Oscilloscope bandwidth
If you’re interested in oscilloscopes you will have noticed one of the main specifications that is given for them is oscilloscope bandwidth. So you’ll probably guess that this is quite important when it comes to using one.
So what is oscilloscope bandwidth?
Oscilloscope band width is probably best understood using the rough comparison of an audio amplifiers bandwidth. As the hearing range of humans is anywhere between 20 Hz to 20 Khz the best audio amplifiers work between those frequencies. If your audio amplifier only goes up to 15 KHz then any sound higher than this wont get through.
With an oscilloscope displaying waveforms its similar. If you have a 10 MHz oscilloscope you wont be able to see waveforms above this frequency properly. Its also worth noting that a wave form can have overtones that are higher than the one you are hoping to view.
So without wanting to get into to a complex explanation there is a rough rule of thumb that your oscilloscopes bandwidth should be 5 times the frequency you want to see. Once the measured signal level drops off by 3 decibels, that’s where the bandwidth measurement is taken from. So for a 20 MHz bandwidth oscilloscope, the level can’t drop below 3 decibels right up to 20 MHz.
Why is oscilloscope bandwidth important?
Oscilloscope bandwidth is important because if you have you have an oscilloscope with an insufficient bandwidth you wont be able to see what your hoping for. The higher the bandwidth of an oscilloscope the better but as you might have already guessed it means a higher bandwidth oscilloscope is more expensive then its lower bandwidth counterpart.
Oscilloscope bandwidth for automotive use
For automotive use its common to see 10 MHz bandwidth oscilloscopes. There’s not much point in getting any higher a bandwidth. Some automotive oscilloscopes even go as low as 5 MHZ. One of the most important things to look for in an automotive oscilloscope is portability. You won’t be bringing you car inside to do tests. You can find quite a few hand held models made specifically for cars
Oscilloscope bandwidth for audio
Using the 5 times rule you’ll find it difficult to find a proper oscilloscope that wont be good enough for audio use. When it comes to electronics even high audio frequencies of 20 KHz are very low. Compare that to the frequencies used in other types of electronics that an oscilloscope caters for. Even a 10 MHz oscilloscope would do. If you only want to use an oscilloscope for the audio range you can make an oscilloscope using a few components and a laptop or PC.
Check out my article “Build a soundcard scope”.
Oscilloscope bandwidth and sample rate
Sample rate is something that is only applicable to digital oscilloscopes. The same as with audio digital samplers. When digital sound samplers became available in the 80’s memory was really expensive and sample rate for audio was a constant compromise between audio quality and memory. The lower the sample rate, the lower the memory but the lower the audio quality.
Sample rate is a specification of the digital oscilloscope and similarly the higher the sampling rate the better. In oscilloscope specifications sample rate is usually given as mega samples per second. With a digital oscilloscope the sample rate is usually shared between the channels. If you choose to use one channel of a dual channel oscilloscope you should get twice the sample rate than if you used two.
Oscilloscope memory depth
When a digital oscilloscope stores its image, it is stored in its buffer memory. Depending on the size of the buffer memory determines how long it can capture the signals for. More memory is obviously better and a higher sample rate means more memory will be used.
Oscilloscope specification examples
A 50 MHz bandwidth oscilloscope will enable you to properly see a 10 MHz waveform.
When it comes to sample rate, to see the 10 Mhz waveform you would need a sample rate of 50 mega samples per second. This is using the guide of 5 points per waveform.
Final thoughts on oscilloscope bandwidth.
If you’re are looking at a digital oscilloscope then sample frequency and memory also go along with bandwidth.
If you have an analogue oscilloscope bandwidth will probably be more of an issue because of the age of the oscilloscope. Some older analogue oscilloscopes that were manufactured a long time ago are still being used today. Although it shows how well made they were to have lasted you may find it has quite a limited bandwidth.
If your looking to buy a new digital oscilloscope check out my article
“is an oscilloscope worth buying”. It contains some more information, some tips and what to look for when buying.
If you are looking to get a new digital oscilloscope check out my buying guide “Best hobbyist oscilloscope”. You don’t have to worry too much about minimum specifications. In the price range between £250 – £400 the lowest bandwidth is 100 MHz with a sample rate of 1 gig samples per second.
There are a few bargain digital oscilloscopes at around the £100 price mark. You may want to check the bandwidth and specifications of those in a little more detail if you are considering buying one of those models.