Information

BANDWIDTH
System bandwidth determines an oscilloscope’s fundamental ability to
measure an analog signal - the maximum frequency range that it can
accurately measure.
What you need
 Entry level scopes will often have a maximum bandwidth of 100 MHz. They can
accurately (within 2%) show the amplitudes of sine-wave signals up to 20 MHz.
 For digital signals, oscilloscopes must capture the fundamental, third and fifth
harmonics or the display will lose key features. So, the bandwidth of the scope
together with the probe should similarly be at least 5x the maximum signal
bandwidth for better than ±2% measurement error – the ‘five times rule’.
This is also needed for accurate amplitude measurements.
 High-speed digital, serial communications, video and other complex signals
can therefore require scope bandwidths of 500 MHz or more.
Bandwidth is defined as the frequency at which a sine-wave
input signal is attenuated to 70.7% of its true amplitude (the
-3 dB or ‘half-power’ point, shown here for a 1 GHz scope).
100
85
70.7
0.1 0.5
Frequency (GHz)
30% Amplitude degration!
Amplitude error (%)
1.0
-3 dB
~2% Amplitude degration
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12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE
CONTENTS
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