) every time!
So how is dB calculated?
bel = lg ( P2 / P1 )
In the above formula, P1 is the first energy (signal) to be compared, and P2 is the second energy (signal) to be compared. The units of P1 and P2 should be the same.
dB = 10 * bel = 10 * lg ( P2 / P1 )
Example: The first signal power is 4Watts, the second signal power is 24Watts, then the gain is:
10 * lg ( 24 / 4 ) = 10 * lg6 = 7.78 dB
OK, let's go back to dBm, so the formula for converting dBm to mW should look like this:
dBm = 10 * lg(mW) or mW = 10^( dBm / 10 )
So here are some examples you can check:
0 dBm = 1 mW
10 dBm = 10 mW
14 dBm = 25 mW
15 dBm = 32 mW
16 dBm = 40 mW
17 dBm = 50 mW
20 dBm = 100 mW
30 dBm = 1000 mW = 1W
If everyone is very smart, they will be able to find the following rules from the basic properties of the log:
Increase in dB by 3dB = mW multiplied by 2 times; decrease in dB by 3dB = mW becomes 1/2; increase by 10dB = multiply by 10 times
This way, you can do quick calculations directly in your head to get rough values:
+3dBm= *2
+6dBm= *4 (2*2)
+7dBm= *5 (+10dB-3dB = 10/2)
+4dBm= *2.5 (+10dB-6dB = 10/4)
+1dBm= *1.25 (+4dB-3dB=2.5/2)
+2dBm=*1.6(+6dBm-4dBm=4/2.5=1.6)
For example, suppose you already know that 0dBm = 1mW, then 3dBm is of course equal to 2mW. So, what about 47dBm? 40dBm → 10^4mW, 7dBm more → 5 * 10^4mW = 50W.
dBc
Sometimes you will also see dBc, which is also a unit that expresses the relative value of power, and it is calculated in exactly the same way as dB. Generally speaking, dBc is relative to the carrier (Carrier) power. In many cases, it is used to measure the relative value of the carrier power, such as used to measure the relative value of interference (co-channel interference, intermodulation interference, intermodulation interference, out-of-band interference, etc.) and coupling, spurious, etc. Where dBc is used, dB can also be used instead in principle.
dBuV
According to the basic formula V^2=P*R between power and level, it can be known that dBuV=90+dBm+10*log(R), and R is the resistance value. In the PHS system, the correct value should be dBm=dBuv-107, because its antenna feed impedance is 50 ohms.
dBuVemf and dBuV
emf: electromotive force (electromotive force)
For a signal source, dBuVemf refers to the port voltage when it is open, and dBuV refers to the port voltage when it is connected to a matched load.
Question: What is the difference between dBi, dBd, dB, dBm, and dBc?
Answer: They are both units of power gain, the differences are as follows:
dBi and dBd are units of power gain, and both are relative values, but the references are different. The reference for dBi is an omnidirectional antenna; the reference for dBd is a dipole. It is generally believed that dBi and dBd represent the same gain, and the value represented by dBi is 2.15 dBi larger than that represented by dBd. For example: for an antenna with a gain of 16 dBd, when the gain is converted into dBi, it is 18.15dBi, and the decimal place is generally ignored, which is 18dBi.
dB is also the unit of power gain, expressing a relative value. When calculating how many dBs the power of A is greater or less than B, it can be calculated according to the formula 10 lg A/B. For example: the power of A is double that of B, then 10 lg A/B = 10 lg 2 = 3dB. That is to say, the power of A is 3dB greater than that of B; if the power of A is 46dBm and the power of B is 40dBm, it can be said that A is 6dB greater than B; if the antenna of A is 12dBd and the antenna of B is 14dBd, it can be said that A is 2dB smaller than B.
dBm is a unit that expresses the absolute value of power, and the calculation formula is: 10lg power value/1mW. For example: if the transmit power is 1mW, the converted value in units of dBm should be: 10 lg 1mW/1mW = 0dBm; for 40W power, then 10 lg (40W/1mW) = 46dBm.
dBc is also a unit that expresses the relative value of power, and it is calculated in exactly the same way as dB. Generally speaking, dBc is relative to the carrier (Carrier) power. In many cases, it is used to measure the relative value of carrier power, such as measuring the relative value of interference (co-channel interference, intermodulation interference, intermodulation interference, out-of-band interference, etc.) and coupling, spurious, etc. Where dBc is used, dB can also be used instead in principle.
Practical Information - About Antenna Gain and Its Considerations
In the practical application of wireless communication, in order to effectively improve the communication effect and reduce the input power of the antenna, the antenna will be made into various structures with radiation directionality to concentrate the radiation power, which leads to the concept of "antenna gain". To put it simply, antenna gain refers to the degree to which an antenna radiates the input RF power. Obviously, the gain of the antenna has a great relationship with its pattern. The narrower the main lobe and the smaller the side lobe, the higher the gain of the antenna. However, antennas with different structures have great differences in their pattern.
In the field of communication technology, like other parameters such as power and level, antenna gain is also expressed by relative comparison and logarithmic simplification. The specific calculation method is: when the same radiation field strength is generated from a certain direction to a certain position, take the logarithm of the ratio of the input power of the lossless ideal reference antenna to the input power of the antenna under consideration and multiply it by 10 (G=10lg (reference Pin/consideration Pin)), which is called the gain of the antenna in the direction of this point. Commonly used units to measure antenna gain are dBi and dBd. For dBi, its reference is an ideal point source antenna, that is, a real "point" as a comparison reference for antenna gain. The radiation of an ideal point source antenna is omnidirectional, and its pattern is an ideal sphere, and the electromagnetic radiation intensity of all points on the same sphere is the same; for dBd, its reference is an ideal dipole antenna. Because the dipole antenna is directional, the two have a fixed constant difference of 2.15, that is, 0dBd="2".15dBi.
It should be noted that the commonly referred to as "omnidirectional antenna" is not a strict term. An omnidirectional antenna should refer to omnidirectionality in a three-dimensional space. However, in engineering circles, an antenna whose pattern in a plane is a circle is often referred to as an omnidirectional antenna, such as a whip antenna. Its main lobe in the radial direction is circular, but it still has an axial side lobe.
Common antenna gain: whip antenna 6-9dBi, GSM base station Yagi antenna 15-17dBi, parabolic directional antenna can easily achieve 24dBi.
The radio frequency signal output by the radio transmitter is sent to the antenna through the feeder (cable), and is radiated by the antenna in the form of electromagnetic waves. After the electromagnetic wave reaches the receiving location, it is received by the antenna (only a small part of the power is received), and sent to the radio receiver through the feeder. Therefore, in wireless network engineering, it is very important to calculate the transmitting power of the transmitting device and the radiation capability of the antenna.
Tx is the abbreviation of Transmits. The transmitted power of radio waves refers to the energy within a given frequency range, and there are usually two measures or measurement standards:
1. Power (W): Relative to the linear level of 1 watt (Watts). For example, the transmit power of a WiFi wireless network card is usually 0.036W, or 36mW.
2. Gain (dBm): Relative to 1 milliwatt (milliwatt) proportional level. For example, the transmit gain of WiFi wireless network card is 15.56dBm.
The two expressions can be converted to each other:
1. dBm = 10 x log [power mW]
2. mW = 10[ gain dBm / 10 dBm]
In wireless systems, antennas are used to convert current waves into electromagnetic waves. During the conversion process, the transmitted and received signals can also be "amplified". The measure of this energy amplification is called "Gain". The unit of measure for antenna gain is "dBi". Since the electromagnetic wave energy in the wireless system is generated by the amplification and superposition of the transmitting energy of the transmitting device and the antenna, it is best to measure the transmitting energy with the same measurement-gain (dB). For example, the power of the transmitting device is 100mW, or 20dBm; the gain of the antenna is 10dBi, then:
Total transmit energy = transmit power (dBm) + antenna gain (dBi)
= 20dBm + 10dBi
= 30dBm
or: = 1000mW
= 1W
In "low power" systems (such as WLAN equipment) every dB counts, especially remembering the "3 dB rule". Every increase or decrease of 3 dB means that the power is doubled or reduced by half: whaosoft aiot http://143ai.com
-3 dB = 1/2 power
-6 dB = 1/4 power
+3 dB = 2x power
+6 dB = 4x power
For example, the wireless transmission power of 100mW is 20dBm, the wireless transmission power of 50mW is 17dBm, and the transmission power of 200mW is 23dBm.
Power/Level (dBm): The output capability of the amplifier, generally in W, mW, dBm. dBm is the absolute power level expressed in decibels, taking 1mW as the reference value.
Conversion formula:
Level (dBm) = 10lgW
5W →10lg5000 = 37dBm
10W →10lg10000 = 40dBm
20W →10lg20000 = 43dBm
It is not difficult to see from the above that every time the power doubles, the level value increases by 3dBm
2. 5V withstand voltage pin in MCU
First give a schematic diagram of the internal structure of a typical IO port. 
Figure 0 Schematic diagram of the internal structure of the IO port
We put the internal structure diagram of the I/O port here, but we will not mainly discuss its details today. Generally speaking, the MCU powered by 3.3V will have some 5V-resistant pins. Through these 5V input pins, it is very convenient to directly connect to 5V devices without the need for level conversion. This article briefly talks about some characteristics of the 5V withstand voltage pin.
For the dsPIC33CK256P508 series, Table 8-2 in the data sheet shows all 5V tolerant input pins, as shown in the shaded ports in the figure below. 
Figure 1 Port of 5V withstand voltage pin
These pins are also described in the absolute maximum rating section of the data sheet, as shown in Figure 2 below. 
Figure 2 Description of the rated voltage of the 5V withstand voltage input pin
It can be seen here that the 3.3V withstand voltage MCU can reach a maximum withstand voltage of 4V on VDD, and for non-5V input withstand voltage pins, its maximum withstand voltage can reach VDD+0.3V. For the 5V withstand voltage pin , there are two cases . When the VDD power supply is less than 3V, even if the pin is a 5V withstand voltage input pin, its maximum withstand voltage is only 3.6V. When the VDD power supply is greater than 3V, the maximum withstand voltage of this pin is 5.5V. Therefore, for a 5V withstand voltage pin, pay attention to its VDD supply voltage to determine whether it has the ability to 5V withstand voltage input.
Figure 3 Device package representation information
From the pin package layout diagram of the device, you can also see the relevant information of the 5V input withstand voltage. As shown in Figure 3, for the 28pin SSOP package device, it can be seen from the figure that the shaded part is the 5V withstand voltage input pin. 
Figure 4 Input voltage specification of 5V withstand voltage input pin
Note that this refers to the input voltage specification of the pin. When the pin is 5V withstand voltage, it will have an impact on the input high level voltage index of this pin. For example, if the general GPIO port is 5V withstand voltage, then its maximum withstand voltage is 5.5V. The dsPIC33 series chip MCLR pin is generally 5V withstand voltage, so it also supports a maximum of 5.5V.
For the I2C pin, when the SMbus function is enabled or not, the lower limit of the high level threshold of this pin is affected , and the upper limit of the high level is 5.5V. 
Figure 5 The impact of 5V withstand voltage pin on leakage current
From the data in the specification, no matter whether the GPIO is 5V withstand voltage or not, its leakage current index is the same, which is +-700nA. The 
injection current index of pin 6 in Figure 6
For GPIO, except for the 5V withstand voltage pin, the maximum injection current index is 5mA when the input is high. For the 5V withstand voltage pin, in fact, it has an important feature. It does not contain an internal high-side diode to connect to VDD power supply, so it cannot tolerate any positive input current .