TABLE OF CONTENTS Noise is so great, that one cannot hear God Thunder
(Howell)
Definitions. According to [238]:
An attenuator is an adjustable passive network that reduces the power level of a signal without introducing appreciable distortion.
A pad (attenuating pad) is a nonadjustable passive network that reduces the power level of a signal without introducing appreciable distortion.
Comments
Usually pads and attenuators operate under matching conditions, i.e., the input and output impedances of the attenuator (pad) are matched to the impedances loading its terminals.
Often, the attenuation (or loss) introduced by the attenuator (pad) is denoted by L and expressed in decibels.
Prove the following theorem: the noise factor of any reciprocal, passive two-port is numerically equal to the loss of the two-port, i.e.,
where G a is the available power gain and L the loss. Comment on the conditions under which this equality holds.
Solution
According to (5.13), the input noise temperature of the matched attenuator is
where T is the attenuator temperature. On the other hand, expression (4.77) establishes an equivalence between the input noise temperature and the noise factor of any two-port:
Equating the previous expressions for T e, we have
At T = T o = 290 K, we obtain
Another implicit condition to be satisfied in order to ensure the validity of the theorem is that the input termination of the two-port must also have a...
