Precautions for using microwave signal analyzer for power measurement

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Precautions for using microwave signal analyzer for power measurement

Microwave signal analyzer is one of the important electronic measuring instruments, the signal power measurement is its important function. In recent years, the signal power measurement accuracy of the microwave signal analyzer is getting higher and higher. However, most instrument operators, especially those for primary use, often cause large measurement errors and even wrong results even with high-performance signal analyzers due to easy-to-overlook detail problems. This article summarizes the top five considerations for power measurement using signal analysis instruments.

 

Pay attention to test cable selection and use

Signal through the test cable from the input to the output, there will be some path attenuation, the amount of attenuation depends on the test cable type, length and frequency of the work signal. The higher the operating frequency, the longer the cable. The greater the attenuation, the attenuation of cables up to 8dB when operating to 50GHz for low loss coaxial cables with length of 1.5m will be greater for other common coaxial cables. Therefore, before the signal measurement, you must use the power meter to test the cable for compensation calibration, the compensation value is saved in the signal analyzer, the use of signal analyzer amplitude correction function, to eliminate the impact of cable loss. For high-frequency small signal measurement, we must choose a good low-loss shielding coaxial cable.

The test cables should be very cautious when making connections. The actual bending radius of the cables when connecting two instruments through a cable can not be less than the static bending radius and dynamic bending radius required by the cable specifications because a small bending radius will make the performance of the connecting cable Parameters change, especially caused by changes in the characteristic impedance parameters, so that the standing wave of the signal path worsened, affecting the accuracy of measuring power. Therefore, the use of cables in the process, as far as possible so that the entire cable retains a larger bending radius, for the hardness of semi-steel cables, after use to try to retain the last use of the shape, do not deliberately to break or maintain a straight State, in order to reduce the cable in the process of repeated bending, the cable performance parameters change, thereby affecting the accuracy of the measurement signal. It can be said that the reasonable choice of test cables and the use of accurate measurement of power to achieve the key.

 

Pay attention to the choice and use of adapter

The test cable is connected to the test instrument via a changeover connector. The quality of the connection can also affect the accuracy of the test signal power. Before making a measurement connection, first check and clean the connector to ensure that the connector is clean and undamaged, especially for the 2.4mm and 1.85mm connectors. During the connection must ensure that the tightness of the cable connector to be moderate, neither loose too loose cause bad contact, loss is too large, can not be too tight screw, easy to damage the connector and adapter. Correct operation is as follows: First align the axis of the two interconnected devices to ensure that the male connector plugs move concentrically into the female connector socket. Then move the two connectors straight together so that they engage smoothly. Rotate the sleeve of the connector (be careful not to rotate the connector itself) until it is tightened. There is no relative rotational movement between the connectors during the connection. Finally use the torque wrench to tighten the connection, pay attention to the torque wrench do not exceed the initial turning point, you can use an auxiliary wrench to prevent the connector rotation, and remember not to be too hard to damage the connector.

 

Attention to reserve a reasonable warm-up time

Before turning on the power to signal measurement, you must reserve a certain warm-up time for the whole machine. This time usually takes more than 10 minutes. During the preheating process, the whole machine keeps the full-bandwidth continuous scanning status. Because the temperature from the boot to the normal working temperature, the performance parameters of the whole microwave components, crystal oscillators and device parameters will have some changes, these parameters will affect the accuracy of the whole measurement. Therefore, to maintain the whole span of the machine before measurement, enough preheating time must be reserved to ensure that the whole microwave components and device parameters no longer change in order to achieve the accuracy of the measurement. In particular, the power of the high-frequency signal is accurate measuring.

 

Reasonable choice of AC and DC switching

Signal Analyzer RF Paths By default, it is important to note that the attenuator includes an AC coupling and DC coupling switch. By default, the attenuator is selected for AC (AC) ) Coupling state, the main is to prevent the input signal contains DC components to break the mixer. However, for RF input signals less than 10MHz and below, direct attenuation through AC coupling will result in large attenuation. Therefore, to achieve accurate measurement of the power of signals under 10 MHz and below, the attenuator needs to be artificially switched to the direct current (DC) coupling state. At the same time, in order to effectively prevent the DC component of the RF signal from entering the mixer, it is necessary to make sure that the input RF signal does not contain the DC component before conducting the test.

 

Attention to the instrument on a regular basis measurement and calibration

When the test instrument is used for a long time (generally refers to more than one year), the performance parameters of the whole device tend to change a certain amount. The factory commissioning data and the actual tuning frequency point will have a certain offset, especially the whole machine Path filter some offset, thus affecting the accuracy of signal power measurement. Therefore, to ensure the high precision of signal analysis instruments, customers need to regularly measure the instrument (usually one year), and measure the factory parameters caused by the aging of the instrument.

 

In summary, the signal analyzer can be used to make the correct power measurement as long as the test worker pays attention to the above precautions. At present, the accuracy of power measurement of signal analysis instruments is in the range of ± 0.5dB ~ ± 3dB. If users need more accurate power measurement, they need to select other specialized power test instruments, such as power meters and measurement receivers.

 
Senior Electrical Engineer
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