Oscilloscope Probe Types

Terminated HF Probes

At higher frequencies, the input capacitance has much less impedance (Xe) than the 10- or 20-MO input resistance of the probe. For the circuit under test, this means that if the internal source impedance is high, low input capacitance of the probe is important indeed.  But in hf techniques very often low source impedances of 50 to 75 n are met and a normal 50-0 coaxial cable can be used as the probe, provided that the cable is terminated with its characteristic impedance at the oscilloscope end. For an oscilloscope with an input impedance of 1 MO in parallel with 20 pF this means that a 50-0 termination resistor is to be connected to its input terminals. Special hf oscilloscopes already have a 50-0 input impedance.

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Plug-in Oscilloscopes

A plug-in oscilloscope is electrically like any other oscilloscope. The mechanical housing of the plug-in instrument is different from that of the compact one, because the former consists of a mainframe to which one or more plug-in units can be added, to vary the oscilloscope’s facilities. The company which has elaborated the plug-in idea the most by far is Tektronix, Inc. The picture below shows an example demonstrating the idea. The choice between a plug-in or a compact oscilloscope can be aggravated by the question: How many different oscilloscope functions do I need, and for how many people?

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The DTB with Digital Delay

The delayed time base is started (or triggered) when the MTB sweep has reached a certain level, which is compared to a preset dc level. The preset level is thus reached a certain time after the MTB has started. This time is determined by the TIME/0 1v setting of the MTB. If the signal possesses a jitter, the display of the OTB will not be stable when operated in the START mode. Usually, selecting the TRIG mode of the OTB will eliminate this trouble.  If, however, the jitter is considerable, it can exceed the time between two adjacent waveforms. This may be the case with mechanical devices, such as tape or disk units of computer systems. Not even in the TRIG mode of the OTB can a unique display be obtained, because one delayed sweep may be triggered at waveform number 67 and the next one may be at waveform number 69. If all waveforms are identical (pulses), however, the display will be stable, although the observer will not know which pulse he or she is viewing.

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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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