periods or even part of a period may be shown as a func
tion of the adjustment of the
TIMEBASE
switch. The time
coefficients on the
TIMEBASE
switch are indicated in
ms/cm
and
pslcm.
Accordingly, the dial is subdivided
into two sectors.
The duration o f a signal period or a portion of the
waveform is ascertained by multiplying the relevant
time (horizontal distance in cm) by the time coefficient
selected on the
TIMEBASE
switch.
The time variable control (small knob on the
TIMEBASE
switch) must be in its calibrated detent
(C)
for accurate measurement
(arrow horizontal and poin
ting to the left).
With the designations
L
= displayed wave
length in cm
of one period,
T = time in seconds
for one period,
F
= recurrence
frequency-in Hz
of the signal,
Tc = time coefficient in s/cm
on timebase switch
and the relation
F
=
1/T,
the following equations can be
stated:
T =
F =
L Tc
1
L Tc
L =
Tc
L =
F T c
Tc = r
Tc =
L F
However, these four values are not freely selectable.
They have to be within the following limits:
L
between 0.2 and 10cm, if possible 4 to 10cm,
T
between
0.0bps
and 2 s,
F
between 0.5Hz and 20MHz,
Tc
between 0.5/is/cm and 200ms/cm in 1 -2 -5 se
quence (with
X-MAGN. X5
button in out position).
With depressed X-
MAGN. X5
pushbutton the Tc value
must be divided by 5.
Examples:
Displayed wavelength L = 7cm,
set time coefficient
Tc
= 0.5^s/cm,
required period T
= 7 0.5 • 10 6 = 3.5/is
required rec. freq. F
= 1: (3.5 • 10 6) =
286kHz.
Signal period
T
= 0.5s,
set time coefficient
Tc
= 0.2s/cm,
required wavelength L
= 0.5:0.2 =
2.5cm.
Displayed ripple wavelength L = 1 cm,
set time coefficient
Tc
= 10ms/cm,
required ripple freq. F = 1:(1 • 10• 1 0 3) = 100Hz.
TV-line frequency
F
= 1 5 625 Hz,
set time coefficent
Tc
= 10yus/cm,
required wavelength L =
1
:
(1 5 625 • 10 5) =
6.4cm.
Sine wavelength
L
= min. 4cm , max. 10cm,
Frequency
F
= 1 kHz,
max. time coefficient Tc = 1 :(4 • 10 3) = 0 .2 5 ms/cm,
min. time coefficient Tc = 1: (10 • 10 3) = 0.1 ms/cm,
set time coefficient Tc = 0 .2 ms/cm,
required wavelength
L = 1 :( 1 0 3 0 .2 -1 0 3) =
5cm
.
Displayed wavelength
L
= 0.8cm,
set time coefficient Tc = 0.5yus/cm,
pressed
X-MAGN. X5
button:
Tc
= 0.1
pslcm,
required rec. freq. F
= 1:(0.8-0.1 • 1 0 6) =
12.5MHz,
required period T
= 1 :(1 2.5 • 10 6) =
80ns.
If the time is relatively short as compared with the com
plete signal period, an expanded time scale should
always be applied
(X-MAGN. X5
button depressed). In
this case, the ascertained time values have to be divided
by
5.
When investigating pulse or square waveforms, the
critical feature is the
risetime o f the voltage step.
To
ensure that transients, ramp-offs, and bandwidth limits
do not unduly influence the measuring accuracy, the
risetime is generally measured between
10%
and 30%
of the vertical pulse height. For peak-to-peak signal
amplitudes of
5cm
height, which are symmetrically ad
justed to the horizontal center line, the internal graticule
of the CRT has two horizontal dotted lines + 2 .5cm from
the center line. Adjust'the Y attenuator switch with its
variable control together with the
Y-POS.
control so that
the pulse height is precisely aligned with these 0 and
100% lines. The 10% and 90% points of the signal will
now coincide with the two lines, which have a distance
of + 2cm from the horizontal center line and an additional
subdivision of 0.2 cm.
The risetime is given by the pro
duct o f the horizontal distance in cm between these
two coincidence points and the time coefficient set
ting
If magnification is used, this product must be divid
ed by 5. The
fall time
of a pulse can also be measured by
using this method.
The following figure shows correct positioning of the
oscilloscope trace for accurate risetime measurement.
100%
90%
10
%
0
M6 20 3-4
