High-resolution oscilloscopes

Test 1: Low-cost signal generator

The pictures below show the time-domain and frequency-domain displays from typical 8-bit, 12-bit and 16-bit PicoScope oscilloscopes. The signal source is an Android smartphone running the FuncGen app, set to generate a 250 Hz sine wave with its maximum amplitude of about 170 mV.

8-bit oscilloscope example

An 8-bit oscilloscope such as the PicoScope 2205 gives a good enough visual representation of the wave, as shown in Figure 1. The frequency and amplitude of the wave can be measured with reasonable accuracy. Zooming in by 64x (Figure 2), however, shows up the limitations of 8-bit resolution.

Figure 3 shows a spectrum analyzer plot (FFT) of the signal. The peak at 250 Hz is the fundamental frequency of the input signal. The SFDR, shown as the delta between the rulers, is about 68 dB. The noise floor masks the true characteristics of the input signal.

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Figure 3

12-bit oscilloscope example

The same signal captured with a PicoScope 4423 12-bit oscilloscope looks the same in the normal scope view. The x64 view now shows no digitization steps, but with 12-bit resolution we can see noise that was invisible with the 8-bit scope. The spectrum analyzer shows the SFDR to be about 72 dB, and distortion peaks at the second (500 Hz) and third (750 Hz) harmonics are just visible.

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16-bit oscilloscope example

With a 16-bit PicoScope 4262 oscilloscope, the x64 trace is cleaner, although of course the noise due to the signal source is still visible. (The inset picture in figure 8 shows the effect of a 10 kHz digital filter applied by PicoScope.)

The spectrum view shows the same harmonic spurs and SFDR as obtained with the 12-bit scope, indicating that the distortion is due to the signal source and not the scope.

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Test 2: Low-distortion signal generator

In this test we have replaced the low-cost signal generator with the PicoScope 4262’s built-in low-distortion signal generator. This allows us to show the benefits of the very low distortion in the PicoScope 4262’s front end. The generator was set to produce a 10 kHz sine wave with 990 mV amplitude.

8-bit oscilloscope example

As in the previous test, the 8-bit scope is adequate for viewing the overall shape of the waveform (figure 10) but shows its limitations when the display is zoomed in 64 times (figure 11).

The FFT spectrum analyzer plot (figure 12) shows the 10 kHz fundamental. If there are any other components in the signal, they are masked by the noise floor about 70 dB below the peak.

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Figure 11

Figure 12

12-bit oscilloscope example

The same signal captured with a PicoScope 4423 12-bit oscilloscope looks the same in the normal scope view. The x64 view looks much cleaner, with very little noise and the digitization steps only just visible. In the spectrum view, the noise floor is now low enough to show some harmonics and other spurious signals reaching to about 76 dB below the 10 kHz peak. At this stage we do not know whether these are due to the scope or the signal generator.

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16-bit oscilloscope example

With a 16-bit PicoScope 4262 oscilloscope, the x64 trace is smooth and noise-free, with no sign of distortion caused by digitization. The spectrum trace shows an SFDR of about 96 dB, with a much lower noise floor than that of the 12-bit scope. The distortion peaks are 20 dB lower than those seen by the 12-bit scope, indicating that most of the distortion seen in the previous test was due to the limitations of the 12-bit scope and not the signal generator.

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Spectrum analyzer settings

The spectrum analyzer views above were created using, as closely as possible, the following settings:

• Frequency range: 0 to 50 kHz
• Spectrum bins: ≥ 8k
• Display mode: Average
• Window function: Blackman–Harris

A suitable number of spectrum bins was chosen to place the FFT noise floor below the signals of interest for each oscilloscope.