Amplitude Modulation
for an
UHL’:rLED
message.) The minimum level is limited by the
detector’s range (approximately -20
dBm).
With deep AM engaged,
the minimum level (where the ALC loop is opened) is set to
-13
dBm.
This guarantees that the detector can still sense the signal
level with no distortion. With the ALC loop open, the minimum
level is limited by the modulator’s range to approximately -50
dBm.
p
out
I
Maximum
Soecified
Power
Figure M-2. Power Accuracy Over the AM Dynamic Range
Calibrating the Linear Modulator
The AM bandwidth calibration feature calibrates the linear
modulator gain at the current CW frequency. This results in more
accurate performance in deep AM mode when the ALC loop is
opened. Figure M-2 shows the calibrated response of the modulator
compared to the uncalibrated response. If you choose to calibrate
“Always”, the swept signal generator will automatically perform the
calibration whenever you change the CW frequency. Although this
feature provides more accurate performance, note that it also slows
the frequency switching time by 20 ms.
ALC Bandwidth
Since the ALC loop is open at power levels less than -13
dBm
in
deep AM mode, power levels at very slow AM rates are subject 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
effects 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 (modulating with deep AM at a slow rate), auto mode
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 such as
M-16 Operating and Programming Reference
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