a. Requirement-Dial accuracy within ± (2 MHz + 1%
of dial reading), sensitivity within the specified limits listed in
Table 5-7. Phase lock must operate through all frequency
ranges.
b. Apply a frequency and amplitude calibrated signal be-
tween -60 dBm and -30 dBm, to the RF INPUT connector
listed in the Table 5-7. Switch the Type 491 Band Selector
to the appropriate band.
c. Set the DISPERSION to 500 kHz/div and the RESO-
LUTION control for a resolution bandwidth of 100 kHz
(fully clockwise).
d. Adjust the GAIN control for an average noise amplitude
of one division. Center the IF CENTER FREQ and FINE RF
CENTER FREQ controls.
e. Tune the signal on screen with the RF CENTER FRE-
QUENCY control. Reduce the signal amplitude with the
signal generator output attenuator control for an on-screen
display, then adjust the MIXER PEAKING control and sweep
rate for optimum amplitude. (Sweep rate approximately
5 ms/div,)
Fig. 5-7. Signal to noise ratio for measuring sensitivity.
f. Calibrate the signal generator output then adjust the
variable output attenuator control on the signal generator,
until the signal amplitude is two divisions (twice the noise
amplitude). See Fig. 5-7.
g. Check the total signal attenuation (in dB) below 0 dBm
as indicated on the signal generator attenuator dial. This
is the sensitivity of the analyzer for the RF center frequency
indicated. Check as listed in Table 5-7 under 100 kHz resolu-
tion. Sensitivity can also be checked for 1 kHz resolution;
however, a very stable signal source is required at higher
frequencies.
NOTE
Cable losses for frequencies of 10 GHz and higher
became significant and must be added for correct
sensitivity measurements. Refer to Fig. 6-45 for
insertion loss of a 6 foot cable.
h. Center the IF CENTER FREQ controls and the FINE
RF CENTER FREQ control, then tune the signal to the center
of the screen with the RF CENTER FREQUENCY control.
(Horizontal sweep must be centered.)
i. Check the dial accuracy as listed in Table 5-7, Must
equal or exceed ± (2 MHz + 1% of the dial reading).
j. As the dial accuracy is checked, depress the LOCK
CHECK button and check for phase lock beats. Check for
a phase lock operation at the center and extreme frequency
position for each scale. Dial accuracy need only be checked
for scales 1, 2 and 4. The remaining scales are harmonic
settings of these fundamental frequency ranges,
k. Check phase lock operation with an external reference
frequency as follows:
1) Apply a 1 V peak to peak, 1 MHz signal, from the
Constant Amplitude Signal Generatar (Type 191)” to the REF
FREQ IN connector, Use a BNC T connector to apply the
input signal to the Type 491 to provide a convenient moni-
toring point for the test oscilloscope. The input signal voltage
level must be measured at the REF FREQ IN connector.
Turn the INT REF FREQ control to the OFF or EXT REF FREQ
IN position,
2) Center the FINE RF CENTER FREQ control. Depress the
LOCK CHECK button and adjust the RF CENTER FREQ control
until a beat frequency is displayed.
3) Adjust the FINE RF CENTER FREQ control for a LO
lock or until the beat reduces to zero (zero beat).
4) Repeat the above procedure with a 5 MHz signal from
the signal generator.
(5) Increase the input signal amplitude to 5 V peak to peak
and repeat the check with the increased signal amplitude
at 5 MHz and 1 MHz.
24B. Alternate Procedure to Check Dial Accu-
racy, Oscillator and Mixer Operation, and
Effectiveness of Local Oscillator Phase Lock
a. Apply 100 MHz (10 ns) markers from the Time-Mark
Generator through a coaxial cable, BNC to GR adapter,
Harmonic Generator [calibration fixture 067-0594-00), 200
MHz Trap (calibration fixture 067-0595-00), GR to N type
adapter, and the 20 dB RF attenuator (011-0086-00), to the
band B RF INPUT.
b, Switch the band selector to B. Set the DISPERSION
RANGE to MHz/DIV, and the DISPERSION to 2 MHz/div.
Switch the VERTICAL DISPLAY selector to LOG.
c. Check dial accuracy for band B as follows:
1) Tune the RF CENTER FREQUENCY through the band.
Observe the 100 MHz harmonics and their image spurii travel
5-11
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