1 MHz MARKERS OUT-EXT REF FREQ IN-A BNC con-
nector that provides 1 MHz marker output signals to cali-
brate the dispersion. With the INT REF FREQ control in
Controls coupled together
the OFF position, an external signal between 1 MHz and
and in the 10 MHz/div
5 MHz (1 to 5 V peak to peak) applied to the connector
becomes the reference frequency for phase lock operation.
IF ATTENUATOR dB
All switches OFF position
RF INPUTS-Coaxial connectors which connect through
IF CENTER FREQ
either a coaxial cable, or (if above 12.4 GHz) a wave-
guide mixer, to the signal source. Band A frequency range
is 10 to 275 MHz. Band B frequency range is 275 MHz to
2 GHz. Band C consists of a coaxial mixer (green label) for
the frequency range 1.5 GHz to 12.4 GHz; or a Wave-
guide Mixer Adapter (black label) which connects through
a two foot cable to one of three external Waveguide
RF CENTER FREQUENCY
Mixers for the frequency range 12.4 GHz to 40 GHz.
FINE RF CENTER FREQ
5 full turns from either
INT REF FREQ
Out of the OFF switch
detent in the VARIABLE
TRIG IN-A BNC connector to apply external triggering
signals. Frequency range: 20 Hz to 100 kHz, signal ampli-
tude equal to or greater than 0.2 V.
Fully CCW in the
switched SEARCH position
SAW OUT-A 70 to 90 mV sawtooth signal output that
is coincident with the analyzer sweep.
3. Connect the Type 491 to a suitable power source and
RECORDER-Signals on the display may be recorded by
turn the POWER switch to ON. Allow a few minutes for
connecting to the RECORDER output. A linear output, equal
the instrument to stabilize.
to or greater than 4 mV per displayed division amplitude
4. Adjust the INTENSITY control clockwise until a trace
of signal, in the LIN mode, inta a load impedance of
is visible, then adjust the FOCUS and ASTIGMATISM controls
for optimum trace definition.
5. Position the trace to the horizontal center and to the
FIRST TIME OPERATION
bottom graticule line of the graticule with the POSITION
The following procedure demonstrates the basic functions
6. Adjust SCALE ILLUM control for the desired graticule
of the controls and connectors for the Type 491. We recom-
mend this procedure for first time familiarization. Careless
or incorrect operation may damage the instrument.
7. Apply a signal with an amplitude between -60 and
-30 dBm, from a Signal Generator or other source that is,
1. Check input power selector positions at the rear panel.
within the frequency range of band B, through a coaxial
Correct selector positions for the different input line voltages
cable to band B, RF INPUT connector.
are given under Voltage Considerations in this section.
2. Preset the front panel controls as follows:
If a signal source within the frequency range af
band B is not available, apply the signal to band
A or C and set the band selector to the appropri-
8. Adjust the GAIN control for a moderate noise level
(1 division) on the display, then tune the RF CENTER FRE-
QUENCY control through the frequency range. Note that
some of the signals move across the screen at different rates,
and the direction of movement (left to right or right to left)
of the signals is not the same. (See Spurious and Image
Frequency displays later in this section.)
9. Tune the dial with the RF CENTER FREQUENCY control
Time Base Controls
to the frequency of the applied input RF signal.
10. Switch the MIXER PEAKING control from SEARCH
position to manual, then adjust for optimum signal ampli-
11. Adjust the GAIN and/or the IF ATTENUATOR dB
switches for a signal amplitude of approximately 6 divisions.