1965 年，戈登.摩尔(Gordon Moore) 预测，集成电路(IC) 中晶体管的密度将会每两年翻一番。该预测被称为“摩尔定律”，至今仍然非常强大。现如今，计算和模数转换技术已经出现爆炸性增长。例如我们已经看到，由于 IC 密度的增加，现场可编程门阵列(FPGA) 和中央处理器(CPU) 的性能一直在呈指数增长。

随着摩尔定律影响着处理器的性能，IC 晶体管密度的增加也极大地提高了模数转换器(ADC) 和数模转换器的性能。如下图所示，摩尔定律对这些设备的性能产生了指数效应。模数转换器（ADC）位数是最广为人知的技术指标之一。

随着数字信号处理技术和数字电路工作速度的提高，以及对于系统灵敏度等要求的不断提高，对于高速、高精度的ADC（Analog to Digital Converter）、DAC（Digital to Analog Converter）的指标都提出了很高的要求。比如在雷达和卫星通信中，所需要的信号带宽已经达到了 2 GHz 以上，而下一代的 5G 移动通信技术在使用毫米波频段时也可能会用到 2 GHz 以上的信号带宽。虽然有些场合（比如线性调频雷达）可能采用频段拼接的方式去实现高的带宽，但是毕竟拼接的方式比较复杂，而且对于通信或其它复杂调制信号的传输也有很多限制。

是德科技今天为您介绍模数转换器(ADC) 在信号分析仪，示波器, 高速数字化仪, 任意波形发生器（AWG） 以及矢量信号发生器（信号源）中的应用。

Let see how the converter in different instruments like spectrum analyzers, oscilloscopes, digitizers and ARBs affects the overall system performance. Understanding these elemental concepts allows us to choose the best instrument for the job and leverage its potential to the fullest.

A signal or spectrum analyzer is a frequency domain instrument that translates the signal down to an intermediate frequency (IF) which can be easily digitized and processed by DSP. The IF is at a much lower frequency (typically between 200 and 300 MHz) compared to the input, thus the ADCs used in spectrum analyzers have relatively lower sample rates (from 100 to 400 MHz) and higher bit resolutions (from 14-bits to 16-bits). The advantage of this architecture is it provides us with better dynamic range than an oscilloscope or digitizer.

Essentially, the signal flow proceeds with the input waveform being first conditioned for optimum performance, then down-converted and afterwards sampled by the ADC. The samples are then processed by the analyzer’s DSP chip. Apart from detection algorithms to display the signal on the screen, the DSP will also include the Hilbert transform to produce complex IQ sample pairs that can be used for vector signal analysis and digital demodulation1 .

Since the ADC is used in the IF section of the analyzer, its operation is often transparent to the user. Signal analyzer datasheets may provide the bit resolution of the ADC, but typically not the ENOB. The more important parameters to consider are the analysis bandwidth and dynamic performance characteristics.

Dynamic performance of a spectrum analyzer includes several characteristics, such as the displayed average noise level, second and third order distortion performance, and the 1 dB compression level. The analysis bandwidth represents the bandwidth of a waveform that can be instantaneously digitized by the analyzer. These parameters are influenced by the ADC but not exclusively – they also depend on the overall architecture of the digital IF section and other system components.

Figure 1. Signal analyzers use attenuators and amplifiers to condition the input signal then down-convert the signal to an IF. The ADC is implemented in the IF section. Digitizing the signal at the IF, we can use ADC’s with high bit resolution (better dynamic range) and lower sample rate. Interleaved ADC architectures are used for a higher combined sample rate.

An oscilloscope （ The discussion here pertains to digitizing oscilloscopes. Purely analog oscilloscopes do not contain an ADC and cannot store or record a signal.） is a time-domain instrument and as such does not down-convert the waveform to an IF, although attenuators, amplifiers and other components are used to condition the input signal. Scopes are designed to support wide bandwidths starting at DC, and hence need to use much faster ADC designs and clock rates (compared to spectrum analyzers) to get the desired bandwidth. To achieve these speeds, higher sample rates must be accommodated leading to lower bit resolution and less dynamic range. Scope ADCs typically have bit resolutions from 8 to 10-bits and sample rates from 1 to 160 GHz.

Oscilloscope datasheets will usually provide specifications for bit resolution and ENOB, as well as sample rate and bandwidth. The bit resolution number typically refers to the ADC, while the ENOB refers to the performance of the scope as a whole. The bandwidth specification refers to the 3 dB compression point of the frequency response of the scope. ADC architectures can get quite complex, and as a result the sample rate may not have a direct relationship with the bandwidth. A data sheet may mention the maximum sample rate supported based on the number of input channels.

Figure 2. Oscilloscopes use attenuators and amplifiers to condition the input signal, similar to signal analyzers, but do not down-convert the signal to IF. Since the ADC is used in the main RF signal path, ADC’s with lower bit resolution are needed to achieve faster sample rates. To help maintain overall system dynamic performance, with lower ADC bit resolution, special ADC interleaving architecture is used along with digital signal processing.

While many instruments have an internal ADC or DAC, a digitizer is fundamentally an ADC with memory, DSP capability, input connectors for the waveform, output connectors for timing and triggering, and an interface bus. Digitizers are most frequently implemented in modular form factor (such as the PXIe and AXIe standards1 ), resulting in a smaller footprint making it easy to scale existing setups. They can be combined with input waveform conditioning modules, such as attenuators, amplifiers and downconverters. They can have multiple inputs, and multiple digitizers can be synchronized together to expand the number of input channels. This simplicity and flexibility makes them ideal for custom setups.

The characteristics of the digitizer are closely tied to that of the ADC inside it, given that it is a direct implementation of the ADC. Digitizer datasheets will typically mention the ENOB and SFDR directly, as well as the sample rate and bandwidth. The ENOB and SFDR may be provided at specific frequencies. The bandwidth is usually provided as the 3 dB compression point of the frequency response of the front end. The sample rate is variable, and hence the bandwidth would be provided for specific values of the sample rate (usually the maximum supported sample rate under specific setup conditions). Digitizers come in a wide range of bit resolutions (8 to 16-bit) and sample rates (2 MHz to 8 GHz).

A capability that may be available in higher performance digitizers is digital down conversion (DDC). This implements real-time digital decimation and filtering on the digitized data, which effectively downconverts the data. This allows us to bring a specific signal of interest to baseband, and leads to other advantages like improved dynamic range and faster measurement speed.

Figure 3. Digitizers use ADC’s directly at the input often without input signal conditioning. Not a lot of onboard DSP is provided, but tools for programming the custom FPGAs may be provided. The purpose is to capture the signal and post-process the samples for custom applications.

是德科技高速数字化仪的核心是强大的 FPGA，它能够实时处理数据，并在数字化仪级别上执行数据精简和存储。

This white paper provides advantages and disadvantages of using oscilloscopes or wideband digitizers for wideband signal applications.

An arbitrary waveform generator, or arb, essentially implements a DAC with on board DSP, large memory, and multiple outputs. The DAC sample clock may be used directly or indirectly to regulate address counter logic, depending on the specific architecture being used.

Arbs are some of the most versatile instruments on the market today for generating signals. High speed and high performance arbs can provide a lot of flexibility in regard to output configurations, sample rate settings, and DAC configurations including changes to the DAC bit resolution and interleaving of the DAC outputs. Because of this, they have the capability to generate a wide variety of signals used across many applications from wireless communications to advanced physics research.

Similar to digitizers and scopes, the main characteristics used to describe arbs are the bit resolution, dynamic range, ENOB and SFDR. Recall that the SFDR number may or may not include harmonics, depending on prevailing industry practice. The analog bandwidth and sample rate are also provided in arb data sheets. Arbs can have sample rates as high as 92 GHz, and bit resolutions from 8 to 14-bits. xAn important consideration for the SFDR parameter is the band covered by the specification. For arbs, the band is usually the first Nyquist region or the analog bandwidth, whichever is lower. Depending on the application, this may not be the band of interest either. If oversampling is being used, any spur beyond the base signal bandwidth will not be relevant as it can be eliminated through filtering.

Arbs may also provide a feature called digital upconversion (DUC)1 . This uses interpolation and filtering to upconvert the signal to a higher frequency and is typically implemented in DSP prior to the DAC. The advantage is being able to produce a wideband signal at an RF frequency without the need for RF hardware.

Figure 4. Arbitrary waveform generators come in many different architectures. This is a simplified block diagram of what you would find in a high-speed arb. It includes an adjustable sample clock, onboard memory, and DSP to process the samples prior to the DAC. Arbs may have multiple output channels, which may be configurable as differential or single-ended.

A vector signal generator (VSG) uses an IQ modulator inserted into the RF path to modulate inphase (I) and quadrature (Q) signals onto an RF carrier. The I and Q signals can be provided by an external arb, but most modern VSGs will include an internal baseband generator, which contains two DACs to supply the I and Q signals. A high performance VSG incorporating high resolution DACs provides excellent signal fidelity and serves as an ideal solution for component and receiver test applications.

Since the higher frequency carrier signal is provided by the generator, the DACs only need to provide the lower frequency baseband components of the signal. This allows us to use lower DAC sample rates, thus enabling the use of high bit resolution DAC’s. DACs used in VSGs can have bit resolutions 2 to 4 bits higher than what might be found in converters used in other types of instruments.

The higher bit resolution helps maintain the overall signal fidelity by providing DAC signals with high SFDR and ENOB performance. Additionally, the carrier frequency of the signal is only limited by the frequency range of the VSG. The VSG will automatically set the sample clock rate, but the user may be given some control over this. A setting for runtime scaling is also often provided. Runtime scaling will optimize the samples so that the DACs are used at or near full scale as much as possible.

Typically, datasheets for VSG’s will provide the DAC sample rate and bit resolution of the baseband generator, however, as in the case of spectrum analyzers, the system performance specifications are more useful for evaluating and selecting a VSG. The more vital specifications include RF modulation bandwidth1 , error vector magnitude (EVM) performance and adjacent channel power ratio (ACPR) performance.

The RF modulation bandwidth indicates the maximum bandwidth of a signal that can be generated by the instrument. EVM performance is an indicator of modulation quality and indicates how much distortion is introduced by the overall system. ACPR performance measures the ACPR for specific kinds of signals and relates to the dynamic range performance of the instrument.

Figure 5. Vector signal generators use DAC’s to generate analog I and Q components at baseband, which are then modulated onto the RF carrier. This lets us use DAC’s with lower sample rates to maintain high bit resolution for best dynamic range of the baseband signals.

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