# Things to consider when choosing a Basic Oscilloscope

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Basic oscilloscopes are used as windows into signals for troubleshooting circuits or checking signal quality. They generally go together with bandwidths from around fifty megahertz to two hundred megahertz and are found in nearly every design laboratory, education lab, service centre and manufacturing environment.

### The digital storage oscilloscope

Oscilloscopes are an essential tool for anyone designing, producing or repairing electronic equipment.

A digital storage oscilloscope (DSO, on which this article concentrates) acquires and stores waveforms. The waveforms show a signal’s voltage and frequency, whether the signal is distorted, the timing between signals, how much of a signal is noise, and much, much more.

### Bandwidth

System bandwidth determines an oscilloscope’s ability to measure an analogue signal. Specifically, it determines the maximum frequency that the instrument can accurately measure. Bandwidth is also a key determining factor in price.

Determine what you need – use the ‘five times rule’

For example, a 100 MHz oscilloscope is usually guaranteed to have less than 30% attenuation at 100 MHz. To ensure better than 2% amplitude accuracy, inputs should be lower than 20 MHz.

For digital signals, measuring rise and fall time is key. Bandwidth, along with sample rate, determines the smallest rise-time that an oscilloscope can measure.

The probe and oscilloscope form a measurement system that has an overall bandwidth. Using a low-bandwidth probe will lower the overall bandwidth so be sure to use probes that are matched to the scope.

### Sample Rate

The sample rate of an oscilloscope is comparable to the frame rate of a motion-picture camera. It determines how much waveform detail the scope will capture.

Determine what you need – use the ‘five times rule’

Sample rate (samples per second, S/s) is how often a scope samples the signal. Again, we tend to advocate a ‘five times rule’: use a sample rate of a minimum of 5x your circuit’s highest frequency component.

Most basic scopes have a (maximum) sample rate of 1 to 2 GS/s. Remember, basic scopes have bandwidth up to 200 MHz, so scope designers usually build in 5 to 10 times oversampling at maximum bandwidth.

The quicker you sample, the less information you’ll lose and the better the scope will represent the signal under test. But the faster you will fill up your memory, too, which limits the time you can capture.

### Channel Density

Digital oscilloscopes sample analogue channels to store and display them. In general, the more channels the better, although adding channels adds to the price.

Determine what you need

Whether to select 2 or 4 analogue channels depends on your application. Two channels let you compare a component’s input to its output, for example. Four analogue channels let you compare more signals and provides more flexibility to combine channels mathematically (multiplying to get power, or subtracting for differential signals, for example)

A Mixed-Signal scope adds digital timing channels, which indicate high or low states and can be displayed together as a bus waveform. Whatever you select, all channels should have good range, linearity, gain accuracy, flatness and resistance to static discharge.

Some instruments share the sampling system between channels to save money. But beware: the number of channels you switch on will scale back the sample rate.

### Compatible Probes

Good measurements begin at the probe tip. The scope and probe work together as a system, so be sure to consider probes when selecting an oscilloscope. During measurements probes actually become a part of the circuit, introducing resistive, capacitive, and inductive loading that alters the measurement. To minimise the effect it’s best to use probes that are designed for use with your scope. Select passive probes that have sufficient bandwidth. The probe’s bandwidth should match that of the oscilloscope.

A broad range of compatible probes will allow you to use your scope in more applications. Check to see what’s available for the scope before you buy.

Use the right probe for the job

Passive probes

Passive probes Probes with 10X attenuation present a controlled impedance and capacitance to your circuit and are suitable for most ground-referenced measurements. They are included with most oscilloscopes – you’ll need one for each input channel.

High-voltage differential probes

High-voltage differential probes allow a ground-referenced oscilloscope to take safe, accurate floating and differential measurements. Every lab should have at least one!

Logic probes

Logic probes deliver digital signals to the front end of a Mixed Signal Oscilloscope. They include “flying leads” with accessories designed to connect to small test points on a circuit board.

Current Probes

Adding a current probe enables the scope to measure current, of course, but it also enables it to calculate and display instantaneous power.

### Triggering

Determine what you need

All oscilloscopes offer edge triggering, and most offer pulse width triggering.

To acquire anomalies and make the best use of the scope’s record length, look for a scope that offers advanced triggering on more challenging signals.

The wider the variety of trigger choices obtainable the more versatile the scope (and the faster you get to the root reason for a problem!):

• Digital/pulse triggers: pulse width, runt pulse, rise/fall time, setup-and-hold
• Logic triggering
• Serial data triggers: embedded system designs use both serial (I2C, SPI,CAN/LIN…) and parallel buses.
• Video triggering

Record Length

Record length is the total number of points during a complete waveform record. A scope can store only a restricted number of samples therefore, in general, the greater the record length the better.

Determine what you need

Time captured = record length/sample rate. So, with a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will capture 4 ms. Today’s scopes permit you to pick the record length to optimize the level of detail required for your application.

A good basic scope can store over 2,000 points, which is quite enough for a stable sine-wave signal (needing maybe five hundred points). But to find the causes of timing anomalies in a complex digital data stream consider 1 Mpoints or more.

Zoom & Pan allows you to zoom in on an event of interest, and pan the area backwards and forwards in time. Search & Mark lets you search through the entire acquisition and automatically mark every occurrence of a user-specified event.

Oscilloscopes with record lengths in the millions of points can show many screens worth of signal activity, essential for examining complex waveforms.

Tektronix TBS2000 Range of Oscilloscopes

Tektronix Basic Oscilloscope Range

Tektronix Probing Solutions

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