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Cause analysis of dark room environment less than 6dB limit

Release time:

2022-10-28 00:00

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　　GBT6113.203 requires that the ambient noise level is at least 6dB lower than the required measurement level. What is the reason? The following is the derivation process, please refer to it.

　　1. Derivation process

　　Assuming that the total noise power received at the antenna is P, the total noise power is composed of two parts, one part comes from the radiation noise Pa of the EUT, and the other part comes from the environmental noise Pb, that is to say, the noise we measured is The result of the superposition of these two noises, the relationship is:

　　Formula 1: P = Pa + Pb

　　Since the input impedance R of the antenna receiver is constant at 50Ω, this relationship can be transformed into:

　　Formula 2: U2 = Ua2 + Ub2, where U is the total electric field strength at the receiving antenna, Ua is the electric field strength of the EUT radiation signal, and Ub is the electric field strength of the ambient noise.

　　1. When the ambient noise level is 6 dB lower than the limit value, for the EUT radiated noise signal equal to the limit value, there are:

　　Ua(dB) = Ub(dB) + 6dB

　　From the logarithmic relationship:

　　Ua = 2Ub 或 Ub = Ua/2

　　Bringing it into the voltage relational formula (Formula 2) just now, we can get:

　　U2 = Ua2 + Ua2/4 = 5U12/4

　　→ U2/Ua2=5/4

　　→ U/Ra=1.118

　　Take the logarithm to get:

　　→ U(dB) – Ua(dB) = 0.97dB

　　2. Similarly, when the ambient noise level is 10 dB lower than the limit value, for the EUT radiated noise signal equal to the limit value, there are:

　　Ua(dB) = Ub(dB) + 10dB

　　From the logarithmic relationship:

　　Ua=3.17U2 or Ub=0.315Ua

　　Bringing it into the voltage relational formula (Formula 2) just now, we can get:

　　U2 = Ua2 + Ub2

　　→ U2 = Ua2 + 0.099Ua2 = 1.099Ua2

　　→ U2/Ua2=1.099

　　→ U/Ra=1.0483

　　Take the logarithm to get:

　　→ U(dB) – Ua(dB) = 0.41dB

　　2. Conclusion

　　1. From the above derivation, when the ambient noise level is 6dB lower than the limit value, the error between the actual radiated signal value of EUT and the measured value is 0.97dB; when the ambient noise level is 10dB lower than the limit value, the actual radiated signal of EUT The error between the measured value and the measured value is 0.41dB, that is, the lower the ambient noise level is, the closer the measured comprehensive value is to the actual emission value of the EUT.

　　2. When the ambient noise level is 6dB lower than the limit value, the error between the actual radiated signal value of the EUT and the measured value is 0.97dB, which is less than 1dB, and this error is acceptable.

　　3. It can be seen from the comparison that when the ambient noise level is 6dB and 10dB lower than the limit value, the difference of the introduced error is 0.5dB. The cost of building an electromagnetic environment that meets the 6dB requirement is much lower than the cost of building an electromagnetic environment that meets the 10dB requirement.

　　4. On the whole, meeting the requirement of 6dB is closer to practical application, and its error is within our acceptable range. At the same time, the cost of building an electromagnetic environment that meets the requirements is relatively low.

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