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59

Switched mode power supplies use what is known as a "flyback converter" to provide voltage conversion and galvanic isolation. A core component of this converter is a high frequency transformer. Practical transformers have some stray capacitance between primary and secondary windings. This capacitance interacts with the switching operation of the converter. ...


39

Advantages of holes in shield: Allows some air flow for better heat dissipation. This is the primary reason. Less weight. Small holes don't really compromise the shield, as long as the holes are significantly smaller than the wavelength of what you want the shield to attenuate. As a aside, you won't ever see long slots in RF shields. If a larger overall ...


24

The split ring is preferable. A ring that goes all the way round acts like a loop antenna or may act as one closed winding of a transformer. The loop antenna may radiate or pick up electro-magnetic interference and if it acts as a primary transformer winding, the circuitry around the microcontroller in the middle may act as the secondary winding and become ...


24

Think about a PCB connection (or wire) between an output and an input. It's basically an antenna or radiator. Adding a series resistor will limit the peak current when the output changes state - that causes a reduction in the transient magnetic field generated and therefore will tend to reduce coupling to other parts of the circuit or the outside world. ...


22

Note that this is an autozero amplifier (also called chopper stabilized) - many very low offset opamps work by periodically sampling the input offset and injecting a compensating offset to counter drift in the front end. To do this there is an oscillator in the opamp together with a set of analog switches at the input. This can result in clock feedthrough ...


18

If proper engineering is followed, the switching noise shouldn't impact the readings and the oscilloscope weighs less, takes up less space, and draws less power. Thus, many benefits to a single increase in engineering difficulty.


17

The right term for this "slow down" feature is slew rate. Adding a resistor reduces the slew rate by forming a low-pass RC filter with the input capacitance. You can see the effect of such resistors in the following oscillogram (green curve with higher slew rate produces much more noise): The power consumption increase you mention is in fact not real. It ...


15

I can't really tell if this is a actually a symptom of what is described in the datasheet: Notice how there's a spike that exceeds \$30 \frac{\text{nV}}{\sqrt{\text{Hz}}}\$ at 65kHz – pretty much half of the frequency you're observing your noise at; they didn't characterize up to 131.5kHz, however. What should I try to remove it? I would like to at least ...


15

They are called via fences, they are placed on the outside of the board to "fence in RF", they do this by creating a barrier smaller than the wavelength that needs to be shielded. At very high frequencies, the area between planes can function as a waveguide/antenna and high frequencies can move between planes and out of the edge of the PCB. In addition to ...


14

Use Schmitt-trigger buffers at the inputs of each board. They will clean up the signals so that any noise won't give false pulses on the clock, for instance. The 74LVC3G17 is a triple non-inverting buffer. Also, pass the buffered signals to the next board. Otherwise all inputs would be parallel and you may exceed the fan-out of the driving microcontroller ...


14

The most cited paper on the subject that I could find is PCB design techniques for lowest-cost EMC compliance Part 1 (not free). Though the part you're interested in is succinctly quoted in Best practice in circuit board design: Armstrong recommends stitching at no more than λ/20, with stub lengths no longer than this. This is actually a very good rule ...


14

Ferrites reduce electromagnetic radiation by reducing common mode currents. First, why does reducing common mode currents reduce radiation? If you have two parallel wires that carry equal and opposite currents, that is, no common mode currents, then at distances significantly more than the distance between the wires, the electric and magnetic fields created ...


14

You're convoluting two entirely different things into one. There are two completely separate phenomenon going on, one of which is switching noise, the other of which is ringing. They have no relationship to each other and are caused by very different things. The noise that results is also very different. First, lets talk about what you linked, the snubber ...


13

This split up arrangement would help from noise traveling between the modules? If you have multiple power voltages and a 4-layer board you don't have much choice. You have to deliver different voltages to the different loads. Whether it reduces or increases noise has a lot to do with the details of how you lay it out, it's not possible to just give a ...


13

The follow-up question... but what I don't understand is why the flow of electrons that are physical entities result in emission of these EM waves Why does "radiation" occur? Let's look at this specifically, because it is a common (and excellent) concern. Here is a simple wire, instantly connected to a voltage source: simulate this circuit – ...


13

So you have 3-phase power. There is noise that is line-to-line and noise that is common mode. The caps across the lines are in the right position to filter the line-to-line noise, and the single cap to ground can filter the common mode noise. If you used your method, the capacitance between phases would be less. There may also be a consideration that the X ...


13

Electromagnetic Interference is a radiated or conducted signal that is unwanted that you are trying to avoid. Electromagnetic Compatibility encompasses the standards and testing of equipment so that it can generally be expected to function properly in a shared environment. This involves testing devices to make sure that the EMI produced is under some limit, ...


13

The theory says that the current return path at high frequency is on the reference plane right under(or above) the signal trace. I know it is true and I have always assumed it was, but I would like to understand it properly. There are far too many ambiguous words describing your scenarios so draw a picture but, in the meantime consider what happens when ...


13

The SMPS advantages over linear supplies, of compactness, high efficiency so low heating, light weight and wide input voltage range, are very valuable in portable equipment like an oscilloscope. The principle disadvantages of SMPS over linear supplies are switching noise on the output and radiated EMI. (Circuit complexity used to be considered another but ...


12

Those are vias. They are connecting that one piece of ground plane to another one on another layer. It may be done for increasing current carrying ability, thermal transfer, EMI reduction, and any number of other reasons. Given that it's an RF board it's most likely to help control unwanted EMI. When you have two ground planes, and high frequency, ...


12

Well one it slows down the rise time of your signal which reduces the high frequency content. So that could help you if you didn't need your edges to be that fast. It also lowers the current flowing through the trace and back through its return path which would lower the strength of the field created around it (and radiated out). I would add that I've ...


12

The first one uses a star grounding scheme, which works well in some circumstances: low frequencies, absence of incoming EMI/RFI... which means it is an increasingly less useful scheme in today's world... However, before talking about the loop, I'd like to point out that your design is single-supply, thus chips draw supply current and dumps it into the ...


12

Does painting (non-conductive) over the top of a grounded metal enclosure affect its ability to absorb/block RF interference? No, the idea of an EMI enclosure is to create a faraday shield around something to block electromagnetic waves around whatever it is enclosing. The shield itself needs to be conductive to be effective and have adequate skin depth, ...


11

7 mm between a 25 MHz crystal and the chip driving it is no big deal. What is far more important is that the ground side of the crystal caps connect back to a ground pin on the part, not just punched thru to the ground plane. You don't want those high frequency currents running accross the ground plane else it will become a center-fed patch antenna. All ...


11

Instead of a rigorous mathematical treatment, here's a somewhat hand-waving explanation: Any wire has a magnetic field around it (perpendicular to the length of the wire) when there's a current flowing through it. However, efficiently launching an electromagnetic wave also requires a voltage drop (E field) at right angles to the M field (along the length of ...


11

Touch screens have transmit (Tx) and receive (Rx) electrodes which are drawn in transparent indium tin oxide (ITO), forming a matrix of crossed traces with each Tx-Rx junction having a characteristic capacitance. The human finger is basically a ground that alters the mutual capacitance between the RX and TX electrodes. This network is very sensitive to ...


11

It's called via stitching, a via fence or a picket fence. It's typically used to control EMI at very high RF frequencies- getting in or getting out, and can also reduce the resistance of the connections at DC. If the copper areas are joined by more widely spaced vias the loop area and inductance would be higher, and at DC more vias in parallel mean a ...


11

Good answers here already but I would also add, holes also significantly change the thermal/mechanical properties of the shield. As you know, when metal gets hot it expands, similarly, it shrinks as it cools. If a "can" type EMI shield is soldered down to the PCB, and said shield is solid, that will introduce a significant difference in expansion rates ...


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