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What does the Y capacitor in a SMPS do?

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. ...
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42 votes
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Why do some EMI/RF shields have holes on the top and some don't?

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 ...
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26 votes
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Why is it good to slow down digital lines with resistors?

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 ...
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24 votes
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What is this spongy material?

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 ...
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21 votes

Why is it good to slow down digital lines with resistors?

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 ...
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21 votes

Identifying Source of Periodic Artifact at Op-Amp Output

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 ...
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21 votes

Hobby breadboard wiring guidelines to stay within legal RF/EMI limits?

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, ...
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21 votes

Is twisting a single ended signal with its own GND really useful?

Yes, it is better. Model the cable as two transmission lines: one representing the differential mode (signal to GND wire), stacked on top of one representing the common mode (GND wire over GND plane)....
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18 votes

Why do oscilloscopes use SMPSes instead of linear power supplies?

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 ...
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18 votes
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Can I send PWM through 1 meter cable?

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 ...
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16 votes
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How can I observe rectifier switching noise?

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 ...
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15 votes
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Identifying Source of Periodic Artifact at Op-Amp Output

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 ...
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15 votes
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Why do some PCBs have exposed plated perimeters?

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 ...
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14 votes

Why are caps configured this way in EMC filters?

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 ...
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14 votes

Why do some EMI/RF shields have holes on the top and some don't?

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 ...
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14 votes
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how can power line 'orientation' influence electronic equipment?

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 ...
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14 votes
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How does a bare PCB product such as a Raspberry Pi pass ESD testing for CE mark?

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 ...
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13 votes
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EMC testing vs. EMI testing

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 ...
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13 votes

Real current return path

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 ...
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13 votes
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Why do oscilloscopes use SMPSes instead of linear power supplies?

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 ...
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12 votes
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What is the purpose of holes on edge of the PCB?

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 ...
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12 votes
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How serial resistors actually reduce EMI?

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 ...
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12 votes
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The curse of the ground loop

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 ...
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12 votes
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Does paint affect EMI ability of enclosure?

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 ...
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11 votes

What is the purpose of holes on edge of the PCB?

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 ...
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11 votes
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Why does radiated emission of a PCB decrease as the frequency of the signal increases?

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 ...
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11 votes
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Why does an SMPS use LCR on INPUT?

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 ...
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11 votes

Why does an SMPS use LCR on INPUT?

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 ...
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11 votes

Is twisting a single ended signal with its own GND really useful?

It strongly helps when done right. Allow me to make a small story arc to explain my answer. The problem Ground planes are good dump-and-forget nodes for return currents at both high and low frequency ...
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10 votes
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Why are caps configured this way in EMC filters?

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 ...
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