We generally use continuous wave (CW) SignalTo describe the high-speed analog-to-digital converter (ADC) And digital-to-analog converter (DAC).The reasons for this are: 1) As far as ADC is concerned, CW signals are easier to generate noiselessly by CW generators and narrow bandpass filters; 2) As far as DACs are concerned, CW signals are easier to analyze; 3) They have many standard references Tests are available in variousDeviceMake a clear comparison between. However, most real systems use high-speed data converters for sampling and modulating waveforms. Bridging the differences between the various specifications based on CW measurements and the system requirements of modulated signals presents certain challenges.

There are two differences between the CW signal and the modulated signal, which affect the behavior of high-speed data converters. First, the CW signal has no bandwidth—the energy is limited to a single frequency; while the modulated signal has a bandwidth, and the energy is distributed in a certain frequency range. One of the results is that the distortion of the CW signal causes CW harmonics at another frequency, and the distortion of the modulation signal causes harmonics and cross-modulation in a wider frequency range outside the signal: second harmonic 2x, third harmonic 3x, etc. The propagation of energy in a certain frequency band with the same bandwidth as the modulated signal brings lower integrity distortion energy.

Secondly, most modulated signals (only modulated signals such as GMSK used in GSMplanExcept) are all modulating the amplitude, and its generation ratio is the largestpowerWant low average power. For the convenience of comparison, the power of the CW signal is constant. Figure 1 shows the difference, which shows the power versus time of the modulated long-term evolution (LTE) signal. The average power is about 7% of the maximum power, which is 11 dB lower than the maximum power.

Figure 1 The relationship between power and time of modulated LTE signal

In most devices, the harmonic distortion results increase as the signal power increases. For example, for every 1dB increase in signal power, the third-order harmonic result increases by 3dB. Therefore, the maximum power CW signal has more obvious distortion than the lower average power modulation signal. Figure 2 depicts this situation, which compares the third-order harmonic distortion of the maximum power CW signal with the modulated LTE signal. The distortion model used is a simple polynomial:

Among them, the harmonic distortion coefficient coeff is chosen arbitrarily, which aims to explain the huge amount of distortion.

The CW signal generates a third-order distortion result of 42 dB below the CW signal, and the LTE signal generates a third-order distortion result of 56 dB below the LTE signal. Please note that the power shown in Figure 2 has been standardized to the maximum power of each signal.

Figure 2 Harmonic distortion CW and modulated LTE signal

Therefore, using the maximum power CW signal to estimate the harmonic distortion of the modulated LTE signal in our theoretical device will estimate the LTE signal distortion by 14 dB.

What is a more accurate CW test? A CW test can never capture exactly the same modulation signal effect, and the modulation signal distortion depends on the statistical distribution of signal power. In our example, a CW signal with a maximum power of –7 dB below the maximum power will produce the same level of third harmonic distortion as the LTE signal (see Figure 2). Since the average power of the modulated LTE signal is ~11 dB below the maximum power or peak power, this is equivalent to setting the CW signal power to 4 dB above the average power of the modulated signal.

Modulated signalperformanceTo be more preciseevaluateA quick rule is to use the dB ratio of the peak power of the modulation signal to the average power, and then set the CW power to 2/3 below the maximum power. For example, if the modulated signal PAR is 6dB, the CW signal should be set to -4 dB below the maximum power, and then the harmonic distortion relative to the signal power should be measured. This rule is effective for various modulation signal types (for example: OFDM, WCDMA, QAM, etc.).

Next time, we will discuss more details, so stay tuned.

references

For more details about data converters, please visit: www.ti.com.cn/dataconverters.

authorIntroduction

Robert Keller is currently the manager of high-speed data converter systems and applications.he iswirelessHigh-speed products for infrastructure communications, test and measurement, and military systemsTechnologyHe has 9 years of work experience in support. He graduated from Washington University in St. Louis with a bachelor’s degree in physics and mathematics, and later graduated from Stanford University with a doctorate in applied physics.He has 10 itemsThe internetandsensorApply US patent.