The signal generator is one of the important equipment for electromagnetic compatibility testing

Signal generator is one of the main equipment for electromagnetic compatibility test. Typical applications lead to standard signal correction and reception links as well as EMS continuous wave and differential signal as interference sources. The EMS test standard continues to evolve in the direction of simulating the real electromagnetic environment, and now also considers signal modes such as amplitude modulation and frequency modulation. EMC testing focuses on the accuracy of power detection, which requires the signal generator to output accurate and pure RF signals under continuous wave or pulse, high power or low power output. IEC61000 and CISPR signal sources have corresponding regulations on harmonic suppression and output power (especially under pulse modulation conditions).

1. Harmonic suppression of signal source

In the detection process of EMS, the signal source needs to output higher power to promote the power amplifier (hereinafter referred to as power amplifier) "power amplifier"). Under the condition of high power output, the harmonic and non-harmonic power corresponding to the signal source is also high. Non-linear components such as signal source harmonics, non-octave position harmonics and spurious (non-harmonic) are very harmful to EMC test. EMI receiver selects the scanning detection principle, and the harmonics of the signal source received by the receiver will cause interference; When the harmonic power is too high, the broadband power amplifier or broadband receiver may be oversaturated.

In this regard, IEC61000-4-3 has clear provisions on harmonic suppression of signal sources. After the output of the standard power amplifier, the harmonic suppression of the signal must exceed 15dBc. Considering the nonlinear effect of the power amplifier, the harmonic suppression of the signal source is much better than 15dBc [1]. IEC61000-4-3 preferably adds a harmonic filter behind the signal source to suppress harmonic power. See Figure 1 for harmonic suppression of signal source with/without harmonic filter. However, in order to adapt to the frequency range of harmonic filter EMC test, a large number of filters will be selected to filter out the harmonics of different frequency bands to ensure the purity of the frequency band. At the same time, a large number of switches should be selected to ensure the conversion of different filters before. Filters and switches will generate additional insertion loss, which will gradually increase with the increase of frequency. This will increase the comprehensive cost and the uncertainty of detection.

The embedded harmonic filter can solve the above problems, and can be completed at all output levels – 65dBc (typical value) harmonic suppression can also suppress broadband noise. Combined with high output power, low harmonic and extremely low broadband noise, it can effectively simplify the detection connection and reduce the total number of detection accessories.

图1.配置/未配置谐波滤波器的信号源谐波抑制

Figure 2. Non-harmonic suppression of SMA100B output in actual test

2. Maintain accurate output level

The EMS pulse modulated signal needs to be used for testing. Strictly control. RF pulse output power is particularly important. CISPR stipulates that the deviation of signal output pulse modulation signal power shall not exceed 16-1-1 ± 0.5dB [2].

Automatic power control (ALC) is the core technology of signal source control output power accuracy. ALC is a closed loop power control circuit, which connects part of the output signal to the signal source to collect and detect power immediately. When power fluctuation is detected, operate the amplifier immediately to complete stable and accurate power output according to closed-loop control. The circuit design diagram is shown in Figure 3.

Figure 3. Typical ALC circuit schematic diagram

ALC processing response time shall be at least the length of signal receiving measurement time Ty-min. The signal opening time of each cycle is very short, and the customized ALC minimum collection time of traditional signal source is widely more than the opening time of differential signal:

Therefore, the traditional signal source ALC is not suitable for differential signals. When the differential signal occurs, the traditional signal source will take place ALC generally adopts the "Sample&Hold" processing method, that is, the signal source is instructed to first cause a continuous wave signal for a period of time, and the ALC samples and records its power error value, that is, the "Sample" process; Then, during all pulse output periods, the ALC signal source normally outputs differential signals, and the ALC signal source amplifier will make constant power adjustment according to the power error value saved in the previous step, that is, the "Hold" process. The gain Gcw pulse output Gpulse after the amplifier outputs continuous wave is generally different, namely:

Therefore, ALC's "Sample&Hold" solution has a huge deviation. The deviation between the real power and the set power of the evaluated signal source output difference signal exceeds 1dB, as shown in Figure 4.

Figure 4. Measured output pulse power error of ALC - "Sample&Hold" mode of a certain type of signal source

In order to provide accurate pulse output power, R&SSMA100B redesigned the pulse modulator and pulse modulator ALC pulse modulator with a rise and fall time of 5ns (typical value) and a switching ratio of>80dB. Quantized short pulse with high level accuracy and level accuracy can be given in a time as low as several nanoseconds, as shown in Figure 5.

Figure 5. SMA100B output pulse signal power error does not exceed 0.3dB

3. Higher power output

The EMC test also stipulates that the signal source has a high output power. For example, here. During the detection process of EMS, the output signal of signal source shall fill the loss caused by the remote transmission cable to fully promote the power amplifier. Cable loss will rise sharply in high frequency band (e.g. Ka), which requires signal source to output enough power to promote power. The three-level output power selection is suitable for applications that are cost sensitive and strictly regulated. The high output power option 35dBm can be activated through the option key, and even the customer can activate the call on the spot. However, the factory installed high output power option can maximize the power level of 38dBm (for 6GHz models).

Figure 6. Measured maximum output power of R&S SMA100B

When the specific output power of the signal source exceeds 27dBm, it is necessary to ensure safety in use. Because most receivers, the safe receiving power limit of the spectrum analyzer is 27dBm or 30dbm.

4. Improve the stability of signal source application

EMC test includes a series of large measurement test reports, during which the operation test equipment is set repeatedly, and the test equipment is required to have high application stability. EMS detection must manipulate the signal source at many detection frequency points, resulting in different power signals, and repeatedly convert the attenuator of the signal source. GHz signal generators have been equipped with electronic solid step attenuators since more than ten years ago. The electronic attenuator has no hardware failure in the level conversion process, and its operating life is far higher than that of the mechanical attenuator, but the mechanical attenuator higher than 6GHz is also widely used in signal sources. R&SSMA100B now brings electronic attenuators to the high-frequency microwave signal generator industry. It is equipped with electronic solid state switches in all frequency ranges (up to 20 at present) to achieve very fast and effective level conversion.