As was discussed in Section 2.5.2, a shield offers no magnetic field safety at low frequency. Twisted pair cables, even when unshielded, are very efficient in reducing magnetic field coupling. Therefore, when terminating a twisted pair, shielded or unshielded, don’t untwist the conductors any more than essential to make the termination. At frequencies above about 100 kHz, or where cable length exceeds one twentieth of a wavelength, it becomes necessary to floor the shield at both ends. The problem of all “ideal” approaches is that, they should be adopted exactly and precisely, and all the mandatory preconditions must even be happy for his or her assumptions to be legitimate. Using one other downside in PCB design for example: In PCB design, there’s a rough consensus that, if the PCB is already partitioned into digital and analog sections, the return present in each part is mostly contained in their very own space. If one decides to make use of RC and ferrites solutions, solving the ESD downside is left as an exercise for the reader. In Williams’ anecdotal observations, floating shield, RC and ferrite bead options performs poorly under ESD strikes, and is a common cause of failure of ESD compliance tests. An important flaw is that if the shield and circuit ground are isolated from one another by way of capacitors or ferrite, during a ESD strike, a large potential distinction is created between the shield and circuit floor, enabling a ESD strike throughout them, and causing the system to fail ESD compliance tests.
After bonding the shield immediately onto the circuit ground, these devices would move ESD exams instantly. The USB Type-C specification includes a blanket prohibition in opposition to these practices (Then again, the USB Type-C specification also consists of extensive description on bonding the shield and the chassis, thus, I imagine that the technical committee behind USB follows a method similar to the one proposed by Henry Ott). CEC was developed as an answer to a technical problem: individuals had a number of devices, shield control cable and hauling around a number of remotes was frustrating. You could possibly hook up multiple computer systems to an HDMI change and they may speak to one another with CEC. CEC was deliberate from the very beginning, although it didn’t get a full specification as a part of the HDMI standard until HDMI 1.2a. Indeed, CEC is older than HDMI, courting to not less than 1998, when it was standardized as part of SCART. Then I still have to stand up to actually put a report onto the turntable, and now the Tv is just on the entire time, so this is not that interesting in observe.
For instance, if you had a DVR linked to your Tv through CEC, you may browse the digital program guide (EPG) in your Tv and choose a program to record. These remotes, typically thrown in with dwelling theater receivers as a perk, have some mixture of a database of remote protocols pre-reverse-engineered by the manufacturer and a “be taught” mode by which they’ll report unknown protocols for naive playback. Conceptually you may even use the Tv to regulate non-video functions. Use ferrite beads to connect the shield to the circuit floor. Therefore, how effective the shield is has quite a bit to do with how properly the cable shield is terminated to the enclosure. Consider the case of a PCB, with an input/output (I/O) cable, mounted inside a metallic enclosure as shown in Fig. 3-24. Because the circuit floor carries current and has a finite impedance, there will be a voltage drop VG throughout it. The signal current flows in the 2 internal conductors, and any induced noise present flows within the shield. At low frequency, shields on multiconductor cables where the shield will not be the sign return conductor are sometimes grounded at just one end. Grounding the cable shield at only one finish to eradicate power line frequency noise coupling, nonetheless, permits the cable to act as a excessive-frequency antenna and be susceptible to rf pickup.
This voltage will drive a standard-mode current out on the cable, and will cause the cable to radiate. If, nevertheless, the circuit floor is linked to the enclosure at the I/O connector, the voltage driving widespread-mode present out onto the cable will ideally be zero. Many devices have no metal enclosure at all, invalidating the complete methodology. I have a personal rationalization of why EMI/EMC problems developments to generate flame wars – the hole between idealized greatest practices and actual methods. When you do not have a selection, a straight connection may be the only compromise right here. The source of my declare was Tim Williams, one participant of the previous debate within the remark part, he was already recognized to me as a prolific poster on the EEVblog forum, and i remember seeing his intensive writing on the subject of the USB shield connection. USB began out as a reasonably pure and easy serial hyperlink, and then extra use-circumstances had been piled on, culminating in the marriage of two completely completely different interconnects (USB and Thunderbolt) in one bodily connector. However, at excessive frequency, the capacitor becomes a low impedance, which converts the circuit to at least one that is grounded at both ends.
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