Agilent Technologies ENA Series User Manual Supplement page 21

Table 1-1
Frequency
100 MHz
500 MHz
1 GHz
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
The series may be reduced to a simple form to leave us with the relationship shown
in Equation 10. The term L is a function of the loss of the cable at a specific
frequency and the wavelength at that frequency.
Equation 10
Vref
---------- - = Γ
SRL =
Vin
2 2
The term (L )/(1−L ) can be thought of as the number of bumps that are
contributing to SRL. It represents a balance between the contribution of loss in a
single bump and further bumps in the cable for the specified frequency and cable
loss. Calculate the distance into the cable by multiplying the term (L
distance between bumps.
Table 1-1 illustrates some calculated values for a typical trunk cable. From the table,
bumps spaced 1.5 meters apart out to 307 meters will contribute to SRL.
SRL Equation Constant
Spacing Loss
(λ/2)m (dB)/m
1.5 0.014
0.3 0.033
0.15 0.05
How to Use Table 1-1
Refer to Figure 1-2 and Equation 10 for the following discussion.
Γ = the reflection coefficient of each bump (V
L = the cable loss between bumps (V
The distance between bumps equals λ/2 (1/2 wavelength).
Typical values:
Γ<< 1
L ≤ 1 for low loss cable
20
 
2
 
L
 -------------- - 
2
 
1 L –
 
dB/bump
L
−0.02
−0.01
−0.0075
reflected
/V
transmitted incident
2
)/(1−L
bumps
Distance(m)
2 2
/(1−L )
205 307
433 129
554 83
/V )
incident
)
Chapter 1
2
) by the