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Pulse Modulation
deviation is no longer limited b y the FM rate, only b y the maximum
limit of 8 MHz.
Pulse Modulation
Pulse mo dulation can b e accepted from an external source at the
PULSE connector or can b e in ternally generated. The damage lev els
of the PULSE input are +10 and 05 V DC. The input imp edance is
50 . A function generator m ust b e capable of driving TTL lev els
in to a 50 load. With no input signal, the pulse input is held low,
so activ ating pulse with no input causes RF output to sh ut o.
The sw ept signal generator can also pro duce a 27.778 kHz square
w a v e for use in Agilent scalar net w ork analyzers. Sw ept signal
generators with Option 002 in ternally generate a syn thesized pulse.
The sw ept signal generator pro vides in ternal pulse mo dulation with
pulse widths adjustable with 1 s resolution (adjustable with 25 ns
resolution with Option 002).
Lev eling
Pulse lev eling p erformance dep ends on the accuracy of the dio de
detector whic h measures the RF amplitude. The ALC blo c k diagram,
Figure M-4, sho ws the pulse mo dulation input signal to the sw ept
signal generator whic h con trols a pulse mo dulator. The pulse input
is represen ted b y 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 con trolled b y the linear mo dulator used for CW lev eling and AM.
T race 2 is the resultant RF pulse, whic h is the RF output. This pulse
is detected b y the dio de detector. It trails the pulse input b ecause of
propagation dela ys in the pulse mo dulator and its driv e circuits.
The output of the detector is amplied b y a logarithmic amplier
(log amp).
T race 3 is the output of the log amp. Note that this signal is
dela y ed from the RF output signal and that the rise time is slow er.
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 lev el D A C and the dierence (error) signal driv es an
in tegrate{and{hold circuit. The in tegrator output driv es the RF
output p o w er lev el via the linear mo dulator. When the sum of the
detected and reference signals is 0 v olts, the output of the in tegrator
is held at a constan t lev el and the RF output is lev eled.
T race 4 is the dela y ed signal from the pulse input whic h con trols the
switch in the in tegrate{and{hold circuit. T race 4 is timed to coincide
with trace 3. Since the in tegrate{and{hold switch is closed only when
trace 3 is high, the in tegrator resp onds to correct the p o w er lev el
only when the RF p o w er is on.
Operating and Programming Reference M-19