6
Subject to change without notice
Type of signal voltage
The oscilloscope
HM407
allows examination of DC voltages
and most repetitive signals in the frequency range up to at
least 40MHz (-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. This error becomes
noticeable at approx. 14MHz. At approx. 18MHz 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 40MHz and 42MHz.
For sinewave signals the -6dB limit is approx. 50MHz.
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 delayed 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 (Vpp)
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
Vrms = effective value; Vp = simple peak or crest value;
Vpp = peak-to-peak value; Vmom = momentary value.
The minimum signal voltage which must be applied to the Y
input for a trace of 1div height is 1mVpp (± 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 400Vpp may be displayed (attenuator set to 20V/div,
variable control to 2.5:1).
With the designations
H = display height in div,
U = signal voltage in Vpp 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 1mVpp and 160Vpp,
D between 1mV/div and 20V/div in 1-2-5 sequence.
Examples:
Set deflection coefficient D = 50mV/div 0.05V/div,
observed display height H = 4.6div,
required voltage U = 0.05x4.6 = 0.23Vpp.
Input voltage U = 5Vpp,
set deflection coefficient D = 1V/div,
required display height H = 5:1 = 5div.
Type of signal voltage
