High-resolution oscilloscopes

PicoScope 4262

PicoScope 4262

Introduction

Since 1991, Pico Technology has played a pioneering role in the development of PC oscilloscopes. These compact devices, along with traditional full-sized digital storage oscilloscopes (DSOs), account for most of the oscilloscopes sold today. The driving force behind all of these instruments has been digital electronics. As clock speeds in digital circuits increase, manufacturers have responded by designing scopes with faster sampling rates and higher bandwidths. Unfortunately, analog designers have been left behind: the quest for higher speed has been at the expense of dynamic performance and resolution. To give analog designers the tools they need, we designed the PicoScope high-resolution and flexible-resolution oscilloscopes.

What is a ‘precision’ DSO?

The precision of an oscilloscope is determined by its resolution and its accuracy. Here are the characteristics of some of the resolutions available:

Oscilloscope resolution Number of steps Smallest change that can be detected Ideal dynamic range
6 bits 64 1.6% (16,000 ppm) 36 dB
8 bits 256 0.39% (4,000 ppm) 48 dB
12 bits 4,096 0.024% (244 ppm) 72 dB
16 bits 65,536 0.0015% (15 ppm) 96 dB

Accuracy is not regarded as important for most oscilloscopes. You can make measurements within a few per cent (most DSOs quote 3% to 5% DC accuracy) but for accurate measurements you have to reach for a multimeter. With precision oscilloscopes, however, accurate measurements are possible at full speed. Most handheld meters have 12-bit resolution (equivalent to 3½ digits), while a 16-bit oscilloscope is equivalent in resolution to a 4½-digit benchtop meter.

As well as resolution and accuracy, noise is an issue. The amplifiers that make up the front end of a conventional DSO are designed to have a high bandwidth, but low noise is not a priority. The designer of a 16-bit oscilloscope has a tougher job, as only 0.0015% of noise is enough to mask the least-significant bit.

Pico Technology has over 20 years’ experience in building high-resolution oscilloscopes. Unlike some competing designs that are just 8-bit scopes with a replacement ADC, PicoScopes are designed from the ground up to achieve low noise and low distortion. The extra resolution on one of our 12-bit or 16-bit scopes comes with significantly improved DC accuracy, dynamic range and signal-to-noise ratio.

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.

High-resolution oscilloscopes

Figure 1

High-resolution oscilloscopes

Figure 2

Spectrum plot of 250 Hz peak with SFDR of about 68 dB

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.

High-resolution oscilloscopes

Figure 4

High-resolution oscilloscopes

Figure 5

High-resolution oscilloscopes

Figure 6

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.

High-resolution oscilloscopes

Figure 7

High-resolution oscilloscopes

Figure 8

High-resolution oscilloscopes

Figure 9

Spectrum analyzer settings

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

  • Frequency range: 0 to 1 kHz
  • Spectrum bins: ≥ 8k
  • Display mode: Average
  • Window function: Blackman–Harris

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.

High-resolution oscilloscopes

Figure 10

High-resolution oscilloscopes

Figure 11

High-resolution oscilloscopes

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.

High-resolution oscilloscopes

Figure 13

High-resolution oscilloscopes

Figure 14

High-resolution oscilloscopes

Figure 15

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.

High-resolution oscilloscopes

Figure 16

High-resolution oscilloscopes

Figure 17

High-resolution oscilloscopes

Figure 18

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.

Testimonials

  • I have been using my 4224 PicoScope for years. I travel abroad so this has been ideal due to its physical size. Storage of waveforms on my Laptop is very easy allowing me to quickly email waveforms to my Colleagues.

    Andrew
  • Not many USB scopes works on Win & Mac & Linux too, so that proves me that guys from Pico really cares about us, customers. This made my decision much easier when I was looking to buy an USB scope.

    Raul Trifan
  • We have been using Picoscope 6404D for quite some time, and are amazed by its accuracy and powerful emulations while working with numerous signal evaluations.

    J Mohanty
  • PicoLog TC-08: This is a very nice unit that works consistently and reliably.

    Jeff Hulett
  • It is a great scope. I had a weird problem - it did not work on one of my PC’s. Customer service gave me first class service. If I could give 6 stars for customer service - I would do so.

    Niels Larsen
  • Perfect Partner for Development of Encoder controlled Stepper Motor Actuators. Since the included software is really stable, this type of device is a great tool for great tasks!

    Helmut Schoettner
  • A superb piece of equipment worth its weight in gold

    Nigel Clinch
  • So simple to use & beats any other I have ever used hands down.

    John D Samsing
  • Great functionality in a compact size. I really like moving the mouse pointer to a position and having the Time and Voltage display the values at that point. Calibration equipment is a breeze with that feature.

    Don Horein

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