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Glossary of terms you will come across during your Phase 2 training.


Oscilloscopes

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Waveforms with captionsAmplitude, or peak voltage, is the maximum signal voltage relative to zero.

The apmlitude or peak voltage of the waveform here is approximately 0.7V.

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In digital electronics, outputs of circuits are either high or low. In an astable circuit, the output is constantly changing between high and low.

An astable square waveAn astable circuit is an electrical circuit that does not stay in one stable condition. If you were to monitor it with an oscilloscope, it produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.

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Changing duty cycleThis is the fraction of time that a system is in an "active" state.

With a square wave, during one cycle, the signal is active for part of the time and inactive for the other part of the time.

If the signal is active for half the duration of the cycle, it is said to have a 50% duty cycle.

If we change the duration of the active state, this is called pulse width modulation - PWM.

Read more about duty cycle on Wikipedia

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Inductive current clamp from PicoOscilloscopes can only measure voltage - the difference in potential between its positive and negative probes.

Oscilloscopes cannot measure current, but they can make use of an inductive current clamp to measure the strength of the magnetic field associated with the current flow. It can then display the voltage induced in the current clamp. It is then a simple matter of converting that voltage into the equivalent current value.

Read more about this clamp on the Pico website...

This video shows an inductive current clamp being used to observe the operation of a fuel injector in a petrol engine:

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A word derived from the Latin oscillum, meaning to swing back and forth, and the Greek word skopien, meaning to observe or examime.

Voltmeters sample electrical current and display the results as numbers on a screen (digital) or as a needle sweeping across a scale (analogue).

Mains electricity traced on an oscilloscopeOscilloscopes sample the electrical circuit much more frequently, and display the results as a moving dot, which leaves a trace on a screen with a graph and graticules as a background.

The oscilloscope has two voltage probes, one of which we connect to the signal we want to observe, and the other which we connect to ground. The oscilloscope then reports in the difference in potential between the two probes.

Many oscilloscopes have a separate ground connection which helps protect both the instrument and the operator. You should always connect this and the probe earth before probing other circuits.

NOTE: Oscilloscopes can only measure voltage, they CANNOT measure current. What oscilloscopes can do is to make use of an inductive current clamp around a current carrying conductor.

As we know, electrical current always has an associated magnetic field, and the oscilloscope can use this magnetic field to measure the corresponding current flow. The oscilloscope can then display a value for the current flow.

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A passive device is basically a device that consumes, but does not produce energy, or a component that is incapable of power gain.

For example, an attenuator is a passive device - it consumes but does not produce energy. A 20:1 attenuator takes a 20 volt signal and passes it on as a 1 volt signal.

A passive device such as an attenuator is incapable of power gain.

An example of a device that is capable of power gain is a transistor.

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Waveforms with captionsThis is the maximum signal level from maximum positive voltage to maximum negative voltage.

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Waveforms with captionsPeriod refers to a length of time, and in the context of oscilloscopes and AC, the period of an AC waveform is the length of time taken to complete one cycle - from 0V to maximum positive value, back down to lowest negative value and back to 0V. This is one complete cycle, which is repeated as long as the circuit is closed.

The period of the waveform shown here is 30 ms.

The number of cycles per second is known as the frequency of AC:

Frequency = 1/period

The frequency of the signal shown here is 33.33 Hz (1/30).


Another example: Mains electricity in Ireland alternates with a frequency of 50Hz. - the period of this supply is 20 ms (1/50).

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Root Mean Square Voltage, also known as V Effective (Veff)

RMS voltage waveform'Mean' is the same as 'average', and the words 'Root' and 'Square' refer to the square root of something.

When a voltmeter is measuring an AC signal, it does not measure the peak voltage, it only registers the effective voltage - the DC equivalent of the voltage being measured.

Consider the AC waveform shown- how do we measure its voltage? We cannot take the peak voltage as the value, becuse the supply is not always at this peak - it is constantly rising toward it, then falling away from it, all the way down to the lowest negative value before once again moving back toward the peak voltage.

Likewise, we cannot take the average between the peak voltage and the lowest value, because this value is always 0V.

The answer is that we take the value for peak voltage (the amplitude) of the signal and we divide it by the square root of 2, which is approximately 1.4:

VRMS = VP ÷ √ 2

VRMS = 0.7V ÷ 1.4 = 0.5V 

where VRMS is the root mean square voltage, and VP is the peak voltage. Note, the √ 2 (the square root of 2) is approximately 1.4

When you use a multimeter to read the voltage of an AC circuit, you are reading the RMS Voltage. The peak voltage in this circuit is higher.

For example, if you measure an AC circuit at 12V, the peak voltage of that circuit is calculated by using the formula:

VP = VRMS x √ 2

VP = 12 x √ 2 = 12 x 1.4 = 16.8V

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