Introduction
A customer requires us to solve the problem of radiation interference of ultrasonic parking radar equipment. The equipment is in 530
The KHZ-2 MHz frequency range did not pass the CISPR 25 4 narrowband radiation noise test. The device consists of a controller with a buzzer and two radar modules.
Wiring includes a controller's power cable and a communication / power cable between the controller and two radar modules.
Customers provide screenshots with test failures measured in a non-return room.
The measured conductive emission spectrum exceeded the restriction of CISPR 25 4 in the range of 530 kHz to about 1.1 MHz. In addition, the customer provides information, that is, the test has failed in the case where the module cable is disconnected.
2. Method
The observation spectrum can be obtained for the first clue.
The given device transmits a low frequency range of the limit, and the frequency point points to the relatively low frequency digital signal. Relatively wider noise, there is no discontinuous spectrum line indicates the transmission from the serial interface between the microcontroller itself or the controller and the radar module.
As the customer mentioned, the device also has failed in the case of disconnecting the module's cable, and the initial suspicion comes from the microcontroller.
3. Test settings and measurements
The EMC near field probe and TEM unit are combined with the spectrum analyzer to identify the basic tools for identifying the source of radiation.
Since the sensitivity of H and E field probes in the KHz range is not very high, the TEKBOX TBTC3 TEM unit is used to test immediately.
The spectrum analyzer involved is Siglent SSA3021X.
The challenge of measuring CISPR25 4 or 5 radiation or conduction noise is a relatively low restriction of the standard.
For open TEM units, environmental noise in the nearby switching power supply and AM broadcast signals may hinder the measurement of low levels within KHz and low MHz ranges.
Open the spectrum analyzer and do the following:
Span 530 kHz - 2 MHz; bandwidth 9 kHz; attenuator 0 dB; front amplifier opens;
The average voltage, the positive peak detector displays the following spectrum:
The bottom noise displayed using this setting is approximately -20 dBμV, which is impressive for such spectrum analyzers. However, it is necessary to consider that the frequency range of interest is full of AM broadcast signals that may be picked up.
Connecting empty TEM units display the following spectrum:
The frequency range is a broadcast signal everywhere.
Next, if the emission of the DUT is higher than the amplitude of the amplitude is between the amplitude, it needs to be apparent.
DUT settings
Next, the DUT containing the cable and the radar module is placed inside the TEM unit and turns on the power:
Spectrum analyzer tracking settings is maximum:
Although some peaks in the AM broadcast signal are present, the "peak" visible in the measured spectrum is well related to the measurement maps in the shielded dark chamber. The magnitude measured in the TEM unit is approximately 20 dBμV than the magnitude measured in the shielded dark chamber.
Next, the cable is removed because the cable removal has also failed according to the customer's statement. For the sake of simplicity, the cable is kept physically connected, but moves to one side with respect to the TEM unit separator.
Surprisingly, mobile cables can greatly reduce DUT radiation interference:
"Remove" cable can reduce radiation noise of approximately 11 dBμV. After verification with the customer, the cable is not unplugged, but disconnects the two radar sensors. This also explains why the relative amplitude of the spectrum "Hills" is different from the TEM unit measurement. The cable is set directly in the microwave darkroom, and in the TEM unit is winding.
4. Conclusion
The unplugable cable can greatly reduce the radiation noise of DUT. Although the client removed the radar module, he still did not pass the test. In fact, even if there is no module, the controller will continue attempting to communicate through a serial cable without timeout. Therefore, it is probably that the serial communication interface between the controller and the radar module is the culprit. In order to solve this problem, it is necessary to filter some of the interface.
Since the ferrite is not very effective under low frequencies, it is determined to increase the resistor connected in series with the cable. This seems to be a particularly practical approach because it has been assembled on the PCB. Use 1k instead of 0 OHM series resistance to reduce the radiation noise level to about 12 dBμV. However, it turns out that the serial interface is no longer reliably within the specified supply voltage range. Finally, there is also a need to adjust some other resistors in the transistorful serial interface circuit to resolve radiation problems and keep the serial interface in the specification. After some modifications and measurements, the results are as follows:
In order to obtain a clearer image, the spectral analyzer has a reduced span and set between two adjacent broadcast channels.
The previous "Zoom" screen screenshot below:
Next is a modified enlarged screen screenshot:
The modified frequency reduces the radiation transmitting level of 12 dBμV
After the modification, it is below the final measurement chart of the test room:
The apparatus passes the radiation noise test in the range of 500kHz to 2 MHz, and has also passed the remaining frequency range of up to 2 GHz.
5. Analysis
The device consisting of SIGLENT spectrum analyzer SSA 3021x and Tekbox TEM units has been successfully used to measure and reduce radiation emission problems. The spectrum of the radiation signal is relatively wide, and there is no sharp peak. Although the amplitude is very low, the range of environmental spectrum range is very wide, but the settings can still be carried out well.
The placement of the TEM unit inside DUT can provide a very repeatable result without having to reset the DUT accurately after each modification. The EMC probe is an ideal choice for precise positioning / identifying the radioactive source on the PCB, and the advantage of the TEM unit is that it accurately measures the improvement of the amplitude of the modified radiation signal to achieve regulatory requirements. The complete emissions of products consisting of several separate devices and interconnect cables are also very practical.
Test measurements require a small space. In order to find an ideal solution that meets EMC and meets functional requirements, it is necessary to improve parking radar needs multiple iterations. If it is necessary to perform re-measure in the microwave dark room after each iteration, the cost is likely to exceed the cost of the purchase spectrum analyzer and TEM unit.
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