Skip to main content

Full text of "generalmanual 000043944"

See other formats



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 

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 

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 


m. - 



| 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- 

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 

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. 


BK PRECISION oscilloscope guide 

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 



Field 1 

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

r ~ i 



0 2 ms/div 


20 (is/d™ 

Field 2 

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






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 

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. 

Thi'ii /";— ■ 














_ JO-MHi 



I 16-M 


4 2-Mh 
— MOGUL ft 





Fig. 46. Typical VITS signal 

Fig. 47. Color TV IF amplifier 
response curve