Added Mode And Common-Mode Measurements; Triggering - Philips PM3320 Operating Manual

4-96E
4.3.7 Add mode and common mode measurements
If the ADD mode is selected, the signal voltages of both vertical channels are added.
Depending on the selection of the INVERT-mode of the A or B channel, either the sum or the
difference of the input signal is displayed. The ADD mode also enables differential measure-
ments to be made. With these measurements, advantage is taken on the common mode rejection
in the ADD mode. When the channel B is selected for inversion (and channel A INVERT not
activated), the common mode portions of the signals on input sockets A and B will only be
subjected to very slight amplification compared with the differential mode portions (see Fig. 4.44}.
In measurements where signal lines carry substantial common mode signals, e.g. hum, the
differential mode will largely cancel out these signals and leave the remainder of interest (A-B).
The capability of the oscilloscope to suppress common mode signals is given by the Common
Mode Rejection (CMRR) factor. To obtain the degree of common mode rejection as specified,
channel A and B gains must first be equalised. This can be done by connecting both channels to
ihe CAL output connector, and adjusting one of the vertical VARIABLE controls for minimum
deflection on the screen.
When passive attenuator probes are used, a similar equalisation process is recommended by
adjusting their compensating control for minimum deflection.
CMR [common mode relection)e-St
As drawn here CMR = 10
MATS74
Figure 4.44 Suppression of common mode signals.
Triggering
Trigger sources
The acquisition system can be triggered on signals which are derived from the vertical channels
A and B. if two time-related signals are displayed via channels A and B, a stable display is
obtained if the trigger source is the signa! with the lower frequency.
The acquisition system can also be triggered on a signal which is applied to the external trigger
input socket EXT TRIG. This input can be set for two different sensitivities EXT or EXT:10.
if LINE triggering is selected, the trigger signal is directly derived from the mains voltage and has
the same frequency as the mains voltage. This trigger mode is usefull, for instance, when
examining the mains voltage ripple on the d.c. output voltage of a power supply.