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Circuit Description-SW 503
Referring to Figure 4-2, when ramp A falls below zero,
4.4
$WEEP OSCILLATOR MODULE
CR210 conducts, thus adding ramp B to U210B . The
The rf sweep oscillator module A20 provides a swept or
amplitude of ramp B is determined by R215, which
Cw signal from 1 MHz to 400 MHz (referto Figure 4-3) . The
therefore controls the slope of the entire right half of the
swept signal is generated by heterodyning the output of a
ramp (the slope can only be made steeper) . When the
1 GHz fixed-frequency oscillator with the output of a
voltage on the cathode ofCR206fallsbelowzerothediode
voltage-controlled
oscillator that
sweeps
from
conducts, thus adding ramp C. The voltage applied to the
approximately 1001 MHz to 1400 MHz. The difference
cathode of CR206 is ramp A with a positive offset ;
frequency is amplified, and the output amplitude is
therefore, a zero-volt level at this point occurs at a later
controlled by the leveler circuit, described in Sub-
point in time than did that at CR210. Inputs D and E are
section 4.5. Center frequency calibration is provided by
formed in a similar manner, using different offset levels .
drivingpin8ofthemodulewithadcvoltagedeterminedby
The offset levels are achieved by applying ramp A through
R235, the Freq 200 adjustment, and buffered by U230B .
a voltage divider to a constant-current source, 0190 .
Since the currents through the diodes are negligible, the
voltage drops across resistors R196, R202, and R206 are
4.5
RF OSCILLATOR LEVELER
constant .
The leveler circuit serves two purposes: It maintains a
constant rf output amplitude ai the sweep generator
module output over the swept frequency range, and it
4.3 .3 Output Amplifier
allowsoutputleveladjustmentormodulationviathefront-
U210B is an inverting operational amplifier. Its input
Panel CAL (attenuation) control of EXT CONTROL AMPL
connector. The output is leveled by a detector, com-
adds the segments of ramp from the ramp shaping
parator, and voltage variable attenuator loop (see
network (F in Figure 4-2) . Its output drives the rf sweep
Figure 4-3) . The detector and voltage variable attenuator
generator module . Frequency adjustment R225 adjusts
are inside the rf oscillator module ; U250A is the com-
the gain, thus setting the zero-frequency point.
parator. The detector outputs a do voltage proportional to
the rf amplitude at the various frequencies. If the voltage
deviates from the do reference, the comparator im-
mediately changes the voltage to the voltage variable
attenuator, thus increasing or decreasing the rf applied to
the amplifier. The result is a flat rf output .
Figure 4-3 . RF sweep oscillator module block diagram .
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