Amplitude Modulation
for an
UNLVLED
message.) The minimum level is limited by the
detector's range (approximately
0
20 dBm). With deep AM engaged,
the minimum level (where the ALC lo op is op ened) is set to
0
13 dBm. This guarantees that the detector can still sense the signal
level with no distortion. With the ALC lo op op en, the minimum
level is limited by the mo dulator's range to approximately
0
50 dBm.
Figure M-2. Power Accuracy Over the AM Dynamic Range
Calibrating the Linear Mo dulator
The AM bandwidth calibration feature calibrates the linear
mo dulator gain at the current CW frequency. This results in more
accurate p erformance in deep AM mo de when the ALC lo op is
opened. Figure M-2 shows the calibrated resp onse of the mo dulator
compared to the uncalibrated resp onse. If you choose to calibrate
\Always", the swept signal generator will automatically p erform the
calibration whenever you change the CW frequency. Although this
feature provides more accurate p erformance, note that it also slows
the frequency switching time by 20 ms.
ALC Bandwidth
Since the ALC lo op is op en at p o
wer levels less than
0
13 dBm in
deep AM mo de, p ower levels at very slow AM rates are sub ject to
integrate{and{hold drift of typically 0.25 dB/s. Setting the ALC
bandwidth to low reduces drift by a factor of 10 by switching a larger
capacitor into the integrator circuit. The larger capacitor reduces the
eects of leakage on the integrator. The ALC bandwidth defaults
at factory preset to the auto selection which normally selects the
appropriate bandwidth (high or low) for your application. However,
in this case (mo dulating with deep AM at a slo
w rate), auto mo de
would have set the ALC bandwidth to
high
where a setting of
low
would decrease drift. To make the bandwidth selection, the swept
signal generator determines which functions are activated suchas
M-16 Operating and Programming Reference
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