Summary of troubleshooting methods for network analyzers

The microwave vector network analyzer is mainly composed of four major parts: synthetic sweep source (excitation source), test device (signal separation part), receiving part, and microprocessor. The schematic block diagram is shown in Figure 1. Its basic working principle is: first divide the signal of the excitation source into two channels, one channel is used as the reference signal R, the other channel is attenuated and sent to the test port as the excitation source of the network under test, and is taken out through the directional coupler, and passes through the network under test. The reflected signal A and the transmitted signal B are used as test signals, and then the sampling frequency conversion method is used to linearly transfer the amplitude and phase information contained in the two microwave signals to the intermediate frequency or low frequency to measure the amplitude and phase relationship. The frequency conversion method is also It is beneficial to realize continuous and step frequency sweep measurements in a very wide frequency band to show how various parameters of the network under test change with frequency. Let's take HP8720C of HP company as an example to analyze the working principle and related characteristics of each part.

As the usage rate of network analyzers increases, the instruments will inevitably malfunction. The maintenance of such instruments brings new challenges to our maintenance personnel. This requires us to have a wide range of professional and technical knowledge and strong logical thinking ability. We must first understand the schematic block diagram of the instrument, analyze the signal flow, and initially locate the instrument fault through the basic operation of the instrument. Now based on the maintenance of many such instruments, Antai network analyzer maintenance experts summarize the classification and repair of common faults of this type of instruments.

(1) Soft faults caused by misoperation. Most vector network analyzers have multi-menu display functions. Sometimes misoperation can cause instrument dysfunction. As long as the menus are used correctly, especially the maintenance menu, the soft faults of the instrument can be eliminated.

(2) An error message appears after the power-on self-test. The instrument self-test mainly checks several core components. The sequence is generally CPU-ROM-RAM-I/O interface-each controlled group key. After checking, the The error message can give you a rough idea of ​​where the fault occurred.

(3) The power-on self-test is normal but the instrument has a hard fault. The location of the fault cannot be clear at a glance. It is often necessary to perform a functional check on the instrument and then infer the fault location based on the schematic block diagram. The following is a brief analysis of the inspection ideas and troubleshooting methods for vector network analyzer failures through maintenance examples. Example 1: After powering on, the instrument passed the self-test. The measured reflection (used channel A/R) works normally, but the measured transmission (used channel B/R) does not work normally. The signal is about 30dBm low and the curve is uneven. According to the analysis of the schematic block diagram, by If the reflection measurement is normal, it can be seen that all the common components of the instrument should be normal, then the fault must be in the directional coupler of port 2, the B-channel receiver, the sampler, the second mixer, and the multi-way switch of the ADC circuit. There are many devices involved in the fault, and the structure of the instrument is relatively compact, so it is not feasible to check one by one. Through research, it is found that the first intermediate frequency is easier to test: set the instrument in continuous wave working state, frequency 1GHz, power level 10dBm, this At this time, the first IF should output a sine wave with a frequency of 10MHz and a peak-to-peak value of 0.15V. However, we measured the signal with an oscilloscope and found that the signal was much lower. Therefore, we can judge that the fault is in the sampler or the front coupler. After further measurement, we found that If the output signal of the directional coupler is normal, it can be judged that the sampler is damaged and needs to be replaced.

Example 2: The instrument works normally after being turned on, but an error message appears after one hour or more of operation. Use a power meter to measure at port 1. The output power level is the maximum and the power level cannot be adjusted. This fault may occur in the automatic level control circuit or the power module. Because the circuit is closed-loop, it is difficult to locate the fault. However, according to the status of the fault, it can be judged that the performance of a certain component is not good or there is a virtual solder joint in the circuit, and it is caused by excessive temperature. Here we use local heating. This method can quickly locate the fault, that is, use a temperature-controllable hot air blower to heat a certain component separately, and then quickly cool it to determine the location of the fault.

Example 3: After turning on the computer, a phase lock error occurs on the screen. There are many circuits involved in the phase-locked circuit. As can be seen from the block diagram, it consists of a signal source part, a signal separation part, an R-channel sampling part and its related circuits. If this error message appears, phase calibration must be performed first. If the calibration fails, it can be judged that the instrument has a hardware failure; then check the cables related to the phase-locked loop. Poor contact of the connecting cables may also cause such failures. Check the reference signal, fundamental wave generator and pulse generator. If they all have normal signal output, the possibility of failure of their related circuits can be ruled out. Open the loop, add an analog input signal to the phase detector, check its output, and no problem is found. However, if the R channel sampler in the phase locked loop is disconnected, a phase lock error will occur, so its impact on the fault cannot be measured. , but because the A, B, and R channels are exactly the same and can be used interchangeably, the replacement method is used to find out that the R channel sampler is damaged.

In the future, Yanet will have faster measurement speeds, stronger software and test functions, and more complex and precise circuit structures. Only by mastering the basic principles and flexibly applying various microwave instrument maintenance methods can we quickly and accurately locate faults. , to achieve twice the result with half the effort.