Models 691A/692A/693A/694A Section I11
Paragraphs 3-71 to 3-88
3-71. RF BLANKING. 3-79. AMPLITUDE MODULATION.
3-72. The RF blanking automatically attenuates the 3-80. EXTERNAL AM.
Oscillator output at least 30 db between sweeps giving
a no-output reference trace on anoscilloscope display 3-81. The Sweep Oscillator RF output may be ampli-
of swept-frequency measurement. Blanking can be tude modulated by signals applied to the front-panel
connector under the EXT AM pushbutton. External
used with start-stop, marker, and A F sweeps in the amplitude modulation is possible with any sweep
AUTO and TRIG modes. With AUTO sweeps the mode or function. Frequency response is dc to
blanking interval ends slightly in advance of RF sweep
start to allow external circuits and equipment to sta- 350 Kc for unleveled RF output and dc to 50 Kc
bilize. Blanking can be disabled with the rear-panel for leveled output. Negative 10 volts reduces RF
BLANKING on-off switch except during power meter output at least 30 db below rated CW output.
RF leveling when blanking is automatically disabled.
3 -82. INTERNAL FREQUENCY MARKERS.
3-73. EXPANDED SWEEP OPERATION.
3-83. Two calibrated frequency markers can be in-
3-74. Certain swept-frequency measurements, such dependently adjusted over the full frequency range of
as bandpass filter evaluation, require rapid examina-
tion of more than one frequency band (e.g., filter the Sweep Oscillator. One is tuned by the MARKER 1
overall response characteristic, pass and stop bands). control and the other by the MARKER 2 control. Each
marker amplitude-modulates the RF output with a
wedge-shaped notch at the frequency indicated on the
3-75. Because the start-stop and marker sweeps are digital counter adjacent to the tuning control. The
independent they can be used in combination with the front-panel MARKER AMP control permits amplitude
internal frequency markers to obtain expanded sweep
presentation. For instance, start-stop sweep can be adjustment of the RF-modulating marker but does
used to cover a broad frequency range such as the not affect the amplitude of the pulses at the rear-
panel MARKER output. The amplitude of these pulses
overall response characteristic of the filter mentioned is typically -5 volts into a 1000-ohm load.
above. If a segment of this range (the stop band, for
example) merits detailed examination, the internal 3-84. Activated separately by the MARK 1 and MARK
frequency markers can be activated and tuned to 2 pushbuttons, the markers can be used individually
bracket the important segment. Then, pressing or simultaneously during start-stop, A F, or external
MARKER SWEEP expands the bracketed segment to FM operation with auto, manual, or triggered mode.
occupy the full presentation and sweep time. With- In addition, markers may be used in combination with
out further adjustment, the original sweep may be external amplitude or internal square-wave modu-
restored at will by pressing START-STOP. lation.
3-76. The foregoing example of expanded sweep op- 3-85. INTERNAL SQUARE WAVE.
eration assumes one sweep range within another.
3-86. Internally-generated square-wave modulation
However, the two sweep ranges need not beone within can be used with any sweep function or sweep mode
the other or even overlap; they may each cover and with marker or external amplitude modulation.
separate, remote segments of the total frequency
range. At rated RF output the square wave on-off ratio ex-
ceeds 20 db. The INT SQ WAVE pushbutton selects
square wave modulation, and INT SQ WAVE FREQ
3-77. FOUR PRESET CW FREQUENCIES. permits continuous adjustment of frequency from
3-78. The manual sweep control, in conjunction with 950 to 1050 cps.
the CW mode and the start-stop and marker sweep
functions, can be used to obtain four preset CW 3-87. RF POWER LEVELING.
frequencies. 3-88. A microwave signal source which tunes auto-
a. Select four different frequencies using START/ matically through a preset frequency range in a pre-
CW, STOP/AF, MARKER 1, and MARKER 2 controls. set time, such as the ($3 690 series Sweep Oscillator,
facilitates rapid, broadband evaluation of microwave
b. Rotate MANUAL SWEEP fully clockwise. devices. An additional requirement for such meas-
urements is constant RF power in the frequency range
c. Set SWEEP SELECTOR to CW. of interest. Characteristically, however, the back-
ward-wave oscillator (BWO) used as the microwave
d. Press START-STOP to obtain CW output at fre- source in the Sweep Oscillator does not generate
quency indicated by start dial pointer. constant RF power throughout the operating frequency
range. The RF power output of a BWO usually con-
e. Press MARKER SWEEP to obtain CW output at sists of minor, narrowband variations superimposed
frequency indicated by MARKER 1 digital counter. on a gross variation, as illustrated in Figure 3-8.
Minor variations are less than 3 db while the gross
f. Set SWEEP SELECTOR to MANUAL to obtain variation may be as great as 10 db. By deriving a
CW output at frequency indicated by MARKER 2 signal which is the inverse of this power character-
digital counter. istic and applying it as amplitude control, the RF
power output of the Sweep Oscillator can be main-
g. Press START-STOP to obtain CW output at fre- tained essentially constant with changing frequency.
quency indicated by stop dial pointer. This control of output power is called leveling.
02280-1 3-11
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