8
Subject to change without notice
Type of signal voltage
With the HM 303, most repetitive signals in the frequency
range up to
at least 30MHz
(
−
3dB) can be examined.
Sinewave signals of 50MHz are displayed with a height of
approx. 50% (
−
6dB). However when examining square or
pulse type waveforms, attention must be paid to the
harmonic content
of such signals. The repetition
frequency (fundamental frequency) of the signal must
therefore be significantly smaller than the upper limit
frequency of the vertical amplifier.
Displaying composite signals can be difficult, especially if
they contain no repetive higher amplitude content which
can be used for triggering. This is the case with bursts, for
instance. To obtain a well-triggered display in this case,
the assistance of the
variable holdoff
and/or variable
time control may be required. Television
video signals
are relatively easy to trigger using the built-in
TV-Sync-
Separator (TV).
For optional operation as a DC or AC voltage amplifier, the
vertical amplifier input is provided with a
DC/AC
switch.
The
DC
position should only be used with a series-
connected attenuator probe or at very low frequencies or
if the measurement of the DC voltage content of the signal
is absolutely necessary.
When displaying very low frequency pulses, the flat tops
may be sloping with
AC
coupling of the vertical amplifier
(
AC
limit frequency approx. 1.6 Hz for 3dB). In this case,
DC
operation is preferred, provided the signal voltage is
not superimposed on a too high DC level. Otherwise a
capacitor of adequate capacitance must be connected to
the input of the vertical amplifier with DC coupling. This
capacitor must have a sufficiently high breakdown voltage
rating.
DC
coupling is also recommended for the display
of logic and pulse signals, especially if the pulse duty
factor changes constantly. Otherwise the display will
move upwards or downwards at each change. Pure direct
voltages can only be measured with
DC
-coupling.
Amplitude Measurements
In general electrical engineering, alternating voltage data
normally refers to effective values (rms = root-mean-
square value). However, for signal magnitudes and voltage
designations in oscilloscope measurements, the peak-to-
peak voltage (V
pp
) value is applied. The latter corresponds
to the real potential difference between the most positive
and most negative points of a signal waveform.
If a sinusoidal waveform, displayed on the oscilloscope
screen, is to be converted into an effective (rms) value,
the resulting peak-to-peak value must be divided by 2x
√
2
= 2.83. Conversely, it should be observed that sinusoidal
voltages indicated in V
rms
(V
eff
) have 2.83 times the potential
difference in V
pp
. The relationship between the different
voltage magnitudes can be seen from the following figure.
Voltage values of a sine curve
V
rms
= effective value; V
p
= simple peak or crest value;
V
pp
= peak-to-peak value; V
mom
= momentary value.
The minimum signal voltage which must be applied to the
Y input for a trace of 1div. height is
1mV
pp
when the
Y-
MAG. x5
pushbutton is depressed, the
VOLTS/DIV
.
switch is set to 5mV/div., and the vernier is set to
CAL
by
turning the
fine adjustment knob
of the VOLTS/DIV.
switch fully clockwise. However, smaller signals than this
may also be displayed. The
deflection coefficients
on
the input attenuators are indicated in
mV/div.
or
V/div.
(peak-to-peak value).
The magnitude of the applied voltage is ascertained
by multiplying the selected deflection coefficient by
the vertical display height in div.
If an attenuator probe x10 is used, a further
multiplication by a factor of 10 is required to ascertain
the correct voltage value.
For exact amplitude measurements, the variable control
on the attenuator switch must be set to its calibrated
detent CAL. When turning the variable control ccw, the
sensitivity will be reduced by a factor of 2.5.
Therefore every intermediate value is possible within
the 1-2-5 sequence.
With direct connection to the vertical input, signals
up to
400V
pp
may be displayed (attenuator set to
20V/div.
,
variable control to left stop).
With the designations
H
=
display height in div.
,
U
= signal
voltage in V
pp
at the vertical input,
D
=
deflection coefficient in V/div.
at attenuator switch,
the required value can be calculated from the two given
quantities:
However, these three values are not freely selectable.
They have to be within the following limits (trigger threshold,
accuracy of reading):
U = D · H
U
D
H =
U
H
D =
Vp
Vrms
Vmom
Vpp
