Analog Oscilloscope vs Digital Oscilloscope

For electronics hobbyists, engineers and technicians, buying a new oscilloscope can be a bit of a challenge because the brand to choose from, and the specification each one of them provide, and when comparing the cost, is in abundance. So we have put together a form of perspective to guide you in choosing an oscilloscope.

As A Beginner

Forget about the specs, the cost, and all other technical aspects of the oscilloscope and ask yourself these questions:

  1. Do you need a portable, light-weight oscilloscope, or do you need a benchtop, idle heavy oscilloscope?
  2. According to your work, how many signals will you need to analyze at an instant?
  3. What are the signal amplitude peaks (maximum and minimum) of the ones you analyze?
  4. What is the signal’s highest measuring frequency?
  5. What kind of signals will you have to deal within mostly – repetitive or single shots?
  6. Do you need to analyze your circuit signals in both frequency and time domains.

If you have a clear answer to the above asked questions you can easily filter most of the oscilloscopes available, and just focus on the few of them remaining. If you still do not have a clear idea, we have gone through all these details intricately. Just keep reading our article to get a better idea.

Analog and Digital Oscilloscope – Comparison and Difference

The basic working of both the analog and digital oscilloscopes are somewhat the same. The internal components used in either of the devices are the same as well. Even the display used in both the devices maybe the same. So, if you ever thought of an upgrade from an analogue oscilloscope to a digital one, you could easily cope up with it.

The main use of both the oscilloscopes is to measure time-based varying signals. Time, being one of the parameters for analyzing the waveform, the signal you obtain at a certain time period will vary. This variation is measured using the oscilloscope and the result is to find anomalies and anomalies and noises, along with the characteristics in it.

The difference between an analog oscilloscope and a digital oscilloscope is that in an analog device the waveform is shown in the original form, while a digital oscilloscope converts the original analog waveform by sampling it and converts them into digital numbers and then stores them in digital format. This is done by an Analog-to-Digital converter.

Analogue Oscilloscopes

Older analog oscilloscopes used the cathode ray tube (CRT) to display waveform and image. Later, the display format was upgraded to LCD’s and :ED’d. There are still a few companies that provide CRT based oscilloscopes called the Cathode Ray Oscilloscopes (CRO).

In a CRO, the CRT displays the signal in the X-Y axes. Here, the Y-axis represents the instantaneous value of the incoming voltage and the X-axis represents the ramp waveform. As the ramp waveform voltage is increased, the trace moves across the screen in a horizontal direction. When the trace reaches the screen end, the waveform trace sets back to the beginning.

Cost is one big factor that is to be considered when buying an oscilloscope. Analogue oscilloscopes are way more economic when compared to their digital counterparts. Digital Oscilloscopes are costlier because the use of high-tech components and their development costs increase the device price.

Instek GOS-630FC General Purpose Portable Analog Oscilloscope 30Mhz Bandwidth

Instek GOS-630FC General Purpose Portable Analog Oscilloscope 30Mhz Bandwidth

Performance wise, an analog oscilloscope proves satisfactory for use in industrial applications, electronics labs and so on. But, when it comes to high-end applications, analog oscilloscopes are not reliable. For advanced research and applications, the only way is the use of digital oscilloscopes. We have compiled some of the best selling digital oscilloscopes and reviewed them in the article  – Best Digital Oscilloscopes 

Digital Oscilloscopes

All the features provided by an analog oscilloscope is clearly surpasses by digital oscilloscopes. Let us consider each and every parameters regarding a digital oscilloscope.

  • Bandwidth (BW)

Bandwidth refers to the signal’s’ maximum frequency that can be passed through the front-end amplifier. This implies that in real-time, the analogue BW of your oscilloscope must be more than the maximum frequency that you want to calculate.

BW is not the only parameter that can prove that a digital oscilloscope can capture a high frequency (HF) signal. The provider of the scope must make sure to achieve a specific type of frequency response with their oscilloscope design. This response is called the Maximally Flat Envelope Delay (MFED). But this response parameter can not be fully attained ideally. This is because a loss could occur due to the presence of differential amplifiers, attenuators, analog-to-digital converters, interconnects and relays.

The user must keep in mind that all digital oscilloscopes clarify the device bandwidth as the frequency at which a sine wave signal will be attenuated to 71% of its true amplitude (-3 Decibel point). In other words, the trace you see in the display will have a 29% error of the input. Before buying a digital oscilloscope you must refer the datasheet to note the BW defined for all voltage ranges.

If your input waveform is not a pure sine wave, it is sure to include higher frequency harmonics. For example, a 500 MegaHertz pure sine wave, when seen on a 50 MegaHertz bandwidth oscilloscope will be shown as an attenuated and errored waveform. So theoretically, the user should go for an oscilloscope that has a BW five times higher than the user’s input waveform. But in practise this is not possible, as higher BW oscilloscopes are really expensive.

  • Sampling Rate and Memory Depth

The sampling rate and memory depth are two of the most important aspects of a digital storage oscilloscope. In a digital oscilloscope, the sampling rate is expressed in  either Mega Samples per second (MS/s) or Giga Samples per second (GS/s). According to Nyquist Criterion, the sampling rate of the device must be equal to or more than twice the maximum measuring frequency. This may be true in the case of a spectrum analyzer, but in an oscilloscope you will need at least 5 samples to precisely alternate and repair a waveform.

The sampling rate of signals for all oscilloscopes are different, and is defined on the basis of real-time sampling and equivalent time sampling (ETS) values. In ETS, the waveform is required to be stable and repetitive, as this sampling process is done by building up the waveform from successive acquisitions.

If a comparison is made between a transient waveform and a repetitive waveform, ETS will not prove worthy for the former, and the only reliable sampling method is the real-time (single-shot) bandwidth, which is typically very low.

The manufacturers of oscilloscopes will always provide specifications that sound perfect for their device. It is recommended that the user looks out that the quoted sampling rate applies to all signals, or only to repetitive ones. The specifications may also vary in dependence to the different number of channels in the oscilloscope.

In the case of memory depth, it simply refers to the buffered memory that captures the sampled waveform. The sampling rate is directly related to the memory depth of the device. That is, for a given sampling rate, the memory size decides on the capturing time of a signal before the memory is over. This connection between these two parameters of the device are significant, as the truth is that a digital oscilloscope with a high sampling rate and low memory depth will not handle its complete sampling rate on the top few timebases.

If a 250 microseconds waveform in video mode is captured using a small memory depth of 1K, the small memory will bound the sampling rate at 5 mega samples per second even though the scope has a capability of sampling at 100 mega samples per second.

It is important to know the base connection between the above three parameters: BW, Sample Rate and Memory Depth. For this consider an example where we are capturing one frame of USB (1.1) data. The data is transmitted serially at a speed of 12Mbps and a frame of data at 1 Ms. Let us consider the case where we have to capture a 12 MegaHertz square wave for 1 Ms. The bandwidth for a 12 MegaHertz  signal would require a 50 MegaHertz oscilloscope as you are likely to get a distorted signal for a scope with a BW of 12 MegaHertz or a little above. To get the sampling rate approximately 5 points per waveform will be needed. This means that the oscilloscope must have a sample rate of 60 MS/s or more. For the said sampling rate and a time of 1 Ms, a memory depth of 60,000 samples and more is needed.

  • Resolution

The resolution of most digital oscilloscopes is up to 8 bits. This resolution is enough to catch about 0.4% of distortion in the signal. That is huge when comparing the 0.1% distortion that can cause an overall error in an audio signal.

For a resolution of 8 bits, the voltage ranges is divided up to 256 vertical steps. This is equal to around 8 millivolts per step, provided that the range selected is around +/- 1 Volt. This will suffice in analyzing digital signals.

Higher resolutions of 12 bits and 16 bits are more ideal for cases where temperature, current or pressure  bases audio noise or vibrations are to be measured.
The accuracy of a digital oscilloscope varies around 3% to 5% for a resolution of 8 bits. As the resolution increases, the accuracy also increases. High resolution digital oscillators which have high DC accuracy are named precision oscilloscopes.

  • Trigger

An oscilloscope’s trigger function is important in reference to the characterization of the signal in analysis. The main idea behind using a trigger is to synchronize the horizontal sweep of the correct point to its signal. Trigger helps the user to stabilize repeating waveforms and capture single-shot waveforms. Triggering options provided by the oscilloscope includes source, level slope, pre-trigger and post-trigger. These options are the same for all oscilloscopes, but varies in the manufacturer’s provision in advanced triggering functions. These advanced options can be utilized according to the type of signal to be analyzed. According to the users preference some special triggers like disk drive testing can be provided by the manufacturer for extra cash and can be upgraded to the oscilloscope software.

  • Input Ranges and Probes

Most oscillators provide input ranges between +/- 50 millivolts to +/- 50 volts. Make sure that the scope has a discrete voltage range for the signals to be measured. Measurement of high voltages can be done using 10:1 and 100:1 attenuating probes. For analyzing very small signals (<50 millivolts), it is advisory to look for oscilloscopes with resolution above 12 bits. This will help in zooming capabilities on millivolts and microvolts level signals.

Please make sure to check the standard of the probe before purchase. Some manufacturers only higher BW probes, which are needed to get the best from the scope, as optional components. Though the probes provided can be switched between 1:1 and 10:1 attenuation, it is preferable to use 10:1 attenuation mostly so as to reduce the circuit load and increase protection against accidental high voltage.

An FET probe can also be bought in cases where high speed signals that are greater than 200 MegaHertz are used. A differential isolating probe can be used for high voltage, three phase applications.

  • Handheld Oscilloscopes, PC-Based Oscilloscopes & Benchtop Oscilloscopes

Handheld oscilloscopes are known for their portable use in industrial sites, service centers and so on. Additional batteries can be used to power them. Only disadvantage is the heavy pricing for these devices. An example for a handheld oscilloscope is shown below:

Owon MSO7102TD, 100MHz Mixed Signal Oscilloscope with 16-Channel Logic Analyzer

Handheld Digital Oscilloscope – Owon MSO7102TD, 100MHz Mixed Signal Oscilloscope with 16-Channel Logic Analyzer

Out of the three, in terms of performance the highest mark goes to benchtop oscilloscopes. At the same time, the additional features like FFT mathematical signal analysis, PC interfaces, disk drives and printer connections has made it very much costly.

A main substitute for benchtop oscillators in terms of price is the PC-based type. Besides, you can export analytic datas to word docs and spreadsheets with just a few clicks. Other advantages include colorful LCD displays, fast processors, drives and many more.

PC-based Oscillators are of two types – Internal and External. Internal PC-Oscillators are much more cost effective and are basically PCI format plug-in cards. But they are distortive, noisy and are to be connected to desktops for advanced signal analysis. They are not suitable for portable purposes.

 

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