BK Precision 2120C Instruction Manual Download Page 17

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This control is usually set to the

MIN

position (fully

counterclockwise) because no additional holdoff period is
necessary. The

HOLDOFF

control is useful when a com-

plex series of pulses appear periodically such as in Fig. 4B.
Improper sync may produce a double image as in Fig. 4A.
Such a display could be synchronized with the

VAR

SWEEP

control, but this is impractical because time meas-

urements are then uncalibrated. An alternate method of
synchronizing the display is with the

HOLDOFF

control.

The sweep speed remains the same, but the triggering of the
next sweep is “held off” for the duration selected by the

HOLDOFF

control. Turn the

HOLDOFF

control clock-

wise from the

MIN

position until the sweep starts at the

same point of the waveform each time.

MAGNIFIED SWEEP OPERATION

Since merely shortening the sweep time to magnify a

portion of an observed waveform can result in the desired
portion disappearing off the screen, magnified display
should be performed using magnified sweep.

Using the

POS

ition control, move the desired portion

of waveform to the center of the CRT. Pull out the

PULL X10

knob to magnify the display ten times. For this type of display
the sweep time is the

Main Time Base TIME/DIV

control

setting divided by 10. Rotation of the

POS

ition control can

then be used to select the desired portion of the waveforms.

−

Y OPERATION

−

operation permits the oscilloscope to perform many

measurements not possible with conventional sweep opera-
tion. The CRT display becomes an electronic graph of two
instantaneous voltages. The display may be a direct com-
parison of the two voltages such as stereoscope display of
stereo signal outputs. However, the

−

mode can be used

to graph almost any dynamic characteristic if a transducer is
used to change the characteristic (frequency, temperature,
velocity, etc.) into a voltage. One common application is fre-
quency response measurements, where the Y axis corresponds to
signal amplitude and the X axis corresponds to frequency.

1. On Model

2125

C & 2160C

, set the

SWEEP MODE

switch t

o the

−

position. On the Model 2120

, de

-press the

−

switch. On both models, set the

Trigge

Source

VERTical MODE

switches to

−

2. In this mode, channel 1 becomes the X axis input and

channel 2 becomes the Y axis input. The X and Y
positions are now adjusted using the

POS

ition and

the

channel 2

POS

ition controls respectively.

3. Adjust the amount of vertical (Y axis) deflection with

the

CH 2 VOLTS/DIV

and

VARIABLE

controls.

4. Adjust the amount of horizontal (X axis) deflection

with the

CH 1 VOLTS/DIV

and

VARIABLE

con-

trols.

VIDEO SIGNAL OBSERVATION

Setting the

COUPLING

switch to the

TV-H

TV-V

position permits selection of horizontal or vertical sync
pulses for sweep triggering when viewing composite video
waveforms.

When the

TV-H

mode is selected, horizontal sync pulses

are selected as triggers to permit viewing of horizontal lines
of video. A sweep time of about 10

s/div is appropriate for

displaying lines of video. The

VAR SWEEP

control can be

set to display the exact number of waveforms desired.

When the

TV-V

mode is selected, vertical sync pulses are

selected as triggers to permit viewing of vertical fields and
frames of video. A sweep time of 2 ms/div is appropriate for
viewing fields of video and 5 ms/div for complete frames
(two interlaced fields) of video.

At most points of measurement, a composite video signal

is of the (

−

) polarity, that is, the sync pulses are negative and

the video is positive. In this case, use

(

−

) SLOPE

. If the

waveform is taken at a circuit point where the video wave-
form is inverted, the sync pulses are positive and the video
is negative. In this case, use

(+) SLOPE

APPLICATIONS GUIDEBOOK

B+K Precision

offers a “Guidebook to Oscilloscopes”

which describes numerous applications for this instrument
and important considerations about probes. It includes a
glossary of oscilloscope terminology and an understanding
of how oscilloscopes operate. It may be downloaded free of
charge from our Web site, www.bkprecision.com.

DELAYED SWEEP OPERATION

(Model

2125

& 2160C)

(Refer to Fig. 5)

Delayed sweep operation is achieved by use of both the

main sweep and the delayed sweep and allows any portion
of a waveform to be magnified for observation. Unlike

X10

magnification, delayed sweep allows selectable steps of
magnification.

1. Set the

Sweep Mode

switch to the