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Why Visualization of Signal? Basics to Understanding Oscilloscopes

The specific characteristics of a signal can be measured by a variety of instruments. For example, a counter can measure a signal’s frequency or its period, and an ac voltmeter can measure the RMS value of the signal. Although these instruments are very useful and can be more accurate than the oscilloscope, their application is mainly limited to the measure­ment of one parameter of the signal. With an oscilloscope, one can visual­ize the signal of interest and also observe whether the signal contains properties that would not be made apparent by most other instruments (for example, whether the signal is superimposed on a de level, or whether there are noise or relative hf oscillations present with the signal at the test point). Thus the oscilloscope is a more valuable instrument because it gives an exact visual representation of the signal waveform.

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Sampling Oscilloscopes

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 various places in the ship. The more specimens are taken, the more information is available about the quality of the cargo overall. It is evident that to be 100% sure about the condition of the cargo, all the sugar present in the ship would have to be checked; however, this is not possible.

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Understanding Specifications Part 2

To prevent interference on receiving apparatus, for example, audio and TV receivers or computer systems, signals generated in the line supply and the radiated electromagnetic field of radio frequency from electrical equipment may not exceed certain limits. For this, the IEC makes recommendations. A special committee of the IEC, the CISPR (International Special Committee on Radio Interference), has published several definitions concerning measuring sets and measurement procedures for the various types of interference-producing equipment.

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Understanding Specifications

The 15-MHz portable dual-trace oscilloscope Philips PM 3226 is a compact, lightweight instrument featuring simplicity of operation, for a wide range of use in servicing, research, and educational applications. Other features include provision for chopped or alternate display of Y signals, automatic triggering, mains triggering, and triggering on the line and frame sync pulses of a television signal. The cathode-ray tube displays a useful screen area calibrated into 8 x 10 divisions by an external graticule.

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Measurement Pitfalls

Very often hum is present on the signals under test. This can be easily determined from the screen because the hum is related to the line frequency. If a signal shows a kind of unexpected amplitude modulation, switching back the time-base setting to about 5 to 10 or 20 ms/div, and switching over the trigger source selector to MAINS (or LINE), will generally result in a stable picture in the event of hum.

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Current Probes and Logic Trigger Probes

CURRENT PROBES

Basically, the current probe is a transformer of which the primary winding is the test lead through which the current is measured. The probe head consists of a ferrox-cube core and the secondary windings of the transformer. The core can be split into two parts to clip it simply around the measuring lead. The white-colored part of the probe head can be moved backward and forwards to clip it around the lead. A voltage is developed in the transformer secondary windings by the magnetic field around the measuring lead. This voltage is fed to an amplifier box, the output of which is fed to the oscilloscope. The output cable from the amplifier must be terminated with 50 fl at the oscilloscope end (low-ohmic system for 75-MHz bandwidth).  Furthermore, if the oscilloscope is set to 50-mV/ div sensitivity, the amplifier box provides calibrated outputs ranging from 1 mA/ div on the screen.

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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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