Service manual

Agilent 382A Operating and Service Manual 5
Operation
When all films are aligned, the E field of the applied field is normal to all
films. When this occurs, no current flows in the films and no attenuation
occurs. If the center film is rotated to some angle ?, the E field can be
considered to be split into two orthogonal components: E sin ? in the plane
of the film, and E cos ? at right angles to it. The E sin ? component is
absorbed by the film, while the E cos ? component oriented at an angle ?
with respect to the original wave, is passed unattenuated to the third section.
When it encounters the third film, the E cos ? sin ? component is absorbed,
and the E cos
2
? component emerges at the same orientation as the original
wave.
The attenuation is thus ideally proportional only to the angle to which the
center film is rotated and is completely independent of frequency. In terms
of dB, the attenuation is equal to 40 log cos ?.
Maximum attenuation of the attenuator exceeds the 50 dB calibrated range
by at least 20 dB but the characteristics in this range are not controlled.
Theoretically, the attenuator is capable of very high attenuation. In practice
this property is modified by the fact that the resistive film in the middle
section cannot completely absorb the E sin ? component. Hence, a small
leakage component is passed to the output. For high attenuation above 50
dB, the leakage component begins to approach the magnitude of the desired
output of the attenuator. Ultimate attenuation of this device thus becomes
limited by the attenuation of the center rotating film which is 70 dB or more.
The accuracy of the attenuator does not depend on the stability of the
resistive films: as long as their attenuation is high and remains high,
performance is not affected.
Sources Of Error One of the principle sources of error in the Model 382A is internal
reflections and finite attenuation in the center film. Reflections are
minimized by tapers on the films and careful design of the transitions and
choke joints. The center film attenuation is made as great as possible,
consistent with low reflections and reasonable length and is at least 70 dB.
Since this attenuation does vary with frequency it is not possible to calibrate
the instrument all the way to 90° rotation (maximum attenuation). A margin
of about 20 dB is necessary to maintain accuracy, so calibration is carried to
50 dB.
Another principle source of error is higher-order mode propagation in the
circular waveguide. The other modes (TM
01
and TE
21)
can exist in addition
to the wanted TE
11
mode. The TM
01
mode is equally attenuated at all angles
of the center section but cannot be attenuated completely. The TE
21
mode
should not be excited if the choke sections and transitions are perfectly
aligned. In practice, however, the choke sections are not perfectly matched