Hot answers tagged

73

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. ...


42

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 ...


26

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. ...


24

It is some kind of soft-open-cell foam, wrapped in conductive tape, to ensure a good grounding and shielding contact between pieces. The foam may or may not have large amounts of carbon particles in it to make the foam itself conductive.


21

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 ...


21

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 ...


21

There isn't a reason to worry about exceeding limits with a breadboard, the FCC allows for building of single devices with no testing: What does this mean for a hobbyist? Very little, actually, depending on what you’re doing. The FCC allows a hobbyist to build up to five devices of a single design for personal use with no testing whatsoever. If you ...


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.


18

The PWM frequency is not so important, what matters is the rise times of the switch controlling the PWM. Assuming it is some sort of MOSFET, one can assume very fast rise times of 100ns to 10ns or even less. 1 meter of wire has a self inductance of ~1.5µH to ~1.8µH for a reasonable range of wire gauges. This inductance will be some what less if you run the ...


15

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 ...


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

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 ...


14

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 ...


14

Background (with a little speculation) Inside the TV's switch mode power supply will be one or several Y capacitors that connect the internally produced DC voltages to either live or neutral. They are there to reduce the common mode noise produced by the high frequency switching transformer from affecting the DC outputs. Without the Y capacitors, all the ...


14

Does this answer your question (emphasis added)? 9.3.2 Test method The test method shall be in accordance with EN 61000-4-2 [2]. For radio equipment and ancillary equipment the following requirements and evaluation of test results shall apply. The test severity level for contact discharge shall be 4 kV and for air discharge 8 kV. All other details, ...


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

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

It is not easier to design for higher frequencies and the confusion you are experiencing is caused by the misconception of thinking that the two solutions compared in the book would be as good at 2 GHz when they are actually as bad. In other words, traditional signal and power traces are great antennas anyway at those frequencies, but usually the signals ...


11

The yellow thing is an X-capacitor, C11 in the above schematic. The resistor is to discharge said X-capacitor within legal requirements (usually 34 V within 1 second after unplugging), R14 in the schematic. L is an common mode choke, X3 in the schematic. They all form the EMI supression from the power supply to the mains, again within legal requirements. ...


11

This is a filter that sits between the mains power line and the SMPS. Its goal is to suppress electromagnetic interference (EMI) and (to some extent) protect the circuit from spikes on the line. The basic structure of such a filter looks like this: These filters use specially-rated capacitors to handle the high AC voltage. The capacitors used between the ...


10

Safety standards (UL, CE, etc.) place a limit on the amount of leakage current allowed to return on the earth ground. By using the 4 capacitor arrangement, the line-to-line filtering is made independent of the line-to-ground filtering. i.e. you can make the line-to-line capacitors a larger value without increasing the ground leakage current. Then the value ...


10

Look at "SDR dongles" - USB dongles made ostensibly for laptop reception of TV broadcasts, but repurposed for amateur radio purposes and sold for tens of dollars. Not all of them cover the spectrum up to GPS frequencies but some do, so shop around. Combine "SDR dongle" with "spectrum analyser" and "open source" search terms and I think you'll quickly have ...


10

While you have freewheeling diodes on the relay coils, you don't have any kind of spike attenuation on the relay contacts. The contacts need transient suppression for the same reason that the coil driving transistors do. The inductance of whatever load you are switching can cause a large (several kilovolts) voltage transient with a very fast rise time when ...


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