Bandwidth and analog front-end determine the performance indicators and core design of the oscilloscope

Although the oscilloscope is not the instrument with the highest hardware requirements, given that the oscilloscope is the single instrument most familiar to many engineers and the largest in the test instrument market segment, we interviewed several representatives from global manufacturers that can achieve GHz-level oscilloscope bandwidth. Let's start with the hardware analysis of the oscilloscope and take everyone to understand the core design unit of the oscilloscope.

Bandwidth, sampling rate and storage depth are the three most intuitive characteristics that determine the market value of an oscilloscope. Bandwidth is the most obvious indicator that reflects the performance of an oscilloscope. There is a certain relationship between the value of sampling rate and bandwidth, and these two The numerical value is directly related to the final selling price of the oscilloscope, and its numerical value is basically determined by the hardware performance of the analog unit.

The architecture of oscilloscopes has experienced decades of development, especially the rapid development of digital oscilloscopes in the past two decades, and has basically stabilized. Xing Fei, vice president of RIGOL, introduced that the basic components of contemporary digital oscilloscopes mainly include : Analog front-end (responsible for signal conditioning)-》Analog-to-digital converter (signal digitization after conditioning)-》Data acquisition/storage/signal processing-》Display and human-machine interface. The first two parts determine most of the performance indicators of the oscilloscope and are also the core of the oscilloscope.

Bandwidth and Analog Front End

Bandwidth is the most basic parameter when choosing an oscilloscope. From the advent of 30GHz in 2004 to the emergence of 45GHz, it took 5 years to wait, but it only took 3 years for the 60GHz oscilloscope to appear, and in the last 12 months , the top three oscilloscopes have updated their top-of-the-line oscilloscopes. Everything seems to be a remake of the oscilloscope performance arms race from 2002 to 2004.

LeCroy has held the title of best digital oscilloscope bandwidth performance for many years since 2004, which is occupied by Agilent. Agilent's Infiniium 90000Q has a maximum bandwidth of 63GHz, exceeding the 60GHz of LeCroy's LabMaster10Zi. When the two main competitors have launched 60GHz oscilloscopes, we are looking forward to how Tektronix Technology will respond in the next step.

The key to determining the bandwidth is the analog front-end of the oscilloscope, including attenuators, amplifiers and related circuits. It is the door for the measured signal to enter the oscilloscope. In many cases, the test signal bandwidth of the oscilloscope is determined by the bandwidth of the analog front-end, which directly affects Determine the noise floor and range of the oscilloscope. The design work of the analog front end actually accounts for more than half of the workload in the hardware design of the oscilloscope, and ultimately determines the hardware performance of the oscilloscope to a large extent.

For the analog front end, the main performance indicators that affect the oscilloscope include:

● Analog bandwidth, including the amplitude-frequency response characteristics of the measured signal, which is expressed in the time domain as rise time indicators and overshoot performance indicators;

● Input signal amplitude dynamic range (the range from the minimum vertical sensitivity to the maximum vertical sensitivity of non-digital processing);

● The initial error characteristics and temperature drift characteristics of the two indicators of DC gain accuracy and offset accuracy;

● Input impedance characteristics (resistance in parallel with parasitic capacitance) affect the circuit under test with or without a probe.

If we give a vivid metaphor to the important position of the oscilloscope's analog front-end design in the entire hardware design, the role of the analog front-end is similar to the lens of a camera. Many photography enthusiasts use SLR cameras. A very important reason is that the lenses of SLR cameras have better optical properties. In a similar way, when the analog front end performs attenuation, amplification and signal conditioning on the input signal, system noise will also be amplified. If the analog front-end design of the oscilloscope is poor and the system noise is high, the tiny signals you want to test will not be captured; if observed in the frequency domain, these noises will reduce the signal-to-noise ratio and increase the noise floor. If the isolation between signal paths is not enough, signals from other channels will cause greater interference to the signal being measured. At the same time, the linearity and anti-saturation capabilities of the analog front-end are also very important.

In the analog front-end design process of digital oscilloscopes, the amplifier is one of the core components of the analog front-end design. Xing Fei introduced that through the selection and design of special amplifier devices, the amplifiers in RIGOL products can not only ensure the high-bandwidth characteristics of the oscilloscope, but also Ensure the high DC gain accuracy characteristics of the oscilloscope. For mixed-signal products, the amplifier design of the digital channel has special technical points that are different from the amplifier design of the analog channel. In addition to maintaining high bandwidth characteristics similar to those of the analog channel, it also has higher requirements for the amplitude-frequency response characteristics during the signal conditioning process. , to reduce time domain overshoot and avoid digital channel acquisition and actual errors of the actual measured signal.

As a dark horse that has entered the oscilloscope market in recent years, Rohde & Schwarz (R&S) China oscilloscope business development manager Jiao Baochun analyzed that in the analog front-end design of the company's oscilloscope products, a large number of R&S's expertise in the field of radio frequency testing are used. Mature technology that integrates RF design into analog design. The most intuitive benefit of doing so is that the noise in the signal path is greatly reduced. Under the lowest signal range condition (1mV/div), the oscilloscope can still maintain extremely low noise levels, and its effective signal noise value is less than 1/4 of similar products. High isolation between channels, especially for high-frequency signals, is also an example of RF design applications.

Of course, in high-end oscilloscopes, bandwidth is not only realized in the analog front-end, but can also be achieved through other digital methods. Generally, there are three methods for high-end oscilloscope bandwidth: one is to realize it directly with the preamplifier circuit; the other is to use DSP to pull the bandwidth. Extend the bandwidth; the third is digital bandwidth multiplexing. Tektronix Technology believes that each of the three methods has its own advantages. Currently on the market, direct implementation of preamplifier and DSP stretching bandwidth technology are used more frequently. In terms of use, the bandwidth implemented by hardware uses less digital technology, has higher signal fidelity, is more flexible in use, has fewer restrictions, has a flatter frequency response and noise spectrum, and supports equivalent sampling and undersampling. The signal can be allowed to extend beyond the screen, etc., but the cost is relatively high; in comparison, digital technology may cause fluctuations in frequency response or noise spectrum, and the effective bits are low at certain frequencies. At the same time, digital technology requires real-time sampling and does not support under-current sampling. During sampling, waveform distortion will occur when the signal exceeds the screen. There are relatively many restrictions and the requirements for users are also high. However, the bandwidth achieved by digital technology is relatively low-priced due to low hardware costs. It also provides users with a cheap solution at the expense of some performance. Overall, preamplifier technology and DSP provide different options for different customer needs.

Sampling rate and analog-to-digital converter

Among the three major performance indicators of an oscilloscope, the analog front end determines the bandwidth, and the analog-to-digital converter (ADC) is the most important part that affects the sampling rate. The ADC is the core device of the digital oscilloscope, and the most critical indicators are the sampling rate and The number of valid ADC bits. The sampling rate of the ADC directly determines the digital bandwidth of the oscilloscope, that is, how high-frequency signals can be effectively collected and displayed.

Jiao Baochun believes that the sampling rate of A/D converters cannot be increased without limit. R&S has the leading single-core 10Gs/s sampling rate A/D converter in the current oscilloscope market. In order to achieve a higher sampling rate, many companies use interleaved sampling technology, which uses multiple low-speed A/Ds to be combined in parallel into a high-speed multi-core A/D. The problem with this technique is the phase error of the signal. To correct this error, most oscilloscope manufacturers use DSP correction technology. However, DSP correction processing takes time, and this correction reduces the waveform capture rate of the oscilloscope.

The sampling of the oscilloscope also includes the accuracy of the sampling rate, which is the effective number of conversion bits (ENOB). The A/D converters of common oscilloscopes are 8-bit. But in actual use, the number of conversion digits that can really play a role cannot reach 8 bits. Some oscilloscopes even degrade to around 4 bits at high bandwidth. This means that users cannot use these oscilloscopes to accurately measure the amplitude information of signals. Jiao Baochun introduced that the effective conversion digits of the R&S A/D converter can be as high as 7 or more.