Bird Technologies 4314B - Manual (Page 29)

17
Attenuation By Open Or Short Circuit
Attenuation can also be determined by the open circuit method. The test set
exhibits good equality between forward and reflected readings when the load
connector is open or short circuited. When this is checked on an open circuit and
an open-circuited length of line of unknown attenuation is connected to the load
connector, the ratio shown is the attenuation in two passes along the line (down
and back).
To convert the reading to decibels, divide it in half or double the line length
because twice the line length is being measured (down and back). This measure-
ment must be supplemented with a reflected power to forward power ratio
check or with a dc continuity check or leakage check, since open circuits or
shorts may exist part of the way along the line.
Attenuation can also be determined as described above by using a short circuit
rather than an open circuit. The open circuit method is preferred because the
initial equality (forward power to reflected power) is more easily achieved in an
open circuit.
Measuring Percentage Of Positive Modulation
To measure the percentage of positive modulation in an amplitude modulation
system, employ the average and peak reading characteristics of the unit since:
By substitution we get:
Where: P
P
= peak power as read with CW/PEAK switch in PEAK mode.
P
C
= carrier power as read with CW/PEAK switch in CW mode
Or:
And by cancellation:
After determining average and peak readings, find the intersection of the carrier
and peak powers on figure 7 to determine the percentage of positive modulation.
PercentagePosMod
E
MAX
E
CARRIER
–
E
CARRIER
---------------------------------------------""100 =
PercentagePosMod
P
P
P
C
–
P
C
----------------------------
 
 
 
""100 =
PercentagePosMod
P
P
P
C
-----------
P
C
P
C
----------- –
 
 
 
""100 =
PercentagePosMod
P
P
P
C
----------- 1 –
 
 
 
""100 =