Tektronix Technology: Low-noise design solution for MSO6 oscilloscope

A common method for reducing display signal noise.

Noise is a common problem that is ubiquitous. Almost everyone who works with circuits has to deal with noise at some point, either by finding the source of the noise and fixing it, or by reducing the effect of noise on measurements. Noise comes from a variety of places, both internal to the design and external to it, and it can overwhelm test signals. You may be having trouble measuring low voltage (mV) signals, such as in radar transmissions or heart monitors, where noise can make it difficult to find the actual voltage of the signal. Noise can increase jitter, making it difficult to perform accurate analysis. Therefore, to accurately measure low voltage signals, not only must the oscilloscope itself have low noise, but also simple and accurate measurement methods are required.

The low noise design of oscilloscopes has always been sought after, not only because of the importance of low noise, but also because of the difficulty of its technical challenges. There are many factors to consider in the design of oscilloscopes. Only by striving for excellence in all aspects can the product be made like a fine work of art. Tektronix oscilloscopes provide corresponding functions and tools to help you deal with noise. This article is divided into two parts. This article will disclose the details of the low noise design of Tektronix oscilloscopes for the first time, as well as the commonly used measurement tools in oscilloscopes.

Low noise design

1. Design of Low Noise Preamplifier

As we all know, the analog front end of the oscilloscope, including the attenuation circuit, buffer circuit and amplifier circuit, will introduce noise. This is also an important source of the oscilloscope's background noise. The design of the analog front end is usually used as an important indicator to evaluate the performance of the oscilloscope's noise. The Tektronix MSO6 uses a newly designed front-end amplifier, the Tek061, which achieves very good noise performance at a smaller volt/division setting. Therefore, when amplifying small signals to observe details, the excellent performance of the Tek061 makes us stand out.

2. Design of ADC analog-to-digital conversion

In the digital oscilloscopes currently in use, the quantization noise of the ADC is another source of oscilloscope noise. MSO6 uses the world's fastest new 12-bit ADC converter, with an internal operating rate of 25GS/s, and a sampling rate per channel that is 25% higher than previous similar oscilloscopes. 12 bits achieve 4096 vertical analog-to-digital conversion levels, and the resolution is 16 times higher than that of oscilloscopes using 8-bit ADCs. Each ADC channel is based on an interleaved continuous approach register (SAR) structure, and the performance of the TEK049 platform based on this ADC can achieve a total throughput of up to 100 GS/s. In this way, a large amount of back-end noise in the channel is dispersed over a larger bandwidth, and a suitable filter is used to filter out the noise to achieve accurate measurement.

3. Selection of crystal oscillator

The oven-controlled crystal oscillator used in the MSO6 implements a first-class phase-locked loop ASIC with very low noise in the clock path. The time base accuracy used is ±20ppb (at calibration, 25°C ambient temperature, on any ≥1 ms interval). The generated clock accuracy is very high and the phase noise is very low. Such low phase noise enables our customers to make more accurate time measurements.

4. Design of signal pathway

Differential path designs are less susceptible to crosstalk from other internal sources. Signals typically enter the oscilloscope as single-ended. All single-ended signals are routed under the RF shield using coplanar waveguides to isolate them from other external noise sources. Once the signal passes through the preamplifier, it quickly becomes a differential signal before it can leave the shielded area. Grounded coplanar waveguides are a common path selection technique in RF design. Choose a path, then put ground planes on both sides of the path and stitch the vias from the board to the ground, which prevents the signal from being coupled from any other external noise sources.

Also, the MSO6 pays a lot of attention to the internal shielding, especially the front end, because that's really the focal point. So the Tek061 front end amplifier has a shield covering all around, which helps both the noise performance and the channel isolation, which is where this generation is better than the previous generation. In fact, our channel-to-channel isolation is better than anything else from our competitors. Start with the low noise Tek061 preamplifier, and then prevent all the other noise sources inside. We developed this RF shielding and paid special attention to the crosstalk from other channels, and other noise sources inside the instrument, such as acquisition memory, can be very noisy, so we have to shield it from the acquisition memory. The power supply is as far away as possible, making sure the noise of the power supply is very low.

5. Power supply design

A clean power supply is a prerequisite for the stable operation of digital circuits. The ripple and noise of the power supply will affect the quality of the power supply. MSO6 pays special attention to the design of low-noise power supplies, conducts a lot of analysis on the power supply signal including spectrum analysis, and conducts detailed frequency domain analysis on different stray signals, and then strives to solve these problems.

Several common methods to reduce noise on displayed signals

First, you need to stabilize the signal. Once you have a stable trigger, you can further adjust the noise display on the oscilloscope. There are many tools to accomplish this: bandwidth limit filters, average acquisition mode, HiRes acquisition mode.

1. Bandwidth limiting filter

The bandwidth limit filter reduces the bandwidth of the oscilloscope to a selected frequency. That is, frequencies above the selected frequency are attenuated or completely removed from the trigger path and the acquisition and display paths. Bandwidth limit filters can be used not only to maintain a stable trigger, but also to reduce the amount of noise displayed on the oscilloscope. Using a bandwidth limit filter is one of the simplest ways to reduce noise in an oscilloscope, and is particularly suitable if all unwanted noise frequencies are above a fixed cutoff frequency. However, it will also remove high-speed glitches that may occur.

2. Average acquisition mode

Average acquisition mode takes several complete acquisitions and averages them point by point to get the average voltage at each time sample in the acquisition. The user can adjust the averaging options, including the number of acquisitions. Noise is generally random during the acquisition process, sometimes rising and sometimes falling. When these random variations are averaged over a sufficient number of acquisitions, they will cancel out and produce a stable signal on the screen. Of course, in order to take advantage of the average acquisition mode, your waveform must be repetitive; non-repetitive waveforms or single-shot events cannot be averaged.

Average acquisition mode reduces all types of uncorrelated signals and random noise, even at very low frequencies. In addition, it works at all oscilloscope time/division settings. Because multiple waveforms must be acquired to create one averaged waveform, the display may update slowly when the input signal changes or the front panel knob changes, which means that occasional glitches may be missed. However, compared to bandwidth-limiting filters, average acquisition mode works better in some applications because the full bandwidth of the oscilloscope can be used to capture high-frequency repetitive events.

3. HiRes acquisition mode

Tektronix oscilloscopes include the HiRes acquisition mode, which is similar to the Average acquisition mode in that it uses averaging to eliminate noise. HiRes performs a rectangular average on each acquisition, averaging multiple adjacent samples within a waveform to produce a single averaged sample. This reduces high frequency noise because the averaging cancels out the high speed changes in voltage caused by noise. It also reduces the sample rate because it converts multiple samples into a single sample. Therefore, the HiRes acquisition mode is only suitable for slower time/div settings where the oscilloscope still has enough sample rate to measure the signal being measured.

Unlike Average acquisition mode, HiRes acquisition mode can be used on both non-repeating and single-shot waveforms. In addition, since only one waveform needs to be acquired, HiRes acquisition mode updates the display much faster after an input or front panel setting change. Combining multiple samples that are adjacent in time also reduces the chance of aliasing at lower time/div settings.

4. DSP filter

In addition, oscilloscopes offer post-processing DSP filters to remove certain frequencies of noise from the signal. You have full control over the filter frequency. Although these filters can be flexible, they are often slow and only suitable for single-shot or slow update rate displays. They can filter out important glitches or anomalies of interest without you knowing it.

About Tektronix

Headquartered in Beaverton, Oregon, USA, Tektronix is ​​committed to providing innovative, accurate and easy-to-use test, measurement and monitoring solutions to solve various problems, release insights and promote innovation. For more than 70 years, Tektronix has been at the forefront of the digital age.