Tektronix 492, 492P, Руководство по обслуживанию

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c. Remove the extender for the Center Frequency Con­
trol board and reinstall the board in the instrument.
d. Refer to the Calibration section to recalibrate the 2nd
LO spans and identify functions.
Troubleshooting Aids for the 2182 MHz
Phaselocked 2nd LO
NOTE
If the Phaselocked 2nd LO assembly is in the instru­
ment, it may be desirable to set the FREQ SPAN/DIV
to 100 kHz or greater so the 2nd LO is not swept or
tuned.
The difference frequency from the 2200 MHz Reference
Mixer is amplified and fed to output port P224. Nominally,
the signal at P224 is 18 MHz with an approximate output
level of - 5 dBm into 50 Ω. This port is convenient for moni­
toring the operation of the 14-22 MHz voltage controlled os­
cillator. When the phaselock is operating, the difference
frequency exactly equals the frequency of the 14-22 MHz
VCO. If the lock loop is not functioning properly, the differ­
ence frequency signal will either disappear completely or
tune to its range limits at approximately 4 MHz or 32 MHz.
Note that when the loop is unlocked, RF leakage from the
14-22 MHz oscillator buffer can be seen at P224, with a
level of approximately —35 dBm. Output from the oscillator
can be monitored more directly, if desired, at TP2032 by
using a high impedance probe with a dc block. Similarly, the
amplified difference frequency can be monitored at TP2035.
Another check of phaselock operation can be made by
measuring the dc voltage on the 2182 MHz tune line, at
feedthrough capacitor C2203. Nominally, this voltage is ap­
proximately —5 V when phaselocked. Use a FREQ
SPAN/DIV of 100 kHz or greater before making the mea­
surement. If there is no difference frequency signal, the volt­
age at C2203 will be approximately 0 V. A voltage of
approximately —13 V at C2203 may indicate loss of signal
from the 14-22 MHz oscillator.
Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
Narrow-band noise on the 2nd LO signal may be due to
noise modulation of the 14-22 MHz VCO. Monitor the signal
at P224 to see if the 14-22 MHz oscillator is noisy. Noise on
this oscillator is often caused by noise on the + or — 12
volt supplies. Use a differential oscilloscope, with 1 Hz to
300 Hz bandwidth limits to check supply noise. Measure the
ac differential between the supply and the 2nd LO housing.
Less than 5
μ \ Ι
p-p noise is typical, while 50
μ \ !
p-p of noise
will cause noticeable performance degradation. Output
noise from the shaper, at the junction of R1067 and R1068,
is typically less than 5
μ \ Ι
p-p..
Tuning problems may be caused by defective operational
amplifiers in the shaper. When the voltages on the sweep
and tune lines are 0 V (i.e., span/div of 100 kHz or greater),
there should be 0 V at pin 6 of U2063 and pins 1 and 7 of
U1062.
When making power measurements of microwave cir­
cuits at circuit board interfaces, use a coaxial probe with
very little stray inductance (see Fig. 4-20). Ground the outer
conductor of the probe to the circuit housing within a few
tenths of an inch of the measurement point. Disconnect oth­
er circuit loads from the measurement point.
)
REVJUN 1983
4-35