- To calculate optical receiver sensitivity, total noise in the receiver is
Substituting these values and solving equation (5.2.3) gives
Substituting these values and solving equation (5.2.3) gives
… (5.3.1)
… (5.3.2)
… ( 5.3.3)
Where,
This equation is known as thermal noise characteristic of an optical receiver.
- The optimum gain to achieve desired BER for receiver is given by –
… (5.3.4)
Assuming no ISI i.e. γ =1 Where,
Q is parameter related so S/N ratio to achieve desired BER.
W is thermal noise characteristic of receiver. X is photodiode factor. I2 is normalized BW.
Mean Square Input Noise Current
- The mean square input noise current is gives as –
… (5.3.5)
- Shot noise Current :
- Thermal Noise :
- Shunt Noise :
- Series Noise :
- Total Noise:
… (5.3.6)
… (5.3.7)
Example 5.3.1 : An InGaAs PIN photodiode has the following parameters at a wavelength of 1300 nm : ID = 4 nA, η = 0.9, RL = 1000 Ω and the surface leakage current is negligible. The
incident optical power is 300 nW (- 35 dBm) an the receiver bandwidth is 20 MHz. Find the various nosie terms of the receiver.
Solution : Given :
λ = 1300 nm ID = 4 nA
η = 0.9
RL = 1000Ω
Pincident = 300nW
B = 20 MHz
Mean square quantum noise current
Ans.
Mean spark dark current
= 2 (1.6 x 10-19) (20 x 106) (4 x 10-9)
= 0.256 x 10-19 Amp … Ans.
Mean square thermal noise current
Where B is Boltzman constant = 1.38 x 10-23 J/K
T = (25o+273o) = 298 K
= 3.28 x 10-16 Amp … Ans.