Aliasing occurs when a too low and therefore wrong sampling frequency is used. The next illustration shows how aliasing occurs.

The input signal is a triangular signal with a frequency of 1.25 kHz (upper most in the illustration). The signal is sampled at a frequency of 1 kHz. The dotted signal is the result of the reconstruction. From that triangular signal the periodical time is 4 ms, which corresponds with an apparent frequency (alias) of 250 Hz (1.25 kHz - 1 kHz).

To avoid aliasing, the sample frequency must be greater than 2 times the maximum frequency of the input signal.

If you have any doubts about the displayed signal, you can set the timebase of the oscilloscope one step faster or slower and check whether the signal at the display changes accordingly. If that does not give any clearance, you can determine the frequency of the input signal with the spectrum analyzer. Therefore you must set the frequency range of the spectrum analyzer to the maximum.

The bandwidth of an oscilloscope or a spectrum analyzer determines the frequency spectrum that can be measured. However, for a signal that contains higher harmonics (e.g. a square wave) it should be noted that the bandwidth should be sufficient to display these harmonics to a certain extent and not just the base frequency of the signal. Otherwise, the original signal will not be accurately restored to the original input.

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When a signal is monitored and a glitch is expected, envelope mode can be switched on to be sure to see the glitch. In envelope mode, for each sample in the record a highest value and a lowest value is stored. Each measurement is checked whether the new value lies between the previous highest and lowest or not. If not, the highest or lowest value is changed to the new value. Then a vertical line is drawn between the lowest and highest value. So a glitch is always captured.

The input sensitivity of a channel determines how large a certain signal will be displayed, the lower the full scale value, the larger the signal will be displayed.

The accuracy of the amplitude of a measured signal is determined by the resolution of the Analog to Digital Convertor. The higher the resolution (number of bits), the more accurate the measurement. For example, an 8 bit ADC devides the vertical range of the input signal into 256 discrete levels, while a 12 bit ADC gives 4096 discrete levels.

Before the TiePie engineering software can use the measured data, the (analog) input data needs to be converted to digital signals. A single value of this digital data is called a sample and therefore the conversion process is called sampling. Sampling usually takes place at a fixed rate per second, known as the sample frequency.

• 1 kiloSample = 1,024 Samples
• 1 MegaSample = 1,048,576 Samples
• 1 GigaSample = 1,073,741,824 Samples

The sample frequency determines the speed at which the instrument will take samples of the input signal. The minimum required sample frequency (also known as sample rate) is determined by the maximum frequency of the input signal multiplied by 2. This is known as the Nyquist theorem.

More information on sampling and the Nyquist theorem can be found here

The signal coupling selection determines whether DC offset signals from the input are allowed to be displayed or not. When AC coupling is enabled, the DC component in the input signal is removed.

The Total Harmonic Distortion (THD) is defined as the ratio between the power of the harmonic frequencies above the base frequency and the power of the base frequency. This ratio is displayed in dB's.

To be able to examine a signal, the moment of displaying the signal has to be adjustable. Therefore an oscilloscope is equipped with a triggering system. This functions as follows:

The input signal is compared with two levels in the trigger system: the arming level and the firing level. When the input signal passes the arming level, the trigger system is armed. If the input signal passes the firing level, the trigger system becomes active and 'fires' a pulse. This pulse is used to start the display of the signal.

The arming level and the firing level are coupled to each other by the trigger hysteresis and their level is determined by the trigger level. The firing level corresponds to the trigger level. The trigger hysteresis defines at which signal size change can be triggered, the change has to be that large that both levels are passed. With a small trigger hysteresis it is possible to trigger on small signals. If a signal contains a lot of noise, a small trigger hysteresis causes triggering on the noise, instead of the original signal, which gives an unstable display. A trigger hysteresis larger than the noise level is then necessary.

In the illustration above, a signal and the two levels are displayed. In this case it is triggered on the rising slope of the signal. The signal passes the two levels from low to high. When triggering on the falling slope of a signal, the two levels are swapped. Then the signal has to pass the two levels from high to low, to generate a trigger.

The moment of trigger, the trigger point, can be placed at any position in the record with measured data. This will result in a certain number of samples before the trigger point (pre samples) and a number of samples after the trigger point (post samples). Pre and post samples together form the record length. The position of the trigger point is referred to as the pre trigger value, indicating how many pre samples are recorded in the total record. Usually (and default) this number is given in a percentage of the record length, but it can be given in a number of samples or in a time value as well.

The oscilloscope software of TiePie engineering allows for different trigger modes to be set:

• Rising slope
• Falling slope
• Inside window
• Outside window
• Peak