ERRORS
In all waveguide attenuators, the excitation of modes other than the
desired one distorts the theoretical attenuation characteristic, particularly
at low attenuation levels. In the Type 874-GAL Attenuator, the TM0
1
mode (capacitive
coupling) is the most serious
offender.
This mode
is attenuated more rapidly with distance from the source end of the wave-
guide than is the desired mode and its intensity is proportional to the volt-
age across the exciting line at the coupling point, while the intensity of
the desired mode is proportional to the current flowing in the exciting line
at the point of coupling.
The effect of the undesired mode on the attenu-
ation characteristic therefore decreases as the attenuation increases. Since
the output
from the attenuator is the sum of the voltage generated in the
loop by both modes, serious errors can result if the voltage in the exciting
line at the coupling point is high compared to the current; that is, if the
impedance is high.
An error correction procedure is required to achieve
the stated accuracy for the 874-GAL Attenuator.
CORRECTION PROCEDURES
There are two configurations that may be employed in using the
874-GAL. First,it may be terminated in a 50-ohm termination (874-W5OBL)
as shown in Figure 2,in which case the special correction given in Fig-
ure 2 is required, and, in addition, the frequency correction of Figure 3 is
required.
In the second configuration recommended to achieve the best
accuracy above 2 GHz, the termination is an adjustable stub employed to
place a voltage minimum at the coupling points. The frequency correction
graph of Figure 3 is required only in this case.
The detailed procedure
is given in the following paragraphs.
If the exciting line is terminated in its characteristic impedance, for
instance with a Type 874-W5OBL Termination, the error resulting from the
coupling of the TM0
1
mode can be corrected by using the correction chart
of Figure 2. The ~ign of the correction depends on which end of the
exciting line is connected to the source of rf power. If the source is con-
nected to the end marked with the letter S, use the correction curve labeled
NORMAL and subtract the correction found for initial setting of the atten-
uator from the initial scale reading.
Then subtract the correction found
for the final setting of the attenuator from the final scale reading.
The
true attenuation is
the difference in the corrected values.
If the source
is connected to the opposite end, add to the indicated values the corrections
read from the curve labeled REVERSED. When these correction charts are
used, the indicated relative attenuation is accurate within ±(O.015 times
the diff0.2)dB at frequencies up to 2000MHz. For accurate results
2
GENERAL RADIO
