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Influence of DUT impedance of spectrum analyzer and EMI measurement receiver on test value

The default unit of the spectrum analyzer is dBm, which is the power level value; EMI measurement receiver default unit dB μ 5. Is the voltage level. Why use different units of measurement? In addition to the different units, how do the two instruments convert the measured values of the same signal? If the same units are set on the instrument, can the readings be consistent? What are the reasons for these questions?

Analysis of measured values of spectrometer and EMI receiver

The power spectrum refers to the power of the signal on each frequency component. The spectrum is actually an amplitude spectrum, which is the voltage amplitude of the signal on each component. So the power is the square of the voltage divided by the resistance value.

The "spectrum" of the spectrometer is actually "power spectrum", and the default power unit is dBm; According to various EMC standards, EMI receiver is required to measure the voltage value at each frequency point, and the default voltage unit is dB μ V。


Spectrum analyzer

Both the spectrometer and EMI measurement receiver adopt superheterodyne architecture, that is, the input signal is attenuated/amplified/mixed/filtered/detected/digitally processed to obtain the measured value, where the detected voltage value is obtained. Therefore, the quantity value obtained from the initial measurement of the two instruments corresponds to the voltage value.

The default impedance of the spectrometer and receiver is 50 Ω. According to the initial voltage value and 50 Ω impedance, other measurement units can be converted. For example, after simple conversion, 0dBm=107dB μ V。 At present, the commonly used spectrum analyzer and receiver can switch to display the test unit and automatically convert to display the correct results. However, it is necessary to pay attention to the different bandwidth and definition methods of the two instruments.

In the field of signal measurement, the definition of "width" means that the measurement point is at the peak linear amplitude of - 50%, regardless of pulse width or bandwidth. The RBW resolution bandwidth of the spectrometer is - 3dB, because the measured power value of the spectrometer is 10lg (0.5)=- 3dB; The IF bandwidth of EMI receiver is - 6dB, because it is the measured voltage value, 20lg (0.5)=- 6dB. In addition, in the CISPR standard, the - 6dB IF bandwidth of EMI receiver needs to be 200Hz/9kHz/120kHz/1MHz according to the frequency.

When the input signal is CW continuous wave, the measured values of the two instruments are consistent under the same unit; If it is a broadband signal input, the measured values of the two are different due to different bandwidth definition methods; If the bandwidth and definition method of the two instruments are set in the same way, the measured values of the two instruments are the same when any input signal is not accompanied by a large signal of other frequencies.


EMI measurement receiver

When the input signal is accompanied by large signals of other frequencies, the spectrometer may be overloaded, while the EMI measurement receiver preselector can filter large signals of other frequencies to maintain high sensitivity while avoiding overload. In addition to the above advantages, the preselector has the disadvantage of additional insertion loss and additional uncertainty caused by filter unevenness.

Influence of DUT impedance on test value

The default instrument impedance of the spectrometer and EMI measurement receiver is 50 Ω. When measuring signals, the default DUT port impedance of the equipment under test is also 50 Ω. When the impedance of the DUT is not 50 Ω and the deviation is large, the reading value only represents the voltage of the input signal on the internal resistance of the 50 Ω instrument. If you want to obtain the output level value of the DUT, you need to calculate it through the partial voltage coefficient when the DUT output impedance is known.

How to calculate the signal power from the I/Q data of the spectrometer

After reading I/Q data by remote control spectrometer, novices often make mistakes: they think that the signal power level P (dBm)=10 · lg (I ^ 2+Q ^ 2), and find that the calculation result is wrong, which is about 13dB different from the actual signal level. Why?

The reason is that the read I/Q data unit is V and the impedance is 50 Ω. The correct power calculation formula should be:


This calculation method is applicable to receiver-related data processing in all RF measurement fields, including various spectrum analyzers, EMI measurement receivers, as well as receiver data processing in instruments such as vector network analyzers and integrated measurement instruments, including logarithmic processing of FFT spectrum analysis.

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