Special Oscilloscope Variants

The Multiplier Oscilloscope

One of the latest oscilloscope features is the multiplication of signals. With this feature, it is possible to study instantaneous power. For instance, during the switching transients in logic circuitry, the collector voltage can be seen as a function of time. Also, the collector current can be shown on the screen. The product of these parameters is then a measure of the collector dissipation. But, it is difficult to study the instantaneous power from the screen. For this, the analog multiplier provides a solution.

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Applications for a Sampling Oscilloscope: Part 2

Measurements on Signals below the Noise Level

The use of a recorder for handling the output signals of the PM 3400 oscilloscope has the advantage of acting as a low-pass filter that effectively reduces noise and jitter. It has been found possible to measure signals that lie considerably below the noise level in this way. Recording signals on an X-Y recording via a sampling oscilloscope will give a considerable reduction of the noise because the X-Y recorder acts as an integrator. If, however, the noise signal contains a component which is coupled in frequency with the trigger signal, this part of the signal is recorded without attenuation.

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Applications for a Sampling Oscilloscope

Recording Signals with an X-Y Recorder

One of the limitations of an oscilloscope is the relatively small size of the screen (generally 8 x 10 cm). If the trace is 0.3 mm thick, this gives a resolution of about 270 x 330 lines. A photographic record of the trace on the screen will be subject to the same limitations as resolution while making extra copies of Polaroid prints (the usual medium used in oscilloscope cameras) is by no means an easy matter and is relatively expensive.

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The Sampling Oscilloscope Part 2

Loop Gain and Smoothing

To ensure smooth operation, the loop gain of the signal path through the transformer, the ac amplifier, the memory, and the feedback attenuator must be unity. When the loop gain is less than unity, more samples are required to ensure a correct reproduction of the input waveform. For example, with only a few samples per centimeter, the shoulder of a displayed square-wave signal is likely to be rounded.

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Random Sampling vs. Sequential Sampling

The two sampling techniques most commonly applied are random sampling and sequential sampling. In the random-sampling technique, no time relation exists between the timing-ramp voltage (trigger-source functioning) and the sampling instant. Owing to this, the picture on the screen is built up with samples which appear at places scattered at random over the waveform. In the sequential-sampling technique, which is the technique most frequently employed, the successive samples appear on the screen at adjacent places over the waveform because a comparison circuit links the sampling instants to the timing ramp voltages when triggered by the input signal.

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The Sampling Oscilloscope

What is Sampling?

Sampling is the taking of a specimen, or a part, to illustrate the whole. For example, when a ship’s cargo of sugar must be checked for the amount (%) of water in the sugar, specimens of the sugar are taken from different places in the ship. The more specimens taken, the more information is available about the quality of the cargo overall. To be 100% sure about the condition of the cargo, all the sugar present in the ship would have to be checked; for obvious reasons, this is not possible.

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Construction of the Variable-Persistence Tube Part 2

The flood guns are located just outside the horizontal deflection plates. A cloud of electrons is emitted by each flood-gun cathode. These clouds are combined, shaped, and accelerated by the two control grids, as well as the collimator. The collimator consists of a coating on the inside of the tube. The positive voltage on the collimator is adjusted so the flood-gun electron cloud only fills the CRT viewing screen. The cloud is further accelerated towards the storage mesh and viewing screen by the collector mesh. After passing through the collector mesh, the flood electrons are further controlled by the potentials of the storage mesh and storage layer. The cathode side of the storage mesh is coated with the nonconductive storage material, which is where the pattern to be displayed is stored. Because of the nonconductive property, only a capacitive coupling exists between the storage layer and the storage mesh. This capacitive coupling is required for the storage and erase functions. The rest potential of the storage mesh is approximately + 1 V with respect to the flood-gun cathodes. In the write and erase routines, the potential of the storage layer varies from O V to negative. This is accomplished through the storage mesh and the capacitive coupling.

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The Storage Oscilloscope: Storage Principles

Storage oscilloscopes are used in applications where the display time at the screen is too short to examine the signals to be measured. If a single-shot signal is to be measured, only one sweep is generated. During this sweep, the screen is excited by the high-energy electron beam. When the beam is suppressed at the end of the single sweep, a phosphorescence remains for some time. The time that the phosphorescence remains visible is dependent upon the type of phosphor used and is referred to as the persistence of the tube. The persistence is the time that the intensity, after the excitation, takes to decay to a level of I/ e of the level attained during excitation (e = 2.72 = base of natural logarithms).

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Triggering the Delayed Time Base

Trigger Modes

The delayed timebase may start immediately after the main timebase has reached the level at the DELAY potentiometer. But now the following may happen. Assume that the signal to be tested is a pulse train and that the time between two successive pulses is not constant, but varies a little around the set repetition rate. The result will be a somewhat unstable display; this is known as jitter. The time between the first and the second pulses varies a little, as does the time between the second and the third pulses. The third pulse varies twice as much with respect to the first one as the second pulse does. The fourth pulse varies three times as much, and so on.

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