How To: Modifying an Oscilloscope for Measurements

Apart from single-shot measurements, the signal that is going to be measured must be repetitive. Signals a, b, and c in Fig. 1.9 are all repetitive because a span of time can be defined so that the same signal is repeated sequentially.

(A)              [\_(‘VV’\_

—–11 T,    Ii———

rep

(B)Jl     JL

-j–Trep          I•

(Cl      -fUl[LJlJl.,,f—fUlJLJLfL

-1– Trep

Figure 1.9

The oscilloscope will obtain a steady display of these waveforms:

1. Every sweep is started (triggered) at the same point on the waveform. The sweep is the horizontal deflection on the screen created by a sawtooth voltage applied to the X-deflection plates. The sweep is triggered by a circuit, which creates a trigger pulse every time the trigger signal (usually taken from the input signal) crosses a certain voltage level. This can be on the signal’s positive or negative slope. The start of the waveform on the screen is then determined by two settings of the oscilloscope front-panel controls: the LEVEL position on the waveform and the SLOPE (positive or negative).
2. When the spot reaches the right side of the screen (the end position), it is swiftly brought back to the left side (the start position). This is referred to as the flyback part of the During the flyback, the beam is blanked. The sweep is started again by the next trigger pulse at the same position where it started the previous time.
3. The sweep circuit guarantees that during the sweep, including the flyback time, any other triggers at the input of the sweep circuit have no effect. This may happen if more than one waveform at the screen is displayed.

Modifying the Oscilloscope for Measurements

The oscilloscope does not function as an ideal differential amplifier because the subtraction is made after a few stages of vertical attenuation and amplification instead of at the input. The degree of common-mode rejection which can be attained is highly reliant on how well the operator equalizes the channels before the subtraction stage. The following operation information applies:

• Use the same input coupling (ac or dc) and deflection coefficients (AMPL or v/mv) for both channels.
• If a common-mode rejection higher than IO is required, equalize the channels in the following way: Link both probes from channel A and B inputs to one of the signals to be measured. Adjust GAIN (adjustment on front) or amplitude vernier of a single channel, and the probe compensation if a IO: I probe is used, for minimum deflection on the Ensure that the input amplifiers of the oscilloscope are not overdriven by the common-mode voltage. In some oscilloscopes, this becomes apparent when the VERTICAL POSITION is altered or when the next more sensitive deflection coefficient for both channels is selected. In these cases, the display shows an unexpected rise in the common-mode signal amplitude.

For example, with the Philips PM 3260 and PM 3265 oscilloscopes, a common-mode rejection of at least I00 is guaranteed, as long as the foregoing measures have been taken. However, it must be kept in mind that this figure is only valid for average frequencies and not at the extremes of the bandwidth.