Tektronix 491, Maintenance Manual

Page: 86 / 226

towards the center of the display, merge over the IF feed-
through response than separate and move off the screen,
as the center frequency is tuned through 100 MHz check points
on the dial. See Fig. 5-8. Error between the dial readings
and the frequency check points must not exceed ± (2 MHz
+ 1% of the dial reading).
2) Adjust the MIXER PEAKING control to optimize signal
amplitude. Note the spectral display of the harmonic gen-
erator signals. Tune the RF CENTER FREQUENCY through
the band, checking for dead spots which could be caused
by either the local oscillator failure or mixer malfunction.
The MIXER PEAKING must be peaked at all check points.
d. Check-Local oscillator phase lock operation as fol-
lows:
1) Turn the INT REF FREQ control on, Decrease the DIS-
PERSION to 500 kHz/div.
2) Depress the LOCK CHECK button and adjust the FINE
RF CENTER FREQ control to position the display to the center
of the graticule area. Release the LOCK CHECK button.
3) Shift the IF feedthrough response approximately 2
g r a t i c u l e d i v i s i o n s o f f c e n t e r w i t h t h e I F C E N T E R F R E Q
control, then tune the RF CENTER FREQ to any harmonic sig-
nal. Depress the LOCK CHECK button and adjust the FINE
RF CENTER FREQ control to establish a lock made on the
harmonic signal. See Operating instructions.
4) Decrease the DISPERSION to 50 kHz/div, keeping the
signal centered on the screen with the IF CENTER FREQ
control.
5) Slowly adjust the FINE RF CENTER FREQ control until
the LO loses the lock. The signal may shift off screen. Re-
establish phase lock by adjusting the FINE RF CENTER FREQ
control to return the signal on screen.
6) Slowly adjust the INT REF FREQ, VARIABLE control. Note
the signal shift across the dispersion window as the reference
oscillator frequency is changed. Range of the control is
approximately 0.1% of the dial frequency. See step 15.
e. Remove the signal and harmonic source from band B
RF INPUT connector and apply the signals to band C, coaxial
mixer. Switch the band selector to C and set the DISPERSION
to 2 MHz/d iv.
f. Check dial accuracy over scale 4, band C. Check
oscillator and mixer operation and local oscillator phase lock
as the dial accuracy is checked. Check these parameters by
repeating the procedure described in steps (d) for band B.
N O T E
There is no need to check dial calibration of the
upper scales of band C because they are multiples
of scale 4.
g. Apply 10 MHz (.1 µs) marker signals and harmonics
to the band A RF INPUT. Switch the band selector to A.
Set the DISPERSION to 1 MHz/div.
h. Check the dial accuracy, oscillator and mixer perform-
ance, and LO phase lock operation through band A. Check
by using the some procedure used to check bands B and C.
Note that range of INT REF FREQ control is less because of
lower ratio between reference oscillator and LO.
i. Check-Phase lock operation with an external reference
frequency. Use the procedure rescribed in step 24A (k).
Fig. 5-8. Harmonic frequency markers used to check dial accuracy.
5-12