Hello.I was trying to figure out why decoupling(or bypass) capacitor was used in the circuit.

Actually, the guy who constructed this circuit said in http://cassiopeia.hk/walkietalkie/
"Due to nRF seems to draw more power , he put a 1000uF capacitor there."

Then I began searching and found that bypass capacitors are used in order to ground AC signals. Okay but where does AC signal come from?

I don't see any AC signal in the above picture or in

.

I took the picture from a website where bypass capacitor explained.

 

Welcome to AAC!

Any variations in current drawn from a non-ideal voltage source can introduce variations in it's voltage. Filter, bypass, and/or decoupling capacitors will smooth out those variations.

 

Any capacitor across the supply de-coupls the signal from both power conductors and makes them common to each other as far as signal is concerned.
Max.

 

Welcome to AAC!

Any variations in current drawn from a non-ideal voltage source can introduce variations in it's voltage. Filter, bypass, and/or decoupling capacitors will smooth out those variations.

Thank you for your answer
just to be sure, because the power drawn from battery or USB to my arduino is not ideal, I needed this capacitor.

 

Every time you draw current from the power supply you are disturbing the ability of the power supply to deliver a constant voltage. Capacitors help to keep the voltage constant.

 

Hello.I was trying to figure out why decoupling(or bypass) capacitor was used in the circuit.

Actually, the guy who constructed this circuit said in http://cassiopeia.hk/walkietalkie/
"Due to nRF seems to draw more power , he put a 1000uF capacitor there."

Then I began searching and found that bypass capacitors are used in order to ground AC signals. Okay but where does AC signal come from?

I don't see any AC signal in the above picture or in

.

I took the picture from a website where bypass capacitor explained.

http://ee.old.no/library/ - download one of the kit manuals and build a 3 transistor audio amplifier, but leave out the R/2C Pi filter on the supply rail. It will "motorboat" because of feed back to the low level stages along the supply rail.

 

A walkie talkie has audio and radio signals which cause AC currents in its power supply. These AC currents cause the power supply voltage to swing up and down without the bypass capacitors. If the phase of the power supply voltage swing is positive at a certain frequency then the circuit oscillates. If the phase is negative then at that frequency the gain is reduced.

 

(Edit: motorboating and other oscillations.) That's another good reason.

When you draw current from the power supply the voltage might drop. The constant voltage regulator senses this and tries boost the output voltage. If the voltage overshoots, the regulator cuts back on the output and this can lead to oscillation. The decoupling capacitor recommended on 3-terminal regulators is to prevent oscillation.

 

“Filter” cap is probably the best terminology to use for that 1000uf cap.

“Bypass” caps usually shunt ac either to ground or around a component.

And decoupling caps are generally used at a chip to dump current to the chip during small voltage drops, until the power supply catches up.

Some people tend to use terms interchangeably. And in some cases this is ok…but not always.

 

Decoupling capacitors are also used across the power rails.
Max.

 

All depends on what it’s doing, if the cap on the main is reducing ripple, then it is a filter cap, if it is reducing noise in the circuit then it could be a bypass or decoupling.

If it is doing both...then you can call it anything, but late for dinner

 

because the power drawn from battery or USB to my arduino is not ideal, I needed this capacitor.

Correct. And, non-ideal in several different ways.

The wire or pc board trace between the power source and the circuit has inductance. This is an impedance between the source and the circuit, so any current drawn by the circuit is goung to create a voltage drop across the impedance. That voltage will vary with the signals in the circuit, and can either spread throughout the system as noise, or act as an unintended feedback path to the circuit's inputs; this can cause erratic operation, oscillation, etc.

That series impedance also is a problem if the circuit draws large amounts of current quickly. A current spike can cause the apparent power supply voltage to dip down low enough to cause a microprocessor to reboot. An electrolytic capacitor right at the chip power pins acts as a mini holdup battery to keep the chip running through these brownouts.

A fundamental assumption of ALL circuit analysis is that any voltage source is theoretically perfect, with a zero ohm output impedance at all frequencies from DC to daylight. a decoupling capacitor near a device's power pins not only compensates for trace inductance, but also for imperfections in the power source itself. And this is not just for analog parts. Look on the underside of a standard mini-tower pc motherboard and you'll see a pile of small decoupling capacitors under the CPU and most of the other large chips.

ak