Digital Frequency Characteristic Tester Based On NIOS â…¡

Frequency characteristic tester also know as frequency sweeper, is used to measure the frequency characteristic of network(circuit) under test, which usually used to measure the frequency characteristic of filter, amplifier, high-frequency tuner, diplexer and antenna. It is one of the common electronic instrument in laboratory. A high-precision medium digital frequency characteristic tester is designed by technologies of system on a programmable chip and Direct Digital Synthesis with20kHz～120MHz sweep range, less than0.5dB amplitude frequency detection error, less than1°phase frequency detection error. It can accurately judge whether the phase lead or lag. The principle and realization of frequency characteristic detection were studied in this paper analyzes, focusing on the reasons and solutions of frequency characteristic detection circuit error, such as the problem of analog channel noise and DC offset, the issue of large detection error near0°and180°and poor judgment of lead and lag. Through two closed-loop control method, the automatic gain control circuit and the closed loop phase detection method to improve the amplitude-frequency and phase-frequency detection accuracy, and system self-calibration to eliminate the error caused by the inconsistency of the two way signal analog channel frequency response and DC offset of the detection chip. Test result indicate that the designed frequency characteristic tester can accurately measure the phase-frequency and amplitude-frequency characteristic of the network under test at the frequency band of20kHz to120MH, especially solve the problem of large detection error near0°and180°and poor judgment of lead and lag of common phase detector.To improve frequency characteristic detection accuracy for the main purpose,the following main work are done in this paper:1) In view of the problem of large detection error near0°and180°and poor judgment of lead and lag of domestic digital sweeper,a closed loop phase detection method based on two-way signal source with arbitrary phase difference is proposed and applied in the phase detection circuit, which solve the problem of large detection error near0°and180°of the phase detector and realize right judgment of phase lead and lag.2) A two-way signal source with arbitrary phase difference is designed. According to the issue of the poor quality of high frequency output signal, some optimizations and improvement are done in such aspects as clock, layout and signal conditioning which makes the signal source be able to output signal with the highest160MHz and good quality when it works at400MHz.3) Aiming at the trouble of great influence on the quality of the frequency sweeping signal and detection signal caused by the noise from voltage and ground, when amplifier is working at high gain, front-end amplifying circuit use the differential circuit form to turn the noise from ground and voltage into common mode signal, and implements the effectively restraint of common-mode noise from voltage and ground by the High common mode rejection ratio characteristic of difference amplifier.4) A new intermediate frequency wide band peak detection circuit designed by high-speed comparator, high-speed digital-to-analog converter and digital logic unit is proposed and applied in prime automatic gain control circuit of the detection module.