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 signal 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 signal 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 signal generator have different RF components)
limit the ALC range available at the high end. The internal
instructions (firmware) of the swept signal generator limit the ALC
level range available for request from -20 to
+25
dBm.
If the
power level requested is higher than the swept signal generator is
capable of producing, the maximum available power is produced,
and the message line displays UNLVLED (unleveled). When the swept
.signal 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
+lO
dBm.
This fact is reflected in the performance
specifications of the swept signal 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 signal generator unless
Uncoupl
Atten
or Leveling Mode
ALCoff
is selected. The proper
combination of ALC level and attenuator setting is decided by the
A-6 Operating and Programming Reference
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