HP 416A - Page 58

d. Check that the SET TO FULL SCALE control is not fully
clockwise. (Completely clockwise is maximum gai n, and it
is necessar y at the start of the calibration procedure that
some marg in of gain be avai lable in the probe channel.)
e. Check that the RANGE switch is at the 0 DB 100% position.
4. With no connection between the slotted - line probe and the Model
416A, check that the reference si g nal is within the Model 416A
operating limits : this check includes a check of the level9 of the
signal and also a check that the modulation frequency on the signal
is correct .
NOTE : It is important that there be no connection between the
slotted - line probe and the Model 416A when the level of
the input signal is checked and adjusted. By observing
this operat ional detail , danger of overloading the ratio
meter and the detectors will be avoided.
Indication that th e input si g nal is within frequency and v oltage
limits is given by the RF POWER MO N ITOR (tunin g eye) on the
Model 416A. Input filters in the Model 416A are sharply tuned
to 1, 000 cycles . Therefore , re ga rdless of input signal le v el,
no significant voltage will reach the tuning eye ci rcuit unless
the signal is within ±4 0 cycles of 1, 000 cycles . However , when
the input vo ltage is of correct frequency , an input signal of even
minimum level will cause the eye to give a definite indication:
the shutter will move toward closure. As the voltage of an input
signal of proper frequency approaches the maximum level , the
eye closes completely . If the eye ov erlaps by more than 1/32" ,
indication is given that the frequency of the voltage is correct
but that the level is too high .
a.
To adjust the modulation frequency on the input signal:
( 1) First , determine that is the modulation frequency on the
signal that is incorrectand not that the signal level is
too low . Check by increasing the level of the output from
the signal generator or by settin g the EXCESS COUPLER
LOSS s witch at 10 DB . If the eye still does not mo ve
toward c losure :
(2) At the signal generator or its power source, adjust the
co ntrol which governs the frequency of the modulating
voltage. This adjustment must be made very slowly
while watchin g the tuning eye on the Mode 1 416A.
SLOWLY operate the frequency - determining control
until the tuning eye shutter peaks.
9 The system should be operated at the low es t practical input power. When
the input to the crystal detector is within the - 50 dbm to - 10 dbm ran g e,
the detec t or is operating largely within its true square - law region
(Fig. 8). A brief discussion of crystal detector characteris t ics is given
under Possible Error if Input Power to Detector Too High, on page 30.
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