Testing and debugging technology of digital modulation signals

Agilent Technologies' MXA series signal analyzers (Figure 1) combine the functions of traditional swept spectrum analyzers and vector signal analyzers. The MXA series is positioned as a mid-range signal analyzer and currently offers four frequency bands, all starting from 20 Hz to 3.6 GHz, 8.4 GHz, 13.6 GHz, and 26.5 GHz. MXA has extremely high measurement speeds and measurement functions based on various communication standards, making it suitable for production line applications. Of course, this product is also very suitable for general research and development and aviation and military research.

When analyzing digital modulated signals, the demodulation bandwidth of the analyzer is a very important indicator. The standard configuration of MXA can provide up to 10MHz demodulation bandwidth, and with the B25 option, the demodulation bandwidth can be extended to 25MHz. Of course, MXA also has the necessary RF performance of a signal analyzer, including: high level and frequency accuracy, extremely low local noise and a wide dynamic range.

MXA has built-in analysis software for many communication standards, including IEEE 802.16e WiMAX, WCDMA, HSDPA/HSUPA and automatic phase noise measurement, and can perform modulation analysis of digital video DV, WiMAX, WLAN and personal wireless communication PMR systems and components. It also supports popular test applications for Bluetooth, GSM, EDGE, TETRA 1 and 2, TD-SCDMA and ZigBee.

MXA provides broad spectrum analysis capabilities and complete signal analysis capabilities, and switching between the two takes only a few tenths of a second. Analyzing the modulation results of wireless signals is challenging, and operators usually have to face vector signal analysis at the beginning. In fact, it is more effective to start with spectrum analysis than to analyze complex modulation signals at the beginning. Using the MXA signal analyzer, the following three steps can effectively measure digital modulation signals and troubleshoot: perform basic spectrum measurements; continue to perform vector measurements in the frequency domain and time domain; and finally perform digital demodulation and modulation quality analysis to isolate and determine the problem. (Figure 2)

The above method is very suitable for the R&D stage, of course, this method can also be used in the trial production stage of the product. This method helps to find problems in the initial stage of product design.

Traditional spectrum measurements can provide basic signal characteristics such as power, distortion, signal-to-noise ratio, and phase noise. Traditional spectrum measurements can also verify the frequency and amplitude accuracy of frequency conversion operations. MXA uses a fully digital IF resolution filter and fully digital IF processing to provide more accurate and clearer signal differentiation and improve measurement accuracy. (Figure 3)

Most digital modulated signals vary with time, such as the intermittent presence of RF pulses and signal changes caused by equalization training and synchronization sequences. Therefore, measuring the characteristics of signals that vary with time can provide a deeper understanding of the signal itself. Fortunately, MXA can provide three tools for analyzing the time characteristics of signals: traditional spectrum analysis; vector signal analysis combined with Agilent 89601A analysis software; many test settings based on communication standards, which help to quickly measure the characteristics of signals. Using fast Fourier transform (FFT) and digital signal processing technology (DSP), 80601A vector signal analysis software provides a wealth of trigger functions for capturing signals within a specific time. These trigger functions include: positive/negative level pulse triggering with adjustable trigger level, hold time, and adjustable delay; adjustable time gate for selecting a specified portion of the measured signal; adjustable frequency resolution and resolution bandwidth filter shape to optimize level accuracy and resolution of time-to-frequency conversion; time gate function for complex measurements such as spectrum, occupied bandwidth, complementary cumulative distribution function (CCDF) and power spectral density; capture and playback of single measurement results (up to 42M measurement points); adjustable FFT record time, up to 409,600 points, for measuring extremely long RF burst signals in one go.

Basic digital modulation signal analysis parameters include: vector magnitude error (EVM), relative constellation error (RCE), I/Q error, etc. In addition, for digital demodulation, it is also important to correctly detect the symbol rate and determine whether the receiver's symbol clock is locked.

Basic digital demodulation requires the correct setting of the analyzer's frequency and time parameters, otherwise it will not demodulate correctly. Basic digital demodulation analysis generates many parameters for evaluating modulation quality, such as vector amplitude error, relative constellation error, modulation error ratio (MER), and parameters directly related to I/Q errors, such as I/Q offset, orthogonal offset, and amplitude gain imbalance. Frequency and amplitude stability can be evaluated through the various vector error components and basic frequency error parameters.

MXA's built-in communication standard test programs are a good starting point for learning digital modulation analysis. Since all parameters have been set according to the standard, you can reduce the time and errors of parameter setting. Each program can display the most concerned parameters of the test and provide a convenient chart display. It can also support user-defined limit lines and pass/fail test functions. (Figure 4)

Graphically displaying vector error is very useful for troubleshooting. Analyzing the relationship between error and time is a good starting point. The MXA can generate a standard reference signal internally and calculate the vector difference between the reference signal and the actual measured signal. Then the vector error is displayed for each symbol or each time point. The vector error shows the difference between the reference signal and the actual measured signal (with the reference signal removed).

Measuring the vector error over time can reveal issues such as on/off events, pulse interference, automatic gain control (AGC) circuits, and fast fading phenomena. This is particularly useful for analyzing signals such as WiMAX (fixed and mobile standards) where the modulation scheme changes between RF bursts. Since these errors are vectors, they can be converted to the frequency domain using an FFT for analysis of vector spectral errors.

The marker coupling function is an effective way to analyze the same phenomenon (Figure 5) using different measurement methods. The MXA 89601A software allows users to set coupled markers on 4 to 6 different traces, which can be different measurement values ​​(such as frequency domain, time domain or I/Q domain). For example, the peak value of a vector error on a time axis (represented by a marker) can be used to find the point on the corresponding constellation diagram or vector diagram (represented by a coupled marker), thereby determining whether the vector error is caused by symbol conversion or amplitude abnormality (such as amplitude compression).

The final step of measurement and troubleshooting generally involves advanced demodulation analysis, which is used to troubleshoot complex faults, such as WiMAX and OFDMA faults. Advanced demodulation analysis techniques include: demodulating signals within a specified time and frequency range, using adaptive equalization, configurable pilot tracking (mainly for OFDM) and other technologies. MXA's 89601A software supports the above advanced demodulation techniques. And the 89601A software can run on the instrument or on an external computer through the USB, LAN and GPIB interfaces.

Some advanced modulation techniques utilize adaptive equalization to combat multipath and selective fading. For systems that use these techniques, it is usually necessary to measure the signal quality after equalization as well as to analyze the signal before equalization. When analyzing the overall performance of the system, equalization is required; however, when testing and optimizing the "raw performance" of components, equalization is not required. MXA's 89601A software includes general adaptive equalization as well as standard-specific equalization functions such as WiMAX.

MXA plus 89601A vector signal analysis software can be called a "three-in-one" player. The system can provide design and problem analysis with: high-speed spectrum analysis; advanced vector signal analysis; measurement functions based on various communication standards. It can quickly and easily analyze digital modulation signals in contemporary wireless communication systems.