Note
Two terms are used in the following discussions: power output and
ALC level. Power output means actual output power including the
effects of the attenuator. ALC level means power levels before the
attenuator. In swept CW generators without attenuators, these two
terms are equivalent.
Internal Leveling
-
Leveling Mode Normal,
Leveling Point
Intrnl
In this configuration (Figure A-l), power is sensed by a detector
internal to the swept CW generator and a dc output from this
detector is fed back to the Level Control Circuits.
The ALC level is limited at the low end by the Level Control Circuits
and at the high end by maximum available power. Noise and
drift limit the range at the low end to -20
dBm
or greater. The
combination of RF frequency and RF components (different models
of swept CW generator have different RF components) limit the ALC
range available at the high end. The internal instructions (firmware)
of the swept CW generator limit the ALC level range available for
request from -20 to
+25
dBm.
If the power level requested is higher
than the swept CW generator is capable of producing, the maximum
available power is produced, and the message line displays UNLVLED
(unleveled). When th
e swept CW generator performs frequency
sweeps at certain ALC levels, maximum available power can be
exceeded during small portions of the sweep; in this case, a flashing
UNLVLED message appears.
ALC leveling accuracy depends on power level. Although the ALC
level is
useable
from -20 to
+25
dBm,
it is most accurate from
-10 to i-10
dBm.
This fact is reflected in the performance
specifications of the swept CW generator.
Coupled Operation.
Since many applications require power output
less than -20
dBm,
an optional step attenuator has a range of
0 to 90 dB, in 10 dB steps. With this option, power output down
to -110
dBm
is achieved when the Step Attenuator and Level
Control Circuits work in conjunction (see Figure A-l). With the
attenuator, the ALC level is normally used over the smaller, more
accurate portion of its range. Since ALC level accuracy suffers below
-10
dBm,
and at some frequencies only
+l
dBm
of RF output is
available, the ALC level is set between -10 and 0
dBm.
For power
less than -100
dBm,
the attenuator is set to 90
dB,
and the ALC
level is used from -10 to -20
dBm.
At frequencies where power
output above 0
dBm
is desired, the attenuator is set to 0 dB, and the
ALC level is used from 0 to
+25
dBm
(or whatever power is available
at the RF frequency in use).
Coupled operation is assumed by the swept CW generator unless
Uncoupl
Atten
or Leveling Mode ALCoff is selected. The proper
combination of ALC level and attenuator setting is decided by the
firmware. In coupled operation, when desired power output is set via
A-6 Operating and Programming Reference
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