Agilent Technologies 8360B, Руководство пользователя, Страница: 269 / 496

Amplitude Modulation
for an UNLVLED message.) The minimum lev el is limited b y the
detector's range (approximately 020 dBm). With deep AM engaged,
the minimum lev el (where the ALC lo op is op ened) is set to
013 dBm. This guarantees that the detector can still sense the signal
lev el with no distortion. With the ALC lo op op en, the minimum
lev el is limited b y the mo dulator's range to appro ximately 050 dBm.
Figure M-2. P o w er Accuracy Ov er the AM Dynamic Range
Calibrating the Linear Mo dulator
The AM bandwidth calibration feature calibrates the linear
mo dulator gain at the curren t CW frequency . This results in more
accurate p erformance in deep AM mo de when the ALC lo op is
op ened. Figure M-2 sho ws the calibrated resp onse of the mo dulator
compared to the uncalibrated resp onse. If y ou c ho ose to calibrate
\Alwa ys", the sw ept signal generator will automatically p erform the
calibration whenev er y ou c hange the CW frequency . Although this
feature pro vides more accurate p erformance, note that it also slows
the frequency switching time b y 20 ms.
ALC Bandwidth
Since the ALC lo op is op en at p o w er lev els less than 013 dBm in
deep AM mo de, p o w er lev els at v ery slow AM rates are sub ject to
in tegrate{and{hold drift of t ypically 0.25 dB/s. Setting the ALC
bandwidth to low reduces drift b y a factor of 10 b y switching a larger
capacitor in to the in tegrator circuit. The larger capacitor reduces the
eects of leak age on the in tegrator. The ALC bandwidth defaults
at factory preset to the auto selection whic h normally selects the
appropriate bandwidth (high or low) for y our application. Ho w ev er,
in this case (mo dulating with deep AM at a slow rate), auto mo de
w ould ha v e set the ALC bandwidth to high where a setting of low
w ould decrease drift. T o make the bandwidth selection, the sw ept
signal generator determines whic h functions are activ ated suc h as
M-16 Operating and Programming Reference