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.
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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.
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.
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.
While measuring complex waveforms in digital techniques, mistakes can be made very easily. In this section, examples of this are presented. Some of them are explained in detail, to gain knowledge about the possible reasons for false triggering, which leads to wrong timing displays on the screen.
PULSE DEFINITIONS
In digital techniques, it can happen that two pulses appear in a timerelated sequence, but that the second pulse appears a little later, with a delay, with respect to the first one.
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.
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.
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?
TTL Triggering
In logic systems employing building blocks (gates, Aip-Aop, etc.), a logic state level may be defined to determine whether a logic signal is supposed to be in its 0 (zero) or in its 1 state.
Continue reading “TTL Triggering and Logic Analyzer Triggering”
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