Photo detector noise
- 1. Government Engineering College, Bharuch Optical communication presentation
- 2. Photo detector NoisePhoto detector Noise Prepared by : Xyz 6th Sem (E.C-A)
- 3. Noise is a term generally used to refer to any undesired disturbances that mask the received signal in a communication system. • Thermal noise • Shot noise
- 4. Thermal Noise •Also known as Johnson Noise or Nyquist noise •The thermal noise current it in a resistor R may be expressed by its mean square value and is given by: • where K is Boltzmann's constant, T is the absolute temperature and B is the post-detection bandwidth. R KTB it 42 =
- 5. • Electrons within any resistor never remain stationary and this constitutes a randomly varying current known as thermal current. • Motion due to their thermal energy. The noise current produced by these random motions of charged carriers is called “Thermal noise current” . It is given by I
- 6. SHOT NOISE • Discrete nature of electrons causes a signal disturbance called shot noise. • Deviation of the actual number of electrons from the average number is known as shot noise. • Present for BOTH current: Signal and dark current.
- 7. Shot Noise due to Dark Current • When there is no optical power incident on the photodetector a small reverse leakage current still flows from the device terminals and this contributes to the total system noise • The shot noise due to the dark current, id is given by: where e is the charge of an electron and Id is the dark current. dd eBIi 22 =
- 8. Shot Noise on the Photocurrent The shot noise, is on the photocurrent Ip is given by: ps eBIi 22 =
- 9. Overall Receiver Noise • Figure shows a block schematic of the front end of an optical receiver and the various noise sources associated with it. • The majority of the noise sources shown apply to both main types of optical detector (p-i-n and avalanche photodiode). • The avalanche photodiode receiver is the most complex case as it includes noise resulting from the random nature of the internal gain mechanism.
- 11. The total shot noise iTS is given by: The thermal noise due to the load resistance RL is given by: L TH R KTB i 42 = ( )dpTS IIeBi += 22
- 12. • The signal to noise ratio (SNR) for the p-n or p-i-n photodiode receiver may be obtained by summing all the noise contributions. • It is given by: ( ) 2 2 4 2 amp L dp p i R KTB IIeB I N S +++ = where is the Photo detector Responsivity, iamp=total noise from amplifier circuit,
- 13. The noise associated with the amplifier, iamp can be combined with the thermal noise from the load resistor it using the noise figure, Fn for the amplifier to give: The expression for the SNR can now be written in the form: L n ampt R KTBF ii 422 =+ ( ) L n dp p R KTBF IIeB I N S 4 2 2 ++ =
- 14. Thank You