Optical SETI Map Conferences Map Illustrations Map Photo Galleries Map Observations Map Constructing Map
Search Engines Contents Complete Site Map Tech. Support Map Order Equip. Map OSETI Network

Google
Search WWW Search www.coseti.org Search www.oseti.net Search www.photonstar.org Search www.opticalseti.org

colorbar.gif (4491 bytes)

 

Carrier-To-Noise Ratio In Incoherent Systems

9008-037

 

Graph 9008-036 illustrates how it is not really possible to achieve the ultimate CNR in conventional direct detection optical receivers.  The ultimate CNR of 31 dB for the -118 dBm signal received from the 1 kW alien transmitter situated 10 light years away, is 3 dB less than obtainable with an ideal optical heterodyne receiver.  It is also 6 dB smaller than obtainable from an ideal optical homodyne receiver.

For the particular parameter values chosen for the Silicon Avalanche Photodetector (APD) system with a gain of 400, only a CNR of 7 dB is realisable.  This assumes that the load resistance RL = 10 Mohm, and that the unity-gain dark (leakage) current Id = 0.  Even this value of CNR is somewhat optimistic, since Id is always finite.  Very small area photodiodes, such as might be used to construct a two dimentional array, can help minimize Id.  No account is made of 1/f noise that will exist at frequencies close to zero.  The quantum-noise limited region of the characteristics are asymptotic about 6 dB below the dotted line, since this line is based on assuming that the APD excess noise factor x = 0.  The characteristics for the 1 Mohm and 10 Mohm loads and 0.2 nA dark current coincide, due to the high level of shot-noise produced by this current.  Clearly, if we need a moderate bandwidth with the very high value of load, a transimpedance front-end, as illustrated in 9008-028, will be required.


Home Glossary
SPIE's OSETI I Conference SPIE's OSETI II Conference
SPIE's OSETI III Conference
The Columbus Optical SETI Observatory
 
Copyright , 1990-2006 Personal Web Site:
www.stuartkingsley.com
Last modified:  10/28/06
Contact Info