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Model8690B
A
B
Figure 2.
(A) Full-band start-stop sweep with markers. (B)
Expansion of 2A display between markers on Marker Sweep.
scale. The frequency markers can be applied to the llF
sweep as well as the start-stop sweep.
As noted above, the Start-CW and Stop-llF controls are
used for the llF sweep. Thus the marker controls can be
used for wideband sweep applications and the start-stop
controls for llF operation. This provides independently
adjustable wide and narrow band sweep modes with full cali-
bration for both modes.
Sweep Modes
Recurrent, triggered, and manual sweeps are available.
Recurrent and triggered sweep times are adjustable from
0.01 to 100 seconds, and the triggered sweeps can be syn-
chronized from an external source or manually (front-
panel pushbutton) for photography or X-Y recording. To
enhance the clarity of oscilloscope presentations, RF power
is blanked during retrace to produce a zero baseline; how-
ever, RF is restored before the start of the subsequent sweep
to eliminate transients during the early part of the sweep.
Blanking can be disabled, when desired, with a rear-panel
switch. Oscilloscope photography of slow sweeps is simpli-
fied by a front-panel sweep indicator. This indicator turns
on automatically at the start of the sweep and turns off at
the end. Thus there is no guesswork about when to close
the shutter and remove the film and no time wasted waiting
to be sure the sweep is finished.
For X-Y recorder presentation, an automatic pen-lift
circuit is provided. The circuit drops the pen during the
stabilizing period prior to the sweep and lifts the pen dur-
ing the second stabilizing period which occurs at the end of
Page 3
the sweep just before retrace. Thus transients and retrace
lines are eliminated from X-Y plots.
On manual sweep, a front-panel control varies the RF
frequency between the limits set on the selected sweep func-
tion. With the use of the manual sweep, X-Y recorder setup
time is reduced to seconds.
The sweepers provide their own voltage ramp concur-
rently with the RF sweep for driving the horizontal systems
of oscilloscopes and X-Y recorders. While this ramp varies
somewhat from unit to unit, it remains the same within a
given unit regardless of sweep mode, duration, or direction.
Once the sweep end points have been set on the recording
device, they will remain the same for any sweep. Thus the
broad flexibility and simple push-button operation of these
units can be fully utilized.
In addition to the sweep voltage from the sweep oscil-
lator, the RF units provide a reference voltage which is
proportional to the output frequency. This reference is
about zero at the lowest frequency and increases about 40
volts per octave. The reference, available at the rear of the
RF units, can be used to drive an external monitor when a
continuous indication of frequency is required. It is also
especially useful with the new HP 8410A Network Analyzer
system to enable accurate phase-lock synchronization at high
sweep speeds.
Leveling
The 8690B Sweep Oscillators are designed for external,
closed-loop leveling. This technique minimizes the varia-
tions in output amplitude with frequency which are inherent
in backward-wave oscillators. A basic leveling setup is
shown in Figure 3. The leveling signal, the result of RF
sampling by the directional coupler and detection by a
crystal detector, is applied to a leveling amplifier built into
the sweep oscillator. The amplified signal is applied to the
PIN diode attenuator in the "B" series of RF units and to
the BWO grid in the· "A" series to maintain a constant
power at the output of the directional coupler. External
leveling therefore eliminates the frequency-dependent trans-
mission characteristics of any components (cables, connec-
tors, amplifiers, filters, etc.) between the oscillator and
sampling point.
External leveling with directional couplers has the further
advantage of virtually eliminating source mismatch. Thus
leveled power can be established at any point in a system,
even though it is remote from the source. The degree of
leveling is determined primarily by the characteristics of the
directional coupler and detector.
86908
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DETECTOR
Figure 3.
Basic leveling configuration in coax.
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