Tektronix 491, Руководство по обслуживанию

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calibration. See Maintenance Section. The probe assemblies
can be oriented within the magnetic field of the oscillator
chamber by loosening set screws and positioning the probe
in or out of the chamber or rotating the probe within the
field.
Usually the probes are rotated for maximum power output
and inserted in or out of the chamber for the specified power
output. It is important to keep the output power below the
maximum specification listed in steps 1 (m) and 1 (o) of the
following procedure. All adjustments interact; therefore, as
each adjustment is made, its effect on the frequency tracking
and output power over the frequency range of the oscillator
must be checked. Adjust to obtain as flat an output as pos-
sible with frequency tracking within ± 1%.
ALIGNMENT PROCEDURE FOR BAND C OSCILLATOR
ASSEMBLY
This procedure should be used only after replacing the
oscillator tube. Refer to Fig, 6-50 for the locatian of sub-as-
semblies and parts. The oscillator assembly must be removed
for calibration.
1. Alignment and Installation of the Probe and Varactor
Assemblies.
a. Install probe assembly (Part No. 119-0107-01 into the
No. 2 port Fig, 6-50). Position the probe assembly approxi-
mately 1 /
16
inch out from full penetration with the notch or
keyway (DC return of the probe towards the plate end of the
oscillator assembly, and secure by tightening one of the two
set screws.
CAUTION
Do not over-tighten set screws. They may warp
the shaft if tightened too much.
b. Install probe assembly (Part No. 119-0107-00) into the
No. 1 port (Fig. 6-50]. Position the probe assembly approxi
mately 1 /
16
inch out from full penetration with the notch or
keyway towards the plate end of the oscillator assembly,
and secure by tightening one of the two set screws.
c. Install the Varactor assembly (Part No. 119-01 05-00) into
the control port. Position the assembly approximately
1
/
16
inch out from full penetration and secure by tightening one
of the two set screws.
d. Position the cathode and plate chokes to the high fre-
quency end of the band (towards the center of the assembly)
but not against the stop.
e. Connect an RF power meter through a 50
Ω,
10 dB at-
tenuator and a 9 inch coaxial cable (clear cable covering) to
the No. 2 port.
f. Connect a frequency counter or accurate frequency
measuring device through a 50
Ω
termination and a 9 inch
lossy coaxial cable (white cable covering) to the No, 1 port.
g. Apply power (B+ and filament supply) to the oscillator.
Allow approximately 10 minutes for the oscillator to stabilize.
h. Loosen the set screw to No. 2 port and rotate probe
assembly for maximum power input. Do not rotate the probe
180° from the preset position. If power output exceed
100 mW, decrease the coupling by pulling the probe as-
sembly out. Tighten the set screws.
i. Loosen the set screws (1) in Fig. 6-50, and position the
plate choke for a frequency of 4.2 GHz.
Force the lead screw against the bearing and tighten the
set screws.
j. Loosen the set screws (2) in Fig. 6-50, and position the
cathode choke for maximum power output. Decrease probe
coupling if power output exceeds 100 mW.
Force the lead screws against the bearing and tighten the
set screws.
k. Connect the Varactor terminal to a +1.5 V to +14 V
bias supply. The instrument bias supply is preferred. Set the
Varactor bias voltage to +7 V and tune the oscillator to its
mid-frequency position.
1. Vary the position of the Varactor assembly, until a bias
swing from +1.5 V to +14 V provides a frequency shift
equal to or greater than 1.5 MHz. Return the bias voltage
to +7 V, by adjusting the FINE RF CENTER FREQ control.
m. Tune the oscillator through its frequency range checking
the output power. Power output aver the range should not
exceed 100 mW or decrease below 5 mW.
n. Remove the RF power meter from the No. 2 port and
connect the meter through a 50
Ω
10 dB attenuator and 9
inch lossy cable to the No. 1 port. Connect the 9 inch (not
lossy) coaxial cable from the instrument-phase lock circuit to
the No, 2 port.
o. Tune the oscillator through its frequency range checking
the output power from the No. 1 part. Power output should
not exceed 16 mW or decrease below 2 mW.
Balancing the output of ports No. 1 and No. 2 may be
necessary to provide the desired output from both ports. This
is done by loosening the 2-56 set screw in the probe as-
sembly ond varying the coupling. Each adjustment interacts
with the other adjustments, therefore, both outputs must be
checked after each adjustment.
2. Tracking the oscillator to the dial assembly.
a. Tune the oscillator to 4.2 GHz. Set the indicated dial
assembly to 4.0 GHz and lock the dial assembly to the oscil-
lator tuning shaft.
b. Tune the dial to indicate 1.5 GHz. Adjust the trimmer
screw (3) in Fig. 6-50, for an oscillator frequency of 1.7 GHz.
c. Repeat these steps until the oscillator frequency cor-
responds to the dial reading at both ends of the frequency
band.
d. Check oscillator tracking through the frequency range.
Must track within ± (1% of the indicated dial reading
+200 MHz). It may be necessary to introduce some error at
the upper or lower frequency limits to bring the tracking
within the ± 1% specification.
e. Check the phase lock operation over the frequency
range. Check for the presence of beat frequency signals and
stable locking operation.
6-49