Propogation delay :

• The output of a logic gate does not change its state instantaneously in response to the change in the state of its input.

• There is a time delay between these two events ,which is called as the propogation delay . Thus propogation delay is defined as time delay between the instant application of an input pulse and the instant of occurrence of the corresponding output pulse

   

Fan Out :

• Fan out is defined as the maximum number of the inputs of the same IC family that a gate can drive without falling outside the specified voltage limits.

• Higher the fan out higher is the current supplying capacity of the gate.

   

Power dissipation : 

• Power dissipation is the amount of power drawn by the IC due to the current flowing current through the IC as a result of the applied voltage.

   

Figure of merit : 

• The figure of merit of a logic family is the product of power dissipation and propogation delay . It is called as speed power product . The speed is specified in seconds and power is specified in W . The figure of merit should be as low as possible .
• The figure of merit is always a compromise between speed and power dissipation . This means if we reduce the propogation delay then the power dissipation will increase and vice - versa.

• Figure of merit is always used as a common means for measuring and comparing the over all performance of different IC families.

   

Noise Margin :

• Noise immunity is defined as the ability of logic circuit to tolerate the noise without causing any unwanted changes in the output.
• A quantitative measure of noise immunity is called as noise margin.