Page 48 - Complete Wireless Design

Wireless Essentials

Wireless Essentials 47

The ratio V /V of the amplitude of the reflected signal, V , to the amplitude
C D C
of the incident signal from the signal generator, V , gives the magnitude of S ,
D 22
the output reflection coefficient. This value will be less than unity. The phase
angle variance between V and V stands for the phase angle of S :
C D 22

C D
So S will equal:
22
V
C
S
22 V C D
D
S , the reverse transducer gain, equals the voltage measured at V by M
12 B 2
divided by V :
D
V
B
S
12 V
D
This is also referred to as the amplifier’s isolation. It should be as low a value
as feasible, since we would like to have a gain in the forward direction only,
with as little reverse gain, and thus little adverse interaction, from reflections
back into the amplifier’s output port as possible.
The phase difference between V and V is measured as
B D

B D
So S will equal
12
V
B
S
12 V B D
D
To convert these values into decibels as desired:

|S | dB 20 log |S |
11 11
|S | dB 20 log |S |
22 22
|S | dB 20 log |S |
21 21
|S | dB 20 log |S |
12 12
The DUT, a BJT transistor, has now been characterized at a single frequen-
cy and bias point by employing S parameters. These S parameters could now
be inserted into an RF circuit simulation program, or they could be used to
design an input and output matching circuit for the BJT. S-parameter match-
ing and simulation will be discussed in detail in Sec. 3.1, “Small Signal
Amplifiers.”

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