In 2006, APD statistical parameters measuring technique was approved as a new standard electromagnetic disturbance measurement method in CISPR16-1 Ed.2. Characteristics of APD measuring instruments, as well as APD measurement method have been clearly defined, but the limit of APD is still under consideration. Limits of APD become a major bottleneck for APD employing. Thus, the main purpose of this paper is to present a practical method to set down the limits of APD.First of all, I gave the theoretical principle of the APD technology, reviewed the historical development and current situation of APD, simulated APD measuring function, deduced and obtained the characteristic of APD for typical random signals, as well as the characteristic of APD has been experimental measured for Hair drier and Bluetooth systems. Secondly, through the theoretical analysis of the relationship between the APD of interference and system's BER, the paper concluded that characteristic of APD is a good parameter to evaluate the performance of communication system. Then actual measurement of the Bluetooth and PHS systems has been done to verify the theoretical derivation. Thirdly, based on the constellation diagram, I present a new method to establish a relationship between the APD and BER. The greatest probability of error may occur, when received vector cross the threshold lines, which is defined as half of the distance between closest constellation points. The APD is defined as the ratio of time when the amplitude exceeds certain amplitude. When this certain amplitude is equal to half of min Euclidian Distance, the relationship between the two can be established with a clear physical meaning and strict mathematical description. Then, based on the method presented above, I simulated the constellation maps of MPSK, QAM, MPAM, MSK systems, and obtained the APD limit for base-band system. Finally, took WCDMA system for an example, show how to define the APD limit for spread spectrum communication systems. Some research has been done about using APD to evaluate the performance of communication system, such as the same interference on the impact of different communication systems and different interference on the impact of same communications system.In conclusion, my paper presented a method to get the APD limits on digital base-band modulation communication systems and described how to get an APD limit for spread spectrum modulation communication system.
