Use Of Probes; Input Coupling Ac, 0, Dc; Offset And Shift - Philips PM3320 Operating Manual

4.3.4
4.3.5
4-95E
Use of probes
To obtain a suitable connection between the oscilloscope and the device under test a probe is
often used.
1:1 passive probes should only be used for d.c. and low-frequency signals. The capacitive
loading of these probes attenuates high frequencies or increases the rise-time of signals to be
measured (depending on the source impedance).
10:1 and 100:1 passive probes have smaller capacitive loading and are therefore better suited to
measurements at higher frequencies. Againstthis, it may be a disadvantage that they decrease
the sensitivity respectively by a factor of 10 and of 100.
For measurements at very high frequencies the high impedance probes are less suitable, due to
reflections in the probe cable. This can be avoided by using probes with an inputimpedance of 5
kOhm and an attenuation factor of 100x. These probes have a very low capacitive loading, buta
high resistive charge (6 kOhm).
FET active probes are superior, especially when measurements are to be taken from high
impedance test points at the upper frequency limit of the oscilloscope bandwidth. Probes of this
type have a very low. capacitive loading, yet do not reduce the sensitivity.
10:1 and 100:1 passive probes must be properly compensated before use. Incorrect compen-
sation leads to pulse distortion or amplitude errors at high frequencies.
For correct adjustment of the 10:1 attenuator probes delivered with this oscilloscope, refer to
Section 8.1.1.4.
For more details about the available PHILIPS probes contact your local PHILIPS sales
organisation.
Input coupling AC, 0, DC
The signals under observation are fed to input socket A and/or B and the AC, 0, DC coupling is
set to either AC or DC depending on the compensation of the signal, As the vertical amplifier is
d.c.-coupled, the full bandwidth of the instrument is available, and d.c. components are displayed
as trace shifts if DC is selected. This may be inconvenient when small signals superimposed on
high d.c, voltages need to be displayed. Any attenuation of the signal will also result in attenuation
of the small a.c. component. This is remedied by using AC coupling, which employs a blocking
capacitor to suppres the d.c. and If. signals. However, some pulse droop will occur when Lf.
square-wave signals are displayed.
The 0 coupling interrupts the input signal and grounds the amplifier input, thus providing a quick
check of the 0 V reference level.
OFFSET and SHIFT
The OFFSET (before attenuator) facility, which can be operated via the VERTICAL COUPLING
menu, can be used to bring channel A and channel Broughly atthe same DC level, so thata better
use can be made of the vertical resolution.
The SHIFT (after attenuator and before memory) frontpanel controls can be used to adjust the
channels against each other with an even better resolution.