Pulse Modulation
deviation is no longer limited by the FM rate, only by the maximum
limit of 8 MHz.
Pulse Modulation
Pulse mo dulation can be accepted from an external source at the
PULSE connector or can be internally generated. The damage levels
of the PULSE input are +10 and
0
5 V DC. The input imp edance is
50 . A function generator must b e capable of driving TTL levels
into a 50  load. With no input signal, the pulse input is held low,
so activating pulse with no input causes RF output to shut o.
The swept signal generator can also pro duce a 27.778 kHz square
wave for use in Agilent scalar network analyzers. Swept signal
generators with Option 002 internally generate a synthesized pulse.
The swept signal generator provides internal pulse mo dulation with
pulse widths adjustable with 1

s resolution (adjustable with 25 ns
resolution with Option 002).
Leveling
Pulse leveling performance depends on the accuracy of the dio de
detector which measures the RF amplitude. The ALC blo c
k diagram,
Figure M-4, shows the pulse mo dulation input signal to the sw
ept
signal generator which controls a pulse mo dulator. The pulse input
is represented by trace 1 in Figure M-5. The pulse mo dulator is
either full on or full o. The amplitude, when the pulsed RF is on,
is controlled by the linear mo dulator used for CW leveling and AM.
Trace 2 is the resultant RF pulse, which is the RF output. This pulse
is detected by the dio de detector. It trails the pulse input b ecause of
propagation delays in the pulse mo dulator and its driv
e circuits.
The output of the detector is amplied by a logarithmic amplier
(log amp).
Trace 3 is the output of the log amp. Note that this signal is
delayed from the RF output signal and that the rise time is slower.
This is a result of the bandwidth of the detector and the log amp.
The amplitude of trace 3 is summed with the reference signal
from the level DAC and the dierence (error) signal drives an
integrate{and{hold circuit. The integrator output drives the RF
output p ower level via the linear mo dulator. When the sum of the
detected and reference signals is 0 volts, the output of the integrator
is held at a constant level and the RF output is leveled.
Trace 4 is the delayed signal from the pulse input which controls the
switch in the integrate{and{hold circuit. Trace 4 is timed to coincide
with trace 3. Since the integrate{and{hold switch is closed only when
trace 3 is high, the integrator responds to correct the p ower level
only when the RF power is on.
Operating and Programming Reference M-19
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