物聯網平台開發人才平日班

物聯網平台開發人才平日班
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2016年3月1日 星期二

什麼是RSSI、SNR、dbm ?



http://www.ti.com/product/CC2500

Low Cost, Low-Power 2.4 GHz RF Transceiver Designed for Low-Power Wireless Apps in the 2.4 GHz ISM B

Device TypeSmart RF Transceiver   
Flash (KB)0   
RAM size (KB)1^ 2^ 4   
Min VCC1.8   
Max VCC3.6   
RX Current (Lowest) (mA)13.3   
Data Rate (Max) (kbps)500   
Modulation TechniquesOOK^2-FSK^GFSK   
Sensitivity (Best) (dBm)-104   
TX Power (Max) (dBm)1   
USB 2.0No   
Key Software FeaturesSimpliciTI Supported   
RatingCatalog   

Receiver Sensitivity (單位: dbm) :這個愈小表示其接收訊號靈敏度愈好。如有一個energy 是-130dbm, Sensitivity 為-140 dbm 的RF, 可以收到感知此訊號, 而Sensitivity 為-120dbm 則感知不到此訊號。



RF power : 功率單位用mW表示, 但常用log表示,在數線上才好表示 dbm 。 dBm=10*log(pf/1mw)

用來表示兩訊號的關係,如SNR (Signal to Noise Ratio)。可以用 Power, 但常用dB來示gain或 decay倍數關係, 如原始訊號被放大一倍, , 用log表示, 則會是說增加了3db
dB=10*log(pf/pi)

 0dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W

log2=0.3010, 10*log(2)=3db
每增加3db 就是功率增加(gain)一倍x2,  所以功率增加6db就是power x4
每滅少3db 就是功率減少(loss)一倍x0.5, 所以功率減少6db就是power /4



用成log,的另一個好處理是, 訊號可以用放大或縮小都可以用加減法來運算。

log(AB)=logA+logB
log (A/B)=logA-logB
Received Power (dbm) = Transmitted Power (dbm) + Gains (db) − Losses (db)
signal -53dbm, noise -90dbm, SNR= -53-(-90)=37db
一般Wi-Fi 品質叫做好, 那SNR 至少肴 20db (signal是noise旳100倍)




RSSI(Received Signal Strength Indication)是指接收的信號強度指示(in dbm ),用來判定鏈結的品質。

RSSI (Recieved Signal Strength Indicator) is a common name for the signal strength in a wireless network environment. It is a measure of the power level that a RF client device is receiving from an access point, for example. RSSI is the relative signal strength in a wireless environment and can be measured in any unit of power. It is often expressed in decibels (db), or as percentage values between 1-100, and can be either a negative, or a positive value.

The closer the figure is to zero, the better. For example, RSSI of -65 is better than -85. As a general example, a good signal would be -50 dbm, a reasonable would be -75, and a bad one would be -90, while -100 would provide no service at all.

To get a meaniful measurement of a good quality signal, you would have to subtract the noise on the line from that signal power level and consider the difference. A higher difference of signal to noise indicates better signal.



Wi-Fi 發射TX功率,美國 FCC/ 台灣NCC 規範 100mW (20dbm), 通常接收到時的功率只有-50 dbm (因為訊訊在中間傳輸時會loss)









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