Tektronix 491 Maintenance Manual page 130

Fig. 6-43. Typical equipment setup to check response flatness, sensitivity, frequency calibration and phase lock operation for band C, scales 7
t h r o u g h 8 .
Fig. 6-44. Signal to noise ratio for measuring sensitivity.
6-42
h. Center the IF CENTER FREQ controls ond the FINE RF
CENTER FREQ control then tune the signal to the center of
the screen with the RF CENTER FREQUENCY control (Hori-
zontal sweep must be centered.)
i. Check the dial accuracy as listed in Table 6-9. 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 display at the center and extreme frequency posi-
tions for each scale. Dial accuracy need only be checked for
scales 1, 2 and 4. The other scales are harmonic settings of
these fundamental ranges.
k. CHECK-Phase lock operation with an external refer-
ence frequency as follows:
1) Apply a 1 V peak to peak, 1 MHz signal, from the Con-
stant Amplitude Signal Generator (Type 191) to the REF
FREQ IN connector. Use a BNC T connector to apply the in-
put signal to the Type 491 to provide a convenient monitor-
ing point for the test oscilloscope. The input signal voltage
must be measured at the REF FREQ IN connector. Turn the
INT REF FREQ control to the OFF or EXT REF FREQ IN posi-
tion.