Design Of 300MHz Isolation Channel Of Handheld Scopemeter

Handheld isolated scopemeter adopts independent electrical insulation signal channels, which can take floating measurements to multiple hybrid circuits with no public reference simultaneously, and avoid the danger caused by accidently grounding short circuits. With the growing demand of isolation test of complex electronic signal,the bandwidth demand of isolated scopemeter is growing rapidly, and the instrument’s safety regulatory requirements are also increasing.The analog channel is one of the key modules of handheld isolated scopemeter, so the channel isolation circuit and communication interface isolation method have been researched in this paper. A 300 MHz isolation channel is designed and realised, and so is security isolation communication interface function. Three main points are as follows:First, the analog signal isolation circuit is designed. Analog signal isolation circuit adopts sub paths circuit as transmission mode, which includes a high frequency isolation transmission path based on broadband isolation transformer and a low frequency isolation transmission path based on linear optocoupler, then superimposed combination. By repeating the design and verification, the DC to 300 MHz analog signal isolation circuit is realised.Second, the power isolation circuit and the digital control signal isolation circuit are designed. According to the design requirements of the channel isolation circuit, the power isolation circuit adopts high efficiency isolated power chips and low ripple low-dropout linear regulator, and designs the power control circuit controlled by software, which can achieve dynamic power management depending on the operating mode of scopemeter reducing the power consumption of the scopemeter. Using low-power digital signal isolation chips, designs the channel control circuit, and realizes the working state control of the analog channel.Third, to further ensure the safety of scopemeter, the external interface of the scopemeter is also made the corresponding electrical isolation design, including USB isolation, external trigger isolation and square-wave calibration signal isolation, which realize the internal circuitry of the scopemeter electrically isolated from the outside world completely, and improve the safety performance.Finally, the circuits designed are tested, and the results show that the bandwidth ofthe analog signal isolation circuit designed reaches 300 MHz, meeting the design requirements, and square-wave calibration output isolation signal and USB isolation interface also meet the functional requirements, thus are used in scopemeter.