generalmanual 000043944

OSCILLOSCOPE GUIDE BK PRECISION

APPLICATIONS

Waveform Magnification Using
Delayed Sweep

The apparent magnification of the delayed
sweep is determined by the values set by the
main and delay Time/Div controls.

1 . Apply a signal to the input jack and set
the oscilloscope to display the channel to
be used. Adjust the controls for an easily
observed display of the waveform.

2. Set the Time/Div control so that several
cycles of the waveform are displayed.

3. Set the scope to delayed sweep. Use the
delay time position control to adjust the
delayed sweep portion of the display. Use
the delay Time/Div control to set the
desired sweep speed for the magnified
portion.

4. Set the oscilloscope to view both the
main and delayed sweep (Mix sweep
mode), or set it to view only the magnified
portion (delayed sweep).

5. Time measurements are performed in the
same manner for the delayed sweep as
for the main sweep (but remember to use
the delay Time/Div control setting as the
sweep speed for calculations).

Pulse Jitter Measurements

(Refer to Fig. 44)

1 . Apply the signal to one of the input jacks
and set the oscilloscope to display the
channel to be used. Use the Volts/Div
control to adjust the waveform so that it is
easy to observe. Special care should be
taken to adjust the trigger controls for a
stable display. Set the variable input
attenuator control to the calibrated
position.

2. Set the oscilloscope to display both the
main and delayed sweep. Adjust the
delay Time/Div and delay time position
controls so that the entire jitter area of the
waveform is magnified.

3. Set the oscilloscope to display just the
delayed sweep. Measure the width of the

DELAYED SWEEP APPLICATIONS

m. -

■

1

| Puis* jitter

Fig. 44. Pulse jitter measurement

jitter area. The jitter time is the width in
divisions multiplied by the setting of the
step delay Time/Div control.

The following equation summarizes the meas-
urement:

Pulse Jitter =
Jitter Width (Div) x Delay Time/Div.

For the example shown in Fig. 44, the jitter
width is 1 .6 divisions and the delay Time/Div
control setting is 0.2 mS. The pulse jitter is 0.32
mS, which was calculated as follows:

Pulse Jitter = 1.6 x 0.2 mS = 0.32 mS

Observing Video Signals Using
Delayed Sweep

(Refer to Figs. 45 through 49)

TV triggering allows you to select either vertical
or horizontal sync pulses so that a stable video
waveform may be observed on the oscilloscope
display. When TV triggering is used in conjunc-
tion with delayed sweep (not all oscilloscopes
have the delayed sweep feature and isolating a
desired part of a video signal without using the
delayed sweep is very difficult to do) the video
signal can be observed and analyzed on an
oscilloscope. The following instructions will help
you set up a delayed sweep oscilloscope to
view video signals. The section on "The NTSC
Color Video Signal", at the end of this chapter,
gives some basics for those who are unfamiliar
with video signals or simply wish to refresh their
memory.

1 . Set up the oscilloscope for the alternate
dual-trace display mode of the main
sweep only and connect both the channel
1 and channel 2 probes to the same point
in the circuit (if you wish, a "T" connector
can be used at the channel 1 input jack to
feed the same signal to each channel).
The alternate dual-trace display mode
aids in viewing the video signal because
the "holdoff" period between sweep
signals is increased enough that one field
of video is displayed on channel 1 and
the other field of video is displayed on
channel 2. If only the single trace display
mode were to be used, both fields of
video would be displayed on one trace
and the display would likely be unusable
since the two fields usually differ.

2. Set the main Time/Div control to 0.2
mS/div (this setting will cause about 32
lines of video to be displayed). This will
be a sufficient number of lines so that the
Vertical Interval Test Signal (VITS) can be
displayed along with the rest of the
vertical interval blanking period and
several lines of video information.

3. Select vertical TV trigger coupling (TVV),
select the channel one signal as the
trigger source, and select negative trigger
slope. Adjust the trigger level control for a
display such as that shown in the top line
of Fig. 45 (although the waveform will be
compressed horizontally, the waveform in
the illustration is expanded for clarity).

4. Set the delay Time/Div control to about
0.2 ms/div and set the oscilloscope to
display the delayed sweep signal or, if
desired, a mix of the main sweep and the
delayed sweep. (A setting of 20 mS/div
will give a display of about three lines of
video — further expansion to less than
three lines of video is possible with faster
sweep speeds.) Adjust the delay time
position control until the desired line(s) of
video are displayed. If you wish to view
the VITS/VIR lines, they are the 17th,
18th, and 19th lines in a video frame. Fig.
45 shows the main and delayed sweep of
both field 1 and field 2.

37

BK PRECISION oscilloscope guide
APPLICATIONS

Most network television signals contain a built-in
test signal (called the Vertical Interval Test
Signal, or VITS) that can be a very valuable tool
in troubleshooting and servicing television sets.
The VITS is transmitted during the vertical
blanking interval and can be used to localize
trouble to the antenna, tuner, i-f, or video
sections. The signal appears as a bright white
line above the top of the television picture when
the vertical linearity or height is adjusted to view
the vertical blanking interval (on TV sets with
internal retrace blanking circuits, the blanking
circuit must be disabled to see the VITS). The
following procedure shows how to analyze and
interpret the oscilloscope displays of the VITS.

The transmitted VITS may vary from channel to
channel, but is similar to Fig. 46. The television
networks use the precision signals for
adjustment and checking of network transmis-
sion equipment, but the technician can use them
to evaluate television performance. The first
frame of the VITS (line 17) may begin with a
"flag" of white video, followed by sine wave fre-
quencies of 0.5 MHz, 1.5 MHz, 2 MHz, 3 MHz,
3.6 MHz (3.58 MHz) and 4.2 MHz. This
sequence of frequencies is called the "multi-
burst". The first frame of Field #2 (line 279) may
contain an identical multiburst. This multi-burst
portion of the VITS is the portion that can be the
most valuable to the technician. The second
frame of the VITS (lines 18 and 280), may con-
tain the sine-squared pulse, window pulse, and
the staircase of 3.58 MHz bursts at progressive-
ly lighter shading, which are valuable to the net-
work, but have little value to the technician.

All frequencies of the multi-burst are transmitted
at the same level, but should not be equally
coupled through the receiver (due to its frequen-
cy response curve). Fig. 47 shows the desired
response for a good television receiver, identify-
ing each frequency of the multi-burst and show-
ing the allowable amount of attenuation for each
(remember that -6 dB equals one half of the ref-
erence voltage — the 2.0 MHz signal should be
used as the reference).

To localize trouble, start by observing the VITS
at the video detector. This will localize trouble to
a point either before or after the detector. If the
multi-burst is normal at the detector, check the
VITS on other channels. If some channels look
OK but others do not, you probably have tuner
or antenna-system problems. Don't overlook the

Vertical Interval Blanking

VITS/VIP.

Video

Field 1

TTrrrt f r r t , ^ r t - .WyWYvW

r ~ i

s

Sweep

0 2 ms/div

Delay
Sweep

20 (is/d™

Field 2

iiju/rTTTTTf t i f t f f f-t f /VVVV^VVfYVVV^ Sweep

np

iinn

Delay
Sweep

Multlburst

Staircase

VIR Signal

Fig. 45. Observing video signals using delayed sweep.

chance of the antenna system causing "holes"
or tilted response on some channels. If the VITS
is abnormal at the video detector on all chan-
nels, the trouble is probably in the i-f amplifier
stages.

As another example, let us assume that we
have a set on the bench with a very poor pic-
ture. Our oscilloscope shows the VITS at the
video detector to be about normal except that
the burst at 2.0 MHz is low compared to the
burst on either side. This suggests that an i-f
trap is detuned into the passband, chopping out
frequencies of about 2 MHz below the picture
carrier frequency. Switch to another channel
carrying VITS; if the same thing is seen, then

our reasoning is right, and the i-f amplifier
requires realignment. If the poor response at 2
MHz is not seen on other channels, maybe an
FM trap at the tuner is misadjusted or faulty,
causing a bite on only one channel. Other traps
at the input of the set could similarly be misad-
justed or faulty.

If the VITS response at the detector output is
normal for all channels, the trouble may be in
the video amplifier. Check for open peaking
coils, off-value resistors, solder bridges across
foil patterns, etc.

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Fig. 46. Typical VITS signal

Fig. 47. Color TV IF amplifier
response curve

38