Buying an oscilloscope - spoiled for choice.

Table of contents.

1. Buying a new oscilloscope - but which one?
2. Decision criteria for a new oscilloscope
3. What criteria should I consider when buying an oscilloscope?
4. A short introduction: The different types of oscilloscope
5. Digital oscilloscope: Sampling methods
6. Special laboratory oscilloscopes

Buying a new oscilloscope - but which one?

Your measurement application defines the technical specifications for the oscilloscope. Depending on the budget, compromises may have to be made when purchasing. However, these compromises should not affect the crucial factors. An oscilloscope can be used to display signals over time. Trigger conditions determine the recording and display of the signals. The two-dimensional display shows the signal amplitudes on the y-axis, while the x-axis shows the time curve.

This leads to five key parameters that influence your choice of oscilloscope:

• Minimum and maximum amplitude range to be measured (volts per scale division)
• Minimum and maximum duration of the display (resolution: seconds per scale division)
• Number of channels: How many independent analog and/or digital signals have to be displayed?

Decision criteria for a new oscilloscope

1. The bandwidth

The bandwidth (given in Hz) describes the frequency range over which the oscilloscope can measure. It is defined as the frequency at which a sinusoidal input signal is attenuated to 70.7% of the original signal amplitude. A sine signal with a frequency of f0 and an amplitude of 1 Vpp is therefore displayed as 0.7 Vpp.

This value is also referred to as the -3 dB point or -3 dB limit (see Fig. 1). If the bandwidth is chosen too low, the oscilloscope cannot detect any high-frequency changes. The amplitude may be distorted and the edges are difficult to see, so that signal details are lost.

To determine the optimum oscilloscope bandwidth that allows you to precisely characterize the signal amplitude, the bandwidth of the oscilloscope should be at least five times the highest frequency component to be measured. This means that the oscilloscope is capable of detecting the 5th harmonic of the clock signal and, for example, of displaying a square wave signal as such.

To determine the optimum oscilloscope bandwidth that allows you to precisely characterize the signal amplitude, the bandwidth of the oscilloscope should be at least five times the highest frequency component to be measured. This means that the oscilloscope is capable of detecting the 5th harmonic of the clock signal and, for example, of displaying a square wave signal as such.

In principle, all oscilloscopes have a frequency response like that of a low-pass filter. They differ in how the frequency response drops off from the -3 dB limit. Broadband oscilloscopes (>1 GHz) generally have a steeper drop of 40 dB/decade (see Fig. 2) and can perform much more accurate measurements of the pulse edge steepness within their bandwidth than oscilloscopes with a bandwidth <1 GHz with a drop of 20 dB/decade. Its filter exhibits Gaussian behavior. Regardless of the frequency response of the oscilloscope, the -3 dB point determines its bandwidth by definition (see Fig. 3).

As far as the bandwidth is concerned, note the following points: The measuring system comprises not only the oscilloscope, but usually also a probe. These two together form the relevant system bandwidth. The low-pass filter with the smallest bandwidth ultimately determines the system bandwidth.

As far as the bandwidth is concerned, note the following points:

The measuring system comprises not only the oscilloscope, but usually also a probe. These two together form the relevant system bandwidth. The low-pass filter with the smallest bandwidth ultimately determines the system bandwidth.

2. Rise time (edge steepness)

The rise time can be a suitable performance criterion if digital signals, e.g. pulse and step signals, are to be measured. A steep edge is important here (rise time/fall time) so that the receiver can decide between 1 and 0 within the unit interval. For this reason, input filters with a steeper slope (see Fig. 2) are generally used with oscilloscopes with a bandwidth >1 GHz.

The rise time describes the usable frequency range of an oscilloscope. The following equation can be used to calculate the oscilloscope rise time required for a particular type of signal:

Oscilloscope rise time ≤ fastest signal rise time x 1/5

The rise time of the input signal is the time that the signal needs for the transition from 10% to 90% of the maximum amplitude (see Fig. 4). The rise time of the oscilloscope should be one third to one fifth of the rise time of the signal to be measured. An oscilloscope with a faster rise time can capture important details of fast transitions more accurately. The relationship between the rise time of the oscilloscope (Tr) and the bandwidth (BW) can be established using the constant "k":

The constant "k" depends on the oscilloscope. For oscilloscopes with a bandwidth of <1 GHz, k = 0.35. For oscilloscopes with a bandwidth of >1 GHz, "k" is typically between 0.4 and 0.45.

One example: Using an oscilloscope with Gaussian frequency characteristics, a signal with a rise time of 1 ns is to be measured with an accuracy of 3%. With fKnee = 0.5 / 1ns and the table value for the BW = 1.9 * fKnee, this results in a bandwidth of 950 MHz. Accordingly, an oscilloscope with a bandwidth of 1 GHz is selected. The clock rate of the digital signal, whether 100 or 500 MHz, is not relevant here; the rise time determines the required bandwidth.

3. Sample Rate

The sample rate (Sa/s, samples per second) is the frequency at which the A/D converter converts the analog signal input values into digital data. The oscilloscope samples the input signal after attenuation, amplification or filtering has been applied to the analog input path. The higher the sample rate, the better the temporal resolution and thus the details that can be recognized in the signal curve. The "Nyquist sampling theorem" explains the relationship between the sample rate and the frequency of the measured signal. It states that the sample rate fs must be at least twice as high as the highest frequency component to be analyzed in the measured signal (also known as the Nyquist frequency) in order to represent it unambiguously.

Real-time sampling is an ideal sampling mode for signals whose frequency range is less than half the maximum sample rate of the oscilloscope, e.g. to capture fast, one-off, transient signals.

4. Number of channels

Most oscilloscopes can measure several signals simultaneously and display them on the screen. Each individual signal recorded by an oscilloscope is fed into a separate channel. Models with two, four or even eight channels for analog signals are common.

Mixed-signal oscilloscopes (MSO) usually also have eight or 16 digital inputs for displaying and analyzing time-correlated analog and digital signals. As logic analyzers, these channels can decode digital signals - including serial signals - and display them in the appropriate mask field in accordance with the standard. Signal anomalies and errors can thus be easily detected.

5. Signal acquisition rate

The signal acquisition rate indicates how often the complete displayed signal or a signal section is acquired per second (given in wfms/s, waveforms per second). The number of acquisitions depends, among other things, on the set time resolution (seconds per scale division, e.g. ns/div) and the memory depth for saving the acquired signal sequences. An oscilloscope requires a time X after each sampling cycle of an incoming signal in order to process the data (storage and display on the screen). The shorter this processing time (dead time) is, the faster the oscilloscope samples a new signal and can therefore, for example, also detect sporadic signal anomalies more quickly (see Fig. 4).

With multi-channel oscilloscopes, the sample rate and signal acquisition rate may be reduced if several channels are in use. This depends on how the internal storage process is implemented. The sample rate and signal acquisition rate often adapt automatically to the storage conditions.

6. Memory depth

The number of scans is limited by the memory depth: In order to be able to use the maximum sample rate for a longer period of time, a deep memory is required. The memory requirement is calculated using a simple formula based on the longest time range setting required and the maximum sample rate at which you want to record a signal: Storage depth = recording duration * sample rate. However, this only applies if each input channel of the oscilloscope has the same memory depth. The recording duration is usually adjustable in order to achieve optimum and detailed recording of signals.

Note: In the case of sporadic signal events, it is advantageous to have the deepest possible memory and a high signal acquisition rate in order to be able to store as many signal sequences as possible.

7. Vertical resolution

The vertical resolution of a digital oscilloscope indicates the accuracy with which the A/D converter converts the input voltage into digital values. Eight to 16 bits are common.

Example: An oscilloscope with a resolution of 10 bits achieves 1024 measurement points in the vertical plane. With the setting "1 volt per scale division" with 10 screen scale divisions in the y-direction, 10 volts ("full deflection") can be measured. With a resolution of the A/D converter of 1024 points, this means that a measurement point is set on the screen at intervals of 9.76 mV. With modern oscilloscopes with 16-bit resolution, the measurement points in the above example are even taken every 0.153 mV.

The effective amplitude resolution of some oscilloscopes can also be increased using digital calculation methods, e.g. in high-resolution acquisition mode (Hi-Res mode, HD mode).

8. Connections (interfaces)

The evaluation of your measurement results is just as important as the efficient exchange of information and reliable documentation. The connectivity of your oscilloscope, e.g. via USB, RS-232, LAN, WLAN or Bluetooth, enables extended analysis functions and simplifies the documentation and exchange of results. In addition, your measuring devices can be integrated into networks via appropriate interfaces and controlled remotely via the web.

9. Trigger functions

Triggers are used to stabilize repetitive signals on the screen, to capture single-shot signals or to mark a specific point of a capture. The trigger functions of an oscilloscope synchronize the horizontal deflection at the correct signal point, which can be crucial for clear signal characterization.

10. Extensions

Your oscilloscope should also be able to meet future, changing measurement requirements in order to be economical. Corresponding oscilloscopes allow the number of channels, bandwidth or memory depth to be expanded at a later date. Software options, e.g. math functions, FFT (Fast Fourier Transformation) or masks for standard transmission protocols, can be used to add application-specific measurement functions and extend the performance and functionality of the device: e.g. with an integrated digital voltmeter, frequency meter, power meter, arbitrary function generator or spectrum analyzer. Oscilloscopes can also be supplemented by a comprehensive selection of probes, clip-on ammeters and application modules.

What criteria should I consider when buying an oscilloscope?

A short introduction: The different types of oscilloscope.

Digital oscilloscopes are divided into digital storage oscilloscopes (DSO), digital phosphor oscilloscopes (DPO), mixed-signal oscilloscopes (MSO) and digital sampling oscilloscopes.

Digital oscilloscope: Sampling methods

During sampling, part of the input signal is converted into a series of discrete electrical values. The value of the displayed sampling points corresponds to the amplitude of the input signal at the time of sampling. Each snapshot therefore corresponds to a specific point in time in the signal. The digital oscilloscope displays a series of sampling points with the measured amplitude on the vertical axis and the time on the horizontal axis, thus reconstructing the input signal. A distinction is made between a real-time oscilloscope and an equivalent time oscilloscope (sampling oscilloscope). Some oscilloscopes offer a choice of sampling method: real-time sampling or equivalent time sampling.