Discussion on the Test Method of Spurious Emissions of Radio Transmitting Equipment

For radio management work, spurious emission of radio transmitting equipment is one of the important reasons for communication interference. In the detection of radio transmitting equipment, spurious emission test is an important and mandatory test item.

The main instrument widely used to measure spurious emission is the swept spectrum analyzer. Therefore, to correctly measure the spurious emission components of the equipment under test, it is necessary to have a deep understanding of the performance and working principle of the swept spectrum analyzer. Referring to the relevant standards of spurious emission test, combined with some experience and experience in actual testing, the test method of spurious emission and some issues that should be paid attention to are proposed.

Definition of spurious emission

According to the description of Article 3.6.9 in the national standard GB/T13622-92 "Radio Management Terminology", spurious emission refers to the emission of one or more frequencies outside the necessary bandwidth, and its emission level can be reduced without affecting the transmission of the corresponding information. It includes harmonic emission, parasitic emission, intermodulation products and frequency conversion products, but excludes out-of-band emission. Out-of-band emission is the emission of one or more frequencies generated in the modulation process that just exceeds the necessary bandwidth, but excludes spurious emission.

Methods of expressing spurious emissions

According to the national standard GB 13421-92 "Limits and measurement methods for spurious emission power levels of radio transmitters", there are two methods of expressing spurious emissions. One is the absolute level expression, which is the average power or peak envelope power of spurious emissions expressed in "mW" or "μW". For example, in the 8.2 section of the test standard YD/T884-1996 for GSM mobile stations, it is stipulated that the spurious limit of the transmitter in the frequency band 100 kHz to 1 GHz in working mode is -36 dBm (equivalent to 0.25 μW). The other expression method is the relative level expression, which is the attenuation of the average power or peak envelope power of spurious emissions relative to the peak envelope power of the transmission expressed in decibels. For example, in the 5.2 section of the test standard GB/T15844.1-1995 for FM wireless telephones, it is stipulated that when the carrier power of the transmitter is greater than or equal to 25W, the spurious RF component of the base station should be less than or equal to 70 dB.

Measurement conditions and requirements for spurious emissions

In general, the AC power supply, DC power supply, environmental conditions, and test load during the test must meet the requirements of Section 5.1.1 of GB 13421-92. In specific experiments, the following important conditions should be met:

(1) Temperature: 15℃～35℃;
(2) Relative humidity: 45%～75%;
(3) Atmospheric pressure: 86 kpa～106 kpa;
(4) Power supply: DC power supply voltage is ±2% of the specified value, AC power supply
voltage is ±2% of the nominal value, and
AC power supply frequency is ±1% of the nominal value;
(5) The test should be carried out in a shielded room.

The test block diagram is shown in Figure 1. If the dynamic range of the spectrum analyzer can meet the requirements of the test standard, the switches are connected to S1 and S,1 respectively, and the signal of the transmitter under test is directly processed by the spectrum analyzer after attenuation. When the dynamic range of the spectrum analyzer does not meet the requirements of the test standard, the switches are connected to S2 and S,2 respectively, and the dynamic range of the test system is improved through the adjustable band-stop filter. However, the frequency response characteristics of the adjustable band-stop filter must be accurately calibrated before the test, otherwise it will introduce a large error to the measurement.

The influence of main test instruments on the test

Currently, the main instrument for measuring spurious emissions is the swept spectrum analyzer. It works based on the principle of superheterodyne scanning tuned receiver. It can analyze stable and periodic signals, provide signal amplitude and frequency information, and is suitable for wide-band fast scanning tests. The block diagram of the swept spectrum analyzer is shown in Figure 2. When we test, we must consider the influence of the device characteristics of the spectrum analyzer on the measurement.

4.1 Resolution bandwidth of IF filter

Based on the scanning working principle of spectrum analyzer, when the input signal is a single-point frequency signal, the signal needs to be mixed with the scanning local oscillator signal. After the swept frequency signal passes through the IF filter (bandpass) and the detector, the output waveform is the frequency response shape of the IF filter. Therefore, the single-point frequency signal is displayed on the spectrum as the frequency response shape of the IF filter. The RBW of the spectrum analyzer is actually the 3 dB bandwidth of the IF filter, so the IF filter is also called the resolution bandwidth filter. If the RBW is set too wide in the test, the spurious signal close to the carrier frequency may be masked. Only when the resolution of the spectrum analyzer is high enough can the characteristics of the signal be correctly reflected on the screen. So how should the RBW be set? According to the experimental results, we know that the RBW setting should satisfy the formula RBW≤2 times the frequency interval/rectangular coefficient of the filter (where the rectangular coefficient of the filter = 60 dB bandwidth of the filter/3 dB bandwidth). [page]

4.2 Smoothing effect of video filter on noise

The video filter performs low-pass filtering on the video signal output by the detector. Reducing the video bandwidth can smooth the noise jitter in the spectrum display, thereby reducing the jitter range of the displayed noise. This is beneficial for the spectrum analyzer to find low-power spurious signals submerged in the noise, and can also improve the repeatability of the measurement. Therefore, when testing, the VBW setting should be minimized as much as possible under the condition permitted by the standard, so as to improve the sensitivity of the spectrum analyzer.

4.3 Internal distortion generated by the mixer

The spectrum analyzer uses superheterodyne to convert the signal, and its mixer circuit will definitely produce nonlinear distortion. Among them, the second-order distortion increases in a square relationship with the rise of the fundamental wave, while the third-order distortion increases in a cubic relationship with the fundamental wave. This means that on the logarithmic scale of the spectrum analyzer, the speed of change of the second-order distortion level is twice that of the fundamental wave, and the speed of change of the third-order distortion level is three times that of the fundamental wave. The amplitude of the high-order distortion signal changes faster than the fundamental wave signal. When testing spurious signals with a spectrum analyzer, the input level of the mixer should be adjusted appropriately to ensure that the various distortions generated inside the instrument are much smaller than the spurious signals generated by the transmitter under test. When it is difficult to meet the test requirements by adjusting the input level of the mixer, consider adding an adjustable band-stop filter to reduce the distortion of the spectrum analyzer and improve the dynamic range of the test system.

4.4 Linkage between the IF amplifier gain and the input attenuator setting

When the spectrum analyzer is working, the IF amplifier gain and the input attenuator setting work in a linked manner. When the input attenuator setting is changed, the input signal display level does not change. For example, when the attenuator setting changes from 10 dB to 20 dB, the IF amplifier gain increases by 10 dB at the same time to compensate for this loss, resulting in the signal amplitude displayed by the instrument remaining unchanged. However, since the internal noise is mainly generated by the first stage of the IF amplifier, the noise of the spectrum analyzer will change. For every 10 dB increase in attenuation of the attenuator, the noise level displayed by the spectrum analyzer increases by 10 dB. Therefore, when testing, the ATT setting should be minimized as much as possible under the conditions permitted by the standard to improve the sensitivity of the spectrum analyzer. In addition, this feature of the spectrum analyzer can also help us judge the authenticity of the test results of the spectrum analyzer. When we change the input attenuator setting, if the display result of the spectrum analyzer changes, it means that there are distortion components generated inside the instrument in the test result of spurious emission.

Measurement uncertainty assessment of spurious emission

Measurement uncertainty is the degree of suspicion about the measurement result expressed by the standard deviation or its multiples. It is a scientific method to characterize the dispersion of the measured value. Uncertainty includes two assessment methods: Class A and Class B. We call the standard deviation obtained by statistical methods the Class A assessment method of uncertainty, and the standard deviation obtained by non-statistical methods the Class B assessment method of uncertainty. The specific method for analyzing and evaluating the measurement uncertainty of spurious emission is as follows.

① After connecting the measurement system as shown in Figure 1, make n independent measurements of the measured transmitting equipment to obtain n sets of independent measurement data.

② Use the Bessel formula to obtain the Class A component of the standard uncertainty (uA)

③ Estimate the Class B component of standard uncertainty (see Table 1):

By strictly following these five steps in radio management measurement work and paying attention to related matters, you can successfully complete the measurement, analysis and evaluation of the uncertainty of spurious emissions and ensure good communication quality.