- In a receiver system errors arises because of noises and disturbances in the signal detection system. Noise is an unwanted electric signal in signal processing. The noise sources can be internal or external to the system. Only the internal sources of noise are considered
- The nose is generated by spontaneous fluctuations of current and voltage (e.g. shot noise, thermal noise). When photons incident on the photodetector are random in nature, quantum noise (shot noise) is generated. This noise is significant for both PIN and APD
- Other sources of photodetector noise are from dark current and leakage current. These noise can be reduced considerably by choosing proper components. Thermal noise is generated from detector load resistances.

- Intersymbol interference (ISI) also contributes to error which is causing from pulse Because of pulse spreading energy of a pulse spreads into neighbouring time slots, results in an interfering signal. Fig. 5.2.1 shows ISI.
- Energy in appropriate time slot is γ. The total energy in adjacent slots is 1 – γ as shown in the 5.2.1.

### Noise Mechanisms

** **

… (5.2.1)

Where,

v_{N} (t) is noise voltage.

v_{out} (t) is mean output voltage.

The noise voltage can be expressed as –

… (5.2.2)

Where,

V_{shot} (t) is quantum or shot noise. v_{R} (t) is thermal or Johnson noise.

v_{I} (t) is resulted by amplifier input noise current source i_{a} (t). v_{E} (t) is requested by amplifier input voltage noise source e_{a} (t).

- The mean square noise voltage is expressed as –

… (5.2.3)

### i) Thermal noise of load resistor R_{b} :

… (5.2.4)

Where,

k_{B} T is product of Boltzmann’s constant and temperature. A is amplifier gain.

B_{bae} is noise equivalent BW.

### ii) Thermal noise due to amplifier input noise current source i_{a} (t) :

… (5.2.5)

** **

Where,

S_{I} is the spectral density of amplifier input noise current source.

### iii) Thermal noise due to amplifier input noise voltage source e_{a} (t) :

** … (5.2.6)**

Where,

S_{E} is the spectral density of amplifier noise voltage source. B_{e} is noise equivalent BW of amplifier.

### iv) Mean square shot noise :

… (5.2.7)

** **

Where,

is mean square avalanche gain.

is photocurrent.

- All constituents of mean square noise voltage are summarized