EMC測試注意問題.docx

EMC測試常用設(shè)備使用時(shí)的注意事項(xiàng) 1. 天線：EMC測試的頻率范圍都很寬，因?yàn)槭孪炔⒉恢莉}擾的頻率是多少，所以，測試必須借助各種探測天線來把場強(qiáng)轉(zhuǎn)換成電壓。一般來講，EMC測試用的天線都工作在近場區(qū)，測試結(jié)果對(duì)測試距離非常敏感。因?yàn)殡妶觥⒋艌鲋龋úㄗ杩梗┰诮鼒鰠^(qū)不再是個(gè)常數(shù)，所以這些天線雖然給出了電場、磁場的校正系數(shù)，但只有當(dāng)這些天線作遠(yuǎn)場測試時(shí)才有效，測量近場干擾時(shí)，電場與磁場測試結(jié)果不能再按此換算。 2. 人工電源網(wǎng)絡(luò)要注意人工電源網(wǎng)絡(luò)與接地平板成低阻抗良好連接。 3. 頻譜分析儀要注意根據(jù)測試要求選取頻率分辨率、靈敏度、動(dòng)態(tài)范圍、掃頻寬度等參數(shù)合適的頻譜分析儀。頻率分辨率：能夠分辨的最小譜線間隔，它表征了頻譜分析儀能將頻率緊挨在一起的信號(hào)區(qū)分開來的能力。靈敏度：表征了頻譜分析儀測量微小信號(hào)的能力。頻譜分析儀的噪聲電平?jīng)Q定于中頻濾波器帶寬，一般在-80dB-130dB。動(dòng)態(tài)范圍：表征了它同時(shí)顯示大信號(hào)和小信號(hào)的真實(shí)頻譜的能力，動(dòng)態(tài)范圍的上限由頻譜分析儀的非線性失真所決定。頻譜分析儀動(dòng)態(tài)范圍一般在60dB以上，甚至100dB。掃頻寬度：頻譜分析儀在一次測量分析過程中顯示的頻率范圍。 4. 測量接收機(jī)頻譜分析儀只有峰值檢波，而測量接收機(jī)除了峰值檢波外，還有準(zhǔn)峰值檢波和平均值檢波，要根據(jù)不同的標(biāo)準(zhǔn)或法規(guī)選擇合適的檢波器。峰值檢波只測試騷擾信號(hào)振幅的最大值，適用于單一脈沖或重復(fù)頻率很低的脈沖騷擾信號(hào)的測試。而有些騷擾信號(hào)對(duì)音頻影響的客觀效果是隨著重復(fù)頻率增高而增大的，對(duì)這種信號(hào)僅僅測量信號(hào)的振幅無法客觀反映其影響效果，因此，這時(shí)就要用準(zhǔn)峰值檢波。準(zhǔn)峰值檢波器其充電時(shí)間常數(shù)比峰值檢波器大，放電時(shí)間常數(shù)比峰值檢波器小，不僅可以測量出騷擾信號(hào)信號(hào)幅度，還反映它的時(shí)間分布。如何避免EMI測量接收機(jī)發(fā)生過載OVLD故障測量接收機(jī)是EMC測試中使用最多的設(shè)備，由于測量信號(hào)未知，經(jīng)常會(huì)發(fā)生過載OVLD故障。一般來說，OVLD故障是由于RF Converter模塊中的第一混頻器受到損壞所引起的。導(dǎo)致第一混頻器損壞有如下幾種可能性：1、輸入功率過載2、輸入脈沖過載3、針尖脈沖4、有直流電平饋入5、EMI接收機(jī)與被測設(shè)備的接地不好6、靜電損傷測量接收機(jī)是非常精密的設(shè)備，維修費(fèi)用高昂，為盡量避免發(fā)生以上的故障，影響試驗(yàn)室的試驗(yàn)進(jìn)度，在日常的使用中，應(yīng)注意的事項(xiàng)如下：1、使用外接脈沖限幅器或者衰減器，型號(hào)ENV216的人工電源網(wǎng)絡(luò)自帶了10dB的衰減器，避免了因忘記接限幅器或者衰減器而造成的損傷2、EMI接收機(jī)與被測設(shè)備的接地要良好，且為低阻抗3、被測設(shè)備已正常工作后，再將人工電源網(wǎng)絡(luò)到測量接收機(jī)的同軸電纜接上，避免因被測設(shè)備突然啟動(dòng)時(shí)的騷擾對(duì)測量接收機(jī)的損壞4、在EMI測量接收機(jī)與人工電源網(wǎng)絡(luò)之間的同軸電纜未斷開前，不要帶電插撥或開/關(guān)被測設(shè)備，避免因被測設(shè)備突然啟動(dòng)時(shí)的騷擾對(duì)測量接收機(jī)的損壞5、在連接人工電源網(wǎng)絡(luò)到測量接收機(jī)的同軸電纜時(shí)，測試人員要注意自己手上、身體上可能帶的靜電電荷以型號(hào)ESCI的測量接收機(jī)為例，RF INPUT端最大的能量只能在如下的限制內(nèi)，使用時(shí)要特別小心。For DC-coupling, the max. DC voltage is 0 V. For ACcoupling, the max. DC voltage is 50 V. The maximum power is 1 W ( 30 dBm at 30 dB attenuation)3m場地的輻射數(shù)據(jù)可以直接代替10m場地的數(shù)據(jù)嗎？One problem with 3m testing is that there is a fall-off projection problem that is usually seen below 200 MHz in vertical polarization. if tested at 3m, the maximum vertical field propagates along the ground plane (normal to the ground plane) due to the reflection. Since the antenna is scanned between 1 and 4m, some of the energy goes under the antenna. At 10m the wavefront is more developed and with the same scan window, 1 to 4m more of the main lobe of the total field propagating on the ground plane is measured. the 3m data has to subtract 10 dB from the measured data to account for the 3 to 10m fall-off. But that is free-space assumption without the ground plane effect. At 10m, more of the energy is measured and when compared to the projected fields done at 3m, up to 7 dB errors have been reported. the 3m data actually underpredicts the field at 10m. This has caused products that were thought to pass when tested at 3m to then fail at 10m facilities. This is not much of a problem in horizontal and actually the opposite problem exists with 3m measuring more energy than will be seen at 10m. But the differences are much less. The other reason this is not a bigger problem at 3m, is that there is not much horizontal emission below 100 MHz since the horizontal fields on the ground plane are zero. the direct and reflected are 180 degrees out of phase on the ground plane and cancel. So emissions are measured up around 4m in height for horizontal plus products dont tend to efficiently radiate horizontal fields below 100 MHz. 以上信息是某位國外資深EM