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Amplitude Modulation
giving 0 dBm output p o w er and greater AM depth p oten tial. The
ALC can no w b e v aried o v er its en tire range and the attenuator
remains at a xed lev el.
Uncoupled mo de can also b e used for the follo wing:
T o increase the a v ailable AM depth if y ou are mo dulating near the
minimum p o w er range of the ALC lo op.
T o oset the p o w er sw eep range.
T o reduce AM noise b y op erating at a higher ALC lev el.
AM Rate
The maximum AM rate a v ailable is limited b y the bandwidth of
the comp onen ts in the RF path. A t rates of ab out 100 kHz, the
in tegrator can no longer resp ond so the ALC lo op is ee ctively
op ened. The feedforw ard path pro vides the capabilit y to mo dulate at
m uc h faster rates.
RF comp onen ts in the ALC lo op limit the ALC bandwidth to
250 kHz. High p o w er and Option 006 sw ept signal generators are also
limited to 100 kHz b y the comp onen ts in the RF path. Sw ept signal
generators with frequency doublers (A32) are limited b y the 100 kHz
bandwidth of the doubler for carrier frequencies greater than 20 GHz.
Note that due to the feedforw ard sc heme, AM bandwidth is not
aected when amplitude and pulse mo dulation are simultaneously
activ ated.
Deep AM
Deep AM mo de is a means of reducing distortion when the desired
AM depth is v ery deep (greater than 90%) or when mo dulating b elow
an ALC lev el of 020 dBm. Amplitude mo dulation is summed with
the reference lev el signal. The detected signal is compared to the
reference. Therefore, the ALC lo op should follo w the AM input.
Ho w ev er, the detector's abilit y to sense low p o w er lev els limits the
maximum AM depth. When the mo dulation signal reduces the
output p o w er lev el to a lev el whic h is b elow the detector's range
limit, the error signal generated sends the in tegrator 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
p o w er lev el of the detected signal. When the signal lev el is out of the
detector's range, the lo op in tegrator switch op ens (op ening the ALC
lo op). The output of the in tegrator is frozen, applying a constan t
driv e to the mo dulator. Since the mo dulator's most linear range is
at low p o w er lev els, the AM en v elop e distortion is minimal. When
the comparator senses a signal that is within the detector's range, the
in tegrator switch is closed, re-engaging ALC lo op lev eling.
Figure M-2 sho ws the lev eled AM c haracteristics in the dieren t
mo des. The maximum lev eled output with ALC engaged is sho wn as
the sw ept signal generator's maximum lev eled output sp ecication.
(Individual sw ept signal generators may ha v e more p o w er; w atch
Operating and Programming Reference M-15