Tektronix 516 series, Instruction Manual

Page: 52 / 132

Calibration — Type 5 1 6
square wave following the termination network. Make sure
that the VOLTS/DIV. control is set to .05 and the VARIABLE
control is set to the CALIBRATED position. Set the TIME/
DIV. control to .2 juSEC. Observe the waveform closely in
the region near the center of the positive portion, and ad­
just the two inductors and the two capacitors in the termin­
ation network for the minimum termination bump. Be sure
to check the inductors for balance as described in the prev­
ious paragraph.
The level of that portion of the square wave preceding the
termination bump is determined by the collective effect of
all the Delay-Line capacitors. Set the TIME/DIV. control to
2 ;U,SEC; this will produce narrow pulses on the crt screen
and make it easy to ascertain any departure from a level
display. Each capacitor must now be adjusted a small
amount in a direction that will make the top of the square
wave level. Start at the terminated end of the Delay Line
and adjust each capacitor a small amount, carefully ob­
serving the top of the waveform for the result. Make sure
that you retain a level top to the waveform as you progress
toward the amplifier end of the line.
After you have been over the line once, and have estab­
lished an average level for the waveform, advance the sweep
rate to 1 ^.sec/div. and repeat the procedure. This time, try
to adjust the capacitors for a smooth transition from bump
to bump, at the same time maintaining the level. Do not try
to obtain a wrinkle-free line at this time. Just try to reduce
the amplitude of all the bumps the same amount. The im­
portant consideration is to maintain the level of the wave­
form.
Removing the Bumps and Wrinkles. After you have
established a level display, and reduced the amplitude of
the bumps a bit, you can start removing the wrinkles and
bumps to a greater degree. Again, start at the terminated
end of the line and work toward the amplifier end.
Set the TIME/DIV. control to .5 jnSEC and adjust the ter­
mination network again to reduce the wrinkles in the vicinity
of the termination bump. Do not try to achieve a perfectly
straight line across the top of the waveform at this time. Just
reduce the amplitude of the bumps by about one-half. Then
advance to the first group (about 4 or 5) of the capacitors in
the line and adjust them for a reasonably smooth line over
that portion of the display they affect. Keep in mind that
each capacitor will only require a slight adjustment, and that
it is combined effect of a group of capacitors with which
you must be concerned. As you advance along the line,
from each group of capacitors to the next, turn the VARI­
ABLE TIME/DIV. control full left from time to time; this will
decrease the sweep rate and narrow the pulse width so that
you may more easily check the level of the waveform. It
is just as important to maintain the level of the waveform as
it is to achieve a wrinkle-free display.
After you have traversed the entire length of the Delay
Line, advance the sweep rate to .2 /xsec/div. and repeat the
process. (Adjust the VARIABLE TIME/DIV. control to obtain
the entire positive half-cycle on the crt.) Be extremely care­
ful in your adjustments at this time. Any capacitors that re­
quire adjusting will only need a slight "touch". Any over­
adjustment might nullify all of your efforts up to this point.
From time to time switch back to a sweep rate of 2 yusec/dfv.
to check the level.
Upon completing this portion of the adjustment procedure,
the display on the oscilloscope should appear similar to Fig.
6-15 (b). That is, the display should be level and free from
bumps and wrinkles, with a pronounced rolloff at the leading
corner. The final step in the adjustment procedure, therefore,
is to square up the corner.
Squaring up the Corner.
With the TIME/DIV. control set
to .2 jx SEC, position the display so that you have a good
view of the leading edge and corner. Then, adjust L455 and
L555 in the Vertical Amplifier, being careful to adjust each
inductor the same amount. Continue adjusting each inductor,
maintaining balance, until the leading edge comes up to a
sharp, square corner. This process will introduce some new
wrinkles in the display, but these can be easily removed by
adjusting the first few capacitors in the Delay Line. When
the Vertical Amplifier and the Delay Line are in proper ad­
justment, the display should appear similar to Fig. 6-15 (a).
A slight readjustment of C450 and C451 may also improve
the shape of the corner without seriously degrading the re­
sponse for which they were adjusted in step 27 of the pre­
ceding procedure. If you find it necessary to adjust them
more than a slight amount, you should go back and check
the adjustment as described in step 27. The final setting
will be a compromise between the squareness of the corner
and the amount of feedthrough allowable; the most emphasis
should be put upon the squareness of the corner, since feed­
through from one channel to the other will not be a problem
in most applications of the oscilloscope.
Apply the 450-kHz output of the square-wave generator to
the B VERTICAL input connector and set the MODE switch
to B ONLY. Note the shape of the leading corner. You may
wish to adjust C550 and C551 and/or the first few capacitors
of the Delay Line to improve the response of the B Channel.
The final setting will be a compromise between the response
obtained through the A Channel and the response obtained
through the B Channel. Usually, it is best to set.up one of
the channels for optimum square-wave response, even if it
means degrading the response on the other channel slightly.
Then all critical high-frequency observations can be made on
the optimum channel.
Check Bandwidth o f Vertical Amplifier.
Connect the
output cable from the constant-amplitude signal generator
to the A VERTICAL input connector of the Type 516. Set
up the front panel controls as follows;
TIME BASE
TRIGGER SELECTOR AC
(red)
TRIGGER SELECTOR +INT.
(black)
STABILITY Fully clockwise (free run)
HORIZ. DISPLAY NORM.
TIME/DIV. 1 mSEC
VARIABLE TIME/DIV. CALIBRATED
® I
6-13