Radio frequency attenuator is a very important instrument used in microwave technology. In many microwave systems, such as radar, multi-channel communication systems and other power transmission loss and received signal measurement, RF attenuators are inseparable. The sensitivity and measurement accuracy of RF attenuators are closely related to the smooth production of microwave equipment. The development of the components, test systems and test technology of the company is inseparable from the radio frequency attenuator. The measurement of the radio frequency attenuator is closely related to the establishment of microwave power, the determination of noise standards and the accuracy of attenuation correction.
Brief introduction of RF attenuator
RF power attenuators are widely used in electronic instrument measurement, electromagnetic compatibility testing, and attenuation inside measuring instruments, etc. Their performance directly affects the accuracy of the test and the accuracy of the measuring instrument. Therefore, designing a precise and excellent RF power attenuator is of great significance to radio measurement.
Circuit form of radio frequency attenuator
RF power attenuators generally use resistive elements, and there are two main circuit forms: π-type and T-type
For RF power attenuators, there is generally no need for impedance conversion, that is: the input impedance and output impedance are equal, and the circuit is symmetrical, so whether it is a π-type circuit or a T-type circuit, R2 and R3 have the same resistance value.
RF attenuator briefly describes the basic requirements
The RF power attenuator must follow the following two basic principles:
1. Impedance matching
The RF power attenuator must have impedance matching to accurately attenuate the signal power, otherwise it will form a standing wave or reflection, which will affect the measurement accuracy. For the attenuator input, the input impedance must match the output impedance of the signal source; for the attenuator output, the output impedance must match the load impedance. Since the RF power attenuator generally does not need to perform impedance transformation, the input impedance, output impedance, load impedance and signal source output impedance are all equal.
2. The attenuation meets the requirements
Voltage attenuation 201gAt=20lg (Vin/Vout) (dB) or power attenuation 101gAtp=10lg (Pin/Pout) (dB) meets the required attenuation value, because the power P=V²/R, since the input impedance is equal to the output impedance, so 10lgAtp=10lg(Pin/Pout)=l0lg(Vin²/Vout²)=20lg(in/Vout)=20lgAt, so the decibels of voltage attenuation (dB) are consistent with the decibels of power attenuation (dB). According to the decibel of attenuation (dB), the multiple At of voltage attenuation can be obtained.
Comparison of π-type attenuator and T-type attenuator
From the above calculation, it can be seen that when the decibel of attenuation is large, R1 will be very small in the T-type attenuator. Due to the influence of the lead and solder joints, the resistance is too small to ensure its accuracy, which affects the accuracy of the attenuation. degree. For example: when the input and output impedance is 50Ω and the attenuation is 60dB, R≈50.1Ω and R1≈0.1Ω in the T-type attenuator, while R≈49.9Ω and R1≈25kΩ in the T-type attenuator. Therefore, use it when a larger attenuation is required π-type attenuator is more suitable, general attenuation, π-type attenuator and T-type attenuator are suitable.
to sum up
The attenuator can be formed into a cascade connection, and the attenuation amount can be selected through a switch or a relay to meet different attenuation needs. The RF power attenuator must be shielded, and non-inductive resistance must be selected to ensure that the frequency response meets the requirements. In order to withstand the corresponding radio frequency power, the resistance power of the attenuator must have a certain margin.
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