鏈路噪聲係數計算公式
光鏈路噪聲係數計算
噪聲(功率或電平)
=10lg (ktBF) =−174dBm/Hz+B+F
=10lg(ktBF)
=−174dBm/Hz+B+F
系統噪聲係數
Fs
Fs
F_{s}=F_{1}+frac{F_{2}−1}{G_{1}}+frac{F_{3}−1}{G_{1}G_{2}}+⋯
F
s
=F
1
+
G
1
F
2
−1
+
G
1
G
2
F
3
−1
+⋯
設前置放大器噪聲係數
NF_{pre}=3dB
NF
pre
=3dB
設後置放大器噪聲係數
NF_{post}=3dB
NF
post
=3dB
激光器噪聲---相對強度噪聲
N_{RIN}
N
RIN
設激光器噪聲係數
N_{RIN}=-155dB/Hz
N
RIN
=−155dB/Hz
又
N_{RIN}=frac{P_{begin{array}{cc}opt&noise end{array}}^{2}(激光器輸出端光噪聲)}{P_{laser}^{2}(激光器平均光輸出功率)}
N
RIN
=
P
laser
2
(激光器平均光輸出功率)
P
opt
noise
2
(激光器輸出端光噪聲)
所以
P_{begin{array}{cc}opt&noise end{array}}=sqrt{NF_{RIN}⋅P_{laser}^{2}} =sqrt{−155dB/Hz⋅(4×10^{−3})^{2}} =7.113×10^{−8}mW
P
opt
noise
=
NF
RIN
⋅P
laser
2
=
−155dB/Hz⋅(4×10
−3
)
2
=7.113×10
−8
mW
引起此RIN光噪聲的RIN輸入噪聲電流(光電流)
因為
P_{begin{array}{cc}opt&noise end{array}}=I_{RIN}⋅T(E/O)
P
opt
noise
=I
RIN
⋅T(E/O)
所以
I_{RIN}=frac{P_{begin{array}{cc}opt&noise end{array}}}{T(E/O)} =frac{7.113×10^{−8}mW}{0.1W/A} =7.113×10^{−10}A
I
RIN
=
T(E/O)
P
opt
noise
=
0.1W/A
7、113×10
−8
mW
=7.113×10
−10
A
其中,T(E/O)為激光器轉換函數(光電流——激光器功率),E為在激光器輸入端的RIN噪聲功率,O為射頻RIN噪聲產生激光器光電流。
P_{RIN(in)}=I_{RIN}^{2}⋅R_{laser} =(7.113×10^{−10})^{2}⋅5 =2.53×10^{−15}mW(−145.97dBm)
P
RIN(in)
=I
RIN
2
⋅R
laser
=(7.113×10
−10
)
2
⋅5
=2.53×10
−15
mW(−145.97dBm)
因為一個器件輸入端噪聲功率
=10lg (ktBF) =−174+F
=10lg(ktBF)
=−174+F
所以
F_{RIN}=−145.97−174=28.03dB
F
RIN
=−145.97−174=28.03dB
光電二極管產生的噪聲
P_{begin{array}{cc}photo&noise end{array}}
P
photo
noise
(在光電二極管輸出端)
P_{begin{array}{cc}photo&noise end{array}}=I_{begin{array}{cc}photo&noise end{array}}^{2}⋅R_{pho} =2q⋅I_{p}⋅B =2×1.6×10^{−19}庫⋅2.95mA =9.444×10^{−17}mW/Hz(−160.25dBm/Hz)
P
photo
noise
=I
photo
noise
2
⋅R
pho
=2q⋅I
p
⋅B
=2×1.6×10
−19
庫⋅2.95mA
=9.444×10
−17
mW/Hz(−160.25dBm/Hz)
其中
I_{p}
I
p
是平均(DC)光功率產生的電流,q是電子電荷
q=1.9×10^{−19}庫侖
q=1.9×10
−19
庫侖
設激光器的平均(D)光功率為4mW,經光纖(1KM)損耗為0.35dB(0.92),光電二極管的轉換函數T(O/E),照到光電二極管的光功率產生的RF電流T(O/E)=0.8A/W。
所以
I_{p}=P_{laser(DC)}⋅L_{cable}⋅T(O/E) =4mW×0.92×0.8A/W =2.95mA
I
p
=P
laser(DC)
⋅L
cable
⋅T(O/E)
=4mW×0.92×0.8A/W
=2.95mA
由光電二極管在其輸出端產生的噪聲,折算其輸出端的噪聲係數
F=-160.25+174=13.75dB。
光鏈路系統噪聲係數:
F_{s}=F_{1}+frac{F_{2}−1}{G_{1}}+frac{F_{3}−1}{G_{1}G_{2}}+⋯ =2+frac{1.288-1}{100}+frac{635.331}{10^{1.89}}+frac{1.1749-1}{10^{2.096}}+frac{23.714-1}{10^{0.626}}+frac{1.117-1}{10^{0.626}}+frac{2-1}{10^{0.58}} =2+0.00288+8.172+0.0014+5.374+0.0277+0.263 =15.84(12dB)
F
s
=F
1
+
G
1
F
2
−1
+
G
1
G
2
F
3
−1
+⋯
=2+
100
1、288−1
+
10
1、89
635.331
+
10
2、096
1、1749−1
+
10
0.626
23.714−1
+
10
0.626
1、117−1
+
10
0.58
2−1
=2+0.00288+8.172+0.0014+5.374+0.0277+0.263
=15.84(12dB)
激光器RF增益(損耗)分析
物理過程:RF功率加到激光二極管上產生電流(光電流)
P_{RF}=I_{laser}^{2}⋅R_{laser}^{}
P
RF
=I
laser
2
⋅R
laser
所以
I_{laser}=sqrt{frac{P_{RF}}{R_{laser}}} =sqrt{frac{0.7762×10^{−3}(W)}{5(Ω)}} =0.01246A
I
laser
=
R
laser
P
RF
=
5(Ω)
0.7762×10
−3
(W)
=0.01246A
此RF光電流,產生調製光功率。光電流轉換至光功率的轉換系數T(E/O),T(光電流——光功率)=T(W/A)
P_{調製光}=I_{laser}T(E/O) =0.01246A×0.W/A =0.001246W
P
調製光
=I
laser
T(E/O)
=0.01246A×0.W/A
=0.001246W
frac{P_{調製光}}{P_{RF}}=frac{1.246mW}{0.7762}=1.605(2.06dB)
P
RF
P
調製光
=
0.7762
1、246mW
=1.605(2.06dB)
接收光電二極管增益(損耗)分析:
照射到光電二極管的光功率轉換為RF電流。
I_{photo(RF)}=P_{光}⋅T(O/E) =1.15mW×0.8A/W =0.92mA
I
photo(RF)
=P
光
⋅T(O/E)
=1.15mW×0.8A/W
=0.92mA
此RF電流流過負載電阻(匹配網絡輸入電阻)產生RF功率
P=I^{2}_{photo}⋅R =(0.92mA)^{2}⋅50Ω =0.043mW
P=I
photo
2
⋅R
=(0.92mA)
2
⋅50Ω
=0.043mW
光電二極管增益
=frac{0.043mW}{1.15mW}=−14.27dB
=
1、15mW
0.043mW
=−14.27dB