Subject to change without notice
8
Basics of signal voltage
Type of signal voltage
The oscilloscope
HM2005
allows examination of DC voltages and
most repetitive signals in the frequency range up to at least
200MHz (-3dB). The vertical amplifiers have been designed for
minimum overshoot and therefore permit a true signal display.
The display of sinusoidal signals within the bandwidth limits
causes no problems, but an increasing error in measurement
due to gain reduction must be taken into account when measuring
high frequency signals. These errors become noticeable at approx.
100MHz
. At approx. 120MHz the reduction is approx. 10% and
the real voltage value is 11% higher. The gain reduction error can
not be defined exactly as the -3dB bandwidth of the amplifiers
differ between
200MHz
and
220MHz
.
For sine wave signals the -6dB limits are approx. 280MHz.
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 repetitive 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 function or the second time base 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, each vertical
amplifier input is provided with a DC/AC switch. DC coupling
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.
The input coupling is selectable by the AC/DC pushbutton. The
actual setting is displayed in the readout with the
“ = “
symbol
for DC- and the
“ ~ “
symbol for AC 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 Vrms
(Veff) have 2.83 times the potential difference in Vpp. 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
(± 5%) when this deflection
coefficient is displayed on the screen (readout) and the vernier is
switched off (VAR-LED dark). However, smaller signals than this
may also be displayed. The deflection coefficients 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 (VAR)
must be set to its calibrated detent CAL position.
With the variable control activated the deflection sensitivity can
be reduced up to a ratio of 2.5 to 1 (
please note “Controls and
Readout”
). Therefore any intermediate value is possible within
the 1-2-5 sequence of the attenuator(s).
With direct connection to the vertical input, signals up to
100Vpp may be displayed (attenuator set to 5V/div,
variable control to 2.5:1).
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):
H
between 0.5 and 8div, if possible 3.2 to 8div,
U
between 1mV
pp
and 40V
pp
,
D
between 1mV/div and 5V/div in 1-2-5 sequence.
Basics of signal voltage
