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When looking at USB oscilloscopes and considering which is the best buy, it is worth taking some time to look at the requirements and the options available.
There is a very wide range of USB oscilloscopes and in particular PicoScopes that are available. They can meet many needs, but which is the best one for your particular situation.
To some there can be a huge selection, and selecting the right option may not be easy, but by following a simple logical pattern it is possible to analyse what is needed and then go through the options to make the right decision.
The first step in making the decision about the best USB oscilloscope or the best PicoScope to buy is to carefully consider what is actually needed.
It is worth looking at the waveforms that need to be measured - top frequencies, i.e. the top frequencies of the constituents on the waveforms, the number of channels that need to be made, the sorts of measurements that need to be made, etc..
It is also worth deciding whether the scope is to be used for a specific application, or will it be used as general purpose equipment in the laboratory in which case it may be worth increasing the specification to accommodate other unforeseen requirements. As this will cost additional money a judgement call may be needed as it is never possible to fully predict the future.
When looking at any oscilloscope whether it is a bench top scope, or a USB scope like the PicoScope, then there are several specifications that need to be looked at to ensure it meets the needs of the various jobs it will be required to perform.
Some of the specifications are relatively obvious, whereas others are a little more obscure. However they need to be considered when looking at buying a PicoScope USB oscilloscope.
- Number of channels: It is worth taking a good look at the number of channels required. Years ago scopes only had one or two channels. Now with far more complicated circuits, more channels are a necessity. Two are often used and four may be needed. For example an application looking at tree phase circuits may need one for each voltage waveform, and one for each current waveform. Combined with another monitoring circuit or two this could conceivably use eight channels. Whilst not all applications will need eight channels, this does illustrate the need.
- Mixed signal oscilloscope: When using a PicoScope for analysing embedded system using MCUs or even when looking at general logic, the operation of the circuit may need to be analysed looking at bus circuits and the like. The mixed signal capability where logic analysis channels are used in conjunction with the analogue scope inputs can be particularly useful. MSO capability with 16 lines is available with most PicoScope families.
- Bandwidth: It is important to be able to ensure that signals are displayed correctly. It is necessary to ensure that the frequency response of the scope does not limit the measurement. Typically the bandwidth will be measured in terms of the -3dB point, i.e. the frequency where the response of the scope falls by 3 dB.
In order to ensure that the oscilloscope specification is adequate it is necessary to ensure that the bandwidth of the scope is higher than the operating frequency. Often a Five Times Rule is used as a rule of thumb. Here the bandwidth of the oscilloscope should be five times the highest frequency component in the signal. Using this rule, the error due to the frequency limitations will be less than ±2%.
- Computer compatibility: When considering the use of any computer based product, it is necessary to consider whether it will work with the computers that are intended to be linked to it. The PicoScope range operates with Windows, Mac iOS, and Linux, making it operate with most computers available.
- Rise time:
Another important oscilloscope specification which needs to be considered when buying a USB oscilloscope such as the PicoScope is its rise time. The oscilloscope must have a sufficiently fast rise time to capture the rapid transitions accurately, otherwise important information may not be displayed properly.
The rise time of the oscilloscope is defined as the time it takes for the image to rise from 10% to 90% of the final value. This is different and in addition to the bandwidth and rather like the slew rate of operational amplifiers where the rate of voltage change is the limiting factor.
The relationship between bandwidth and rise time for a first order estimation can be estimated from using rise time = bandwidth / 0.45. This is a rule of thumb and the value of 0.45 is used for modern digital scopes.
- Sample rate: The sample rate of a digital scope like a PicoScope USB scope is measured in terms of sample per second, S/s. As modern scopes take many more samples to provide greater resolution and detail on the waveform, sample rates are much more likely to be measured in terms of MS/s or GS/s.
While the maximum sample rate tends to be the headline rate, the minimum sample rate may also be important. This is occurs when looking at slowly changing signals over longer periods of time. It is also important to note that the displayed sample rate changes with changes made to the horizontal scale control. This is to maintain a constant number of waveform points in the displayed waveform display.
For most applications it is necessary to define the minimum number of samples that are needed to accurately represent the waveforms that need to be displayed. To calculate this it is necessary to understand that the scope takes in the waveform from the voltage input and then digitizes it, after which it is processed. For the display it is necessary to construct the waveform and to avoid aliasing, the Nyquist theorem dictates that the sampling frequency should be twice that of the highest frequency components to be displayed. However this makes some assumptions about repetitive waveforms, anomalous events such as glitches and interpolation methods.
In reality it is better to assume that when using sin(x)/x interpolation (a common option). As a result the industry has adopted a rule of thumb that the sample rate should be 2.5 x highest frequency.
- Memory depth: The memory depth is an important specification to consider when looking at buying a PicoScope USB oscilloscope. The larger the memory the more sample that can be stored. In other words if you need to store a longer sample you need more memory. Also if you have a high sample rate then you need more memory. Essentially the memory depth = acquisition time window times the sample rate.
- Size & weight: Although for many laboratory based systems, size and weight may not be too much of an issue, although the fact that a PicoScope does not take up too much room can be an advantage.
PicoScopes being relatively small can easily fit onto a work bench or into a laptop case - some of the 2000 series versions are approximately credit card sized and can easily be carried for field investigations, etc.
Selecting the PicoScope
Once the requirements have been assembled it is possible to look at what meets the needs. Often this may require a bit of iteration of the cost is too high.
Fortunately PicoScopes offer excellent value for money and it is possible to get a high specification oscilloscope for a reasonable cost.
There are several ranges of PicoScopes and within the ranges a host of different models. It is possible to determine something about the scope from its model number.
The basic number has four numerals and possibly a suffix letter. The first character gives the series to which the scope belongs. The second character gives the number of input scope channels. The third and fourth numerals give the scope model within the range and the final letter is generally either A or B which indicates the series within the range or model number.