Amplitude Modulation
giving 0 dBm output p ower and greater AM depth p otential. The
ALC can nowbe varied over its entire range and the attenuator
remains at a xed level.
Uncoupled mo de can also be used for the following:
To increase the available AM depth if you are mo dulating near the
minimum p ower range of the ALC lo op.
To oset the p ower sweep range.
To reduce AM noise by operating at a higher ALC level.
AM Rate
The maximum AM rate available is limited by the bandwidth of
the comp onents in the RF path. At rates of ab out 100 kHz, the
integrator can no longer respond so the ALC lo op is
eectively
opened. The feedforward path provides the capability to mo dulate at
much faster rates.
RF comp onents in the ALC lo op limit the ALC bandwidth to
250 kHz. High p ower and Option 006 swept signal generators are also
limited to 100 kHz by the comp onents in the RF path. Swept signal
generators with frequency doublers (A32) are limited by the 100 kHz
bandwidth of the doubler for carrier frequencies greater than 20 GHz.
Note that due to the feedforward scheme, AM bandwidth is not
aected when amplitude and pulse mo dulation are sim
ultaneously
activated.
Deep AM
Deep AM mo de is a means of reducing distortion when the desired
AM depth is very deep (greater than 90%) or when mo dulating belo
w
an ALC level of
0
20 dBm. Amplitude mo dulation is summed with
the reference level signal. The detected signal is compared to the
reference. Therefore, the ALC lo op should follow the AM input.
However, the detector's ability to sense lowpower levels limits the
maximum AM depth. When the mo dulation signal reduces the
output p ower level to a level which is below the detector's range
limit, the error signal generated sends the integrator to rail, resulting
in gross AM distortion. This is where deep AM mo de should b e used.
Deep AM engages a comparator circuit (see Figure M-1) to sense the
power level of the detected signal. When the signal lev
el is out of the
detector's range, the lo op integrator switch opens (op ening the ALC
lo op). The output of the integrator is frozen, applying a constant
drive to the mo dulator. Since the mo dulator's most linear range is
at lowpower levels, the AM envelop e distortion is minimal. When
the comparator senses a signal that is within the detector's range, the
integrator switch is closed, re-engaging ALC lo op leveling.
Figure M-2 shows the leveled AM characteristics in the dierent
mo des. The maximum leveled output with ALC engaged is shown as
the swept signal generator's maximum leveled output sp ecication.
(Individual swept signal generators mayhave more power; watch
Operating and Programming Reference M-15
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