The high-power lumped parameter attenuator is a common RF and microwave control device, and one of its main functions is to reduce the amplitude of the RF signal. Attenuators are often used in tests and measurements. It cannot be said that the use of such devices is second only to cables and connectors.
Usually, whether the manufacturer is in the factory inspection, when the user receives the goods at the time of acceptance, or when the measurement institute calibrates the attenuator, the vector network analyzer is used for measurement. The main indicators are the input VSWR and attenuation. This is an industry rule that has been in place for many years.
Have you considered such a question: Since the attenuator is operating under high power conditions, and the output of the vector network analyzer is only 0dBm, then whether the measured index under this condition truly reflects the performance of the attenuator? ? Obviously, this is a "blind spot" that RF engineers have "visible" for many years. Not everyone is not willing to further study the high-power characteristics of the attenuator. Based on the author's communication with peers and experience, there are roughly two reasons:
1. It has been used for so many years. Even if the high-power and low-power tests are somewhat inaccurate, it is difficult to perceive that no one will "true" the final test results;
2. Real-time RF high-power testing is a bit cumbersome.
The author conducted some exploratory tests on the high-power characteristics of the attenuator and found some interesting results that were submitted to peers for discussion.
Why study the high power characteristics of attenuators
When a 50W high-power signal passes through an attenuator with an attenuation of 20dB, the signal is attenuated 100 times, leaving 0.5W signal from the output terminal. Where is the 49.5W power energy going? Of course, you will immediately say that the energy of these RF signals is absorbed by the attenuator and converted into heat, which is eventually consumed by the heat sink of the attenuator (Figure 1).
Figure 1, the basic role of the attenuator
Completely correct! At high power conditions, the attenuator's surface temperature gradually increases over time (above 70oC), while the internal temperature is higher (above 200oC). In other words, under high power conditions, the physical environment of the device has changed, so the performance of the device will inevitably change! How much has changed? Will it affect the final test results? This is the topic of this article.
In this paper, the performance of an attenuator under high power conditions is described through experiments.
Test methods and resultsThe test object is a 50W, 6GHz, 30dB fixed attenuator. We used the PM2000LS60 high-power test platform for testing (Figure 2).
Figure 2. High-power characteristic test of attenuator
In Figure 2, the amplifier generates 2GHz, 47dBm (50W) of continuous wave power continuously applied to the measured attenuator, and the input sampling circuit measures the signal input to the measured attenuator by 47dBm (a1) and the signal reflected by the attenuator. B1; After 30 dB attenuation of the measured attenuator, there is still about +17 dBm (b2) detected by the output sampling circuit.
Subtract b1 from a1 to obtain the attenuation of the attenuator under test in the high power state. The ratio of b1 to a1 is the VSWR.
The biggest benefit of using this method is to fully offset the instability of the amplifier output. At the same time, in order to ensure the accuracy of the test results, a high temperature resistant PTFE dielectric cable was used in the system and normalized and calibrated in advance.
When the test was started, the test system showed that the attenuator under test showed a good VSWR performance, and the attenuation was also very stable, but the temperature was constantly rising. When the test took more than three minutes, there was a failure phenomenon - the VS ratio suddenly increased from 1.15 to 8 and the attenuation amount drastically changed from 30.1 dB to 34.3 dB! (image 3)
Figure 3 a) Change in standing wave ratio
Figure 3 b) Changes in attenuation
Figure 3 c) Changes in surface temperature
Figure 3. Variation of attenuator characteristics over time in high power state
The test system observes and records three sets of data at the same time. Obviously, this attenuator fails under the power of 50W!
The more interesting phenomenon is still behind. After the above high-power test, we used the network analyzer to re-test the attenuator and found that it was normal! Its standing wave is about 1.11@2GHz and the attenuation is about 30.1@2GHz (Figure 4).
Again using the high-power test, the phenomenon of Figure 3 again appears.
Figure 4a) Standing wave ratio at low power
Figure 4b), attenuation at low power
Figure 4, network analyzer test results
Analysis of test resultsThe above experimental results show that under the continuous action of high power, the attenuation of the measured attenuator changes significantly and fails, which is obviously closely related to the change of the internal temperature of the attenuator.
However, it returned to normal under low-power conditions. The explanation of the mechanism of this phenomenon still needs to be further analyzed by this attenuator.
ConclusionThere is no suitable standard for evaluating the high-power stability of the lumped-parameter attenuator. The common method is to use the DC replacement method. However, I believe that this method does not match the actual environment of the tested attenuator. The tests described in this article are conducted under real-life conditions and have practical application value.
If this attenuator is used in a test system or a communication system, the impact on the system can be imagined.
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