BK Precision 5105B, Instruction Manual, Page: 10 / 48

10
Subject to change without notice
Basic signal measurement
Signals which can be measured
The following description pertains as well to analog as to DSO
operation. The different specifications in both operating modes
should be kept in mind.
with a fundamental repetition frequency of at least 150 MHz.
The frequency response is 0 to 150 MHz (-3 dB). The vertical
amplifiers will not distort signals by overshoots, undershoots,
ringing etc.
Simple electrical signals like sine waves from line frequency
ripple to hf will be displayed without problems. However, when
measuring sine waves, the amplitudes will be displayed with
an error increasing with frequency. At 100 MHz the amplitude
error will be around –10 %. As the bandwidths of individual
instruments will show a certain spread (the 150 MHz are a
guaranteed minimum) the actual measurement error for sine
waves cannot be exactly determined.
Pulse signals contain harmonics of their fundamental
frequency which must be represented, so the maximum useful
repetition frequency of nonsinusoidal signals is much lower
than 150 MHz. The criterion is the relationship between the
rise times of the signal and the scope; the scope’s rise time
should be
< 1/3 of the signal’s rise time if a faithful reproduction
without too much rounding of the signal shape is to be
preserved.
The display of a mixture of signals is especially difficult if it
contains no single frequency with a higher amplitude than
those of the other ones as the scope’s trigger system normally
reacts to a certain amplitude. This is e.g. typical of burst
signals. Display of such signals may require using the HOLD-
OFF control.
Composite video signals may be displayed easily as the
instrument has a tv sync separator.
The maximum sweep speed of 5 ns/cm allows sufficient time
resolution, e.g. a 100 MHz sine wave will be displayed one
period per 2 cm.
The vertical amplifier inputs may be DC or AC coupled. Use
DC coupling only if necessary and preferably with a probe.
Low frequency signals when AC coupled will show tilt (AC low
frequency – 3 dB point is 1.6 Hz), so if possible use DC coupling.
Using a probe with 10:1 or higher attenuation will lower the
–3 dB point by the probe factor. If a probe cannot be used due
to the loss of sensitivity DC coupling the scope and an external
large capacitor may help which, of course, must have a
sufficient DC rating. Care must be taken, however, when
charging and discharging a large capacitor.
Dc coupling is preferable with all signals of varying duty cycle,
otherwise the display will move up and down depending on
the duty cycle. Of course, pure DC can only be measured with
DC coupling.
The readout will show which coupling was chosen: = stands
for DC, ~ stands for AC.
Amplitude of signals
In contrast to the general use of rms values in electrical
engineering oscilloscopes are calibrated in Vpp as that is what
is displayed.
Derive rms from V
pp
: divide by 2.84. Derive V
pp
from rms:
multiply by 2.84.
Values of a sine wave signal
V
rms
= rms value
V
PP
= pp – value
V
mom
= momentary value, depends on time vs. period.
The minimum signal for a one cm display is 1 mVpp ±5 %
provided 1 mV/cm was selected and the variable is in the
calibrated position.
The available sensitivities are given in mV
PP
or V
PP
. The cursors
allow to indicate the amplitudes of the signals immediately on
the readout as the attenuation of probes is automatically taken
into account. Even if the probe attenuation was selected
manually this will be overridden if the scope identifies a probe
with an identification contact as different. The readout will
always give the true amplitude.
It is important that the variable be in its calibrated position.
The sensitivity may be continuously decreased by using the
variable (see Controls and Readout). Each intermediate value
between the calibrated positions 1–2–5 may be selected.
Without using a probe thus a maximum of 400 V
PP
may be
displayed (20 V/div x 8 cm screen x 2.5 variable).
Amplitudes may be directly read off the screen by measuring
the height and multiplying by the V/div. setting.
STOP
Please note: Without a probe the maximum
permissible voltage at the inputs must not exceed
400 Vp irrespective of polarity.
In case of signals with a DC content the peak value DC + AC
peak must not or – 400 V
P
. Pure AC of up to 800 V
PP
is
permissible.
STOP
If probes are used their possibly higher ratings are
only usable if the scope is DC coupled.
In case of measuring DC with a probe while the scope input is
AC coupled the capacitor in the scope input will see the input
DC voltage as it is in series with the internal 1 M resistor.
This means that the maximum DC voltage (or DC + peak AC)
is that of the scope input, i.e. 400 V
P
! With signals which contain
DC and AC the DC content will stress the input capacitor while
the AC content will be divided depending on the AC impedance
B a s i c s i g n a l m e a s u r e m e n t
The oscilloscope 5105B can display all repetitive signals