# Is this wave an AC or DC..

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#### demetal

##### Full Member level 5
Hi,

I know this is a stupid question... but i am still confused about dc and ac... here are two wave forms... i want to know what type of (AC or DC) wave is it....

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#### demetal

##### Full Member level 5
i know basic definitions about AC and DC but still a confusion regarding square waves.....?

#### alexan_e

But the link I gave you explains this very well, it even has a triangle AC to show that AC has nothing to do with the actual shape.

An AC voltage is continually changing between positive (+) and negative (-).
A DC voltage is always positive (or always negative), but it may increase and decrease.

Clearly you have a signal that goes from positive to negative and back again and another that is constantly positive

Alex

#### ninju

##### Full Member level 3
Lets put it in a simple way.

AC signals have equal areas in negative and positive cycles. Duration of positive and negative cycles may be different.

If any signal doesn't satisfy the above, then its a combination of AC and DC.

#### alexan_e

Lets put it in a simple way.

AC signals have equal areas in negative and positive cycles. Duration of positive and negative cycles may be different.

If any signal doesn't satisfy the above, then its a combination of AC and DC.

Are you saying that if his first signal had a duty cycle of 10% or 20% instead of 50% it wouldn't be called an AC?
I don't agree with that.

Alex

#### ninju

##### Full Member level 3
i said equal areas. If it had a duty cycle of 10 or 20 % but had a correspondingly larger amplitude during that duration, it will be a pure AC signal.

Try getting fourier series or a fourier transform and check for yourself.

#### demetal

##### Full Member level 5
Now i have got another doubt.... in digital communication do we use AC signal or DC signal...? also what type of Signal (AC or DC) will we get after the fourier transform or such transforms.... please help... it seems that my engineering skills are still poor....

#### KerimF

##### Advanced Member level 4
Now i have got another doubt.... in digital communication do we use AC signal or DC signal...? also what type of Signal (AC or DC) will we get after the fourier transform or such transforms.... please help... it seems that my engineering skills are still poor....

In digital systems we don't see signals as AC and DC, we consider them as true/false, high/low or 1's/0's etc... These signals could be AC only (as amplitude modulation of a carrier), DC only (as the 0V-5V between CMOS logic ICs) and both AC-DC (as the RS232 signals +/- 12V for example).

In brief:
A signal that has a zero average value is said to be AC only.
A signal that its amplitude doesn't change its sign is said to be DC only (constant or varying).
A signal that doesn't have a zero average value and its sign changes, is said to be a combination of AC and DC.

Kerim

#### ninju

##### Full Member level 3
By digital communication, if you mean communication inside a wired digital system, its just DC. Any digital system is just a network of switches that switch on and off in a particular manner so as to do what ever function is required. But, any Dc signal switching that fast will not be a pure DC signal and will have plenty of transient components in it. If you look at the out put pin of even a simple and gate when it switches, with an oscilloscope, you'll not find an exactly clean transition there.

The output of fourier transform gives the frequency components of the wave on which its applied.

#### demetal

##### Full Member level 5
In digital systems we don't see signals as AC and DC, we consider them as true/false, high/low or 1's/0's etc... These signals could be AC only (as amplitude modulation of a carrier), DC only (as the 0V-5V between CMOS logic ICs) and both AC-DC (as the RS232 signals +/- 12V for example).

In brief:
A signal that has a zero average value is said to be AC only.
A signal that its amplitude doesn't change its sign is said to be DC only (constant or varying).
A signal that doesn't have a zero average value and its sign changes, is said to be a combination of AC and DC.

Kerim

Thank for the information.... so can you please tell what will be the difference between (i,e; different effects) the full wave as shown in figure 1 and the half wave in figure 2 posted above in digital communication....

#### Miguel Gaspar

##### Advanced Member level 1
Well . . .. . . if you use a waveform generator . . and you see a pulse train of one signal and after teh second one.
For the first signal: DC component = 0, and has Ac signal It is an AC sinal
If you use a Voltmeter in DC it will gives you 0. In AC tha Voltmeter will gives your the RMS value of the square signal.

For the second: DC coponent >0 and AC >0 It is a complex signal = DC signal and AC signal
If you use a Voltmeter in DC it will gives you half the maxium value. In AC tha Voltmeter will gives your the RMS value of the square signal.

#### KerimF

##### Advanced Member level 4
The first (figure 1) has a zero average value while the other (figure 2) does have an average positive value.
So when the digital link cannot transmit a DC signal (actually AC with DC), then figure 1 (AC only) SHOULD be used that is the transmitted/received signal should have zero average.
On the other hand, it is always simpler to use a single supply in a circuit but this lets the signals swing in a positive or negative range only (0 and 5V for example). In this case, even if the received data is AC (or AC with DC) it should be limited to the proper range say 0 to 5V for example.

In brief, there is no signal better than another in general, but there is a signal that fits better a specific transmission line of a system than others...

Kerim

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#### thannara123

##### Advanced Member level 4
the first one is AC and second one is Dc supply.there is no doubt for me

#### Syncopator

##### Full Member level 6
... this is a stupid question...
No, it isn't. That criticism, could, however be applied to may of the answers we see on this forum.

... but i am still confused about dc and ac... here are two wave forms... i want to know what type of (AC or DC) wave is it...
Think of the definition of d.c. and a.c.

d.c. flows in only one direction. a.c. alternates its direction, i.e. it changes direction repetitively.
It doesn't matter what the waveform of the current (or voltage) is, if it doesn't change direction it's d.c. If its direction alternates, it's a.c.

While it's true that a d.c. with ripple, for instance, can me shown to consist of the sum of a d.c. and an a.c., that's academic, and does nothing to help you.

#### KerimF

##### Advanced Member level 4
While it's true that a d.c. with ripple, for instance, can me shown to consist of the sum of a d.c. and an a.c., that's academic, and does nothing to help you.

Just to be sure I understood your definitions.
What about the waveform of the current (or voltage) that does change direction AND has an average value, is it called an a.c. signal irrespective of its DC component?

#### lostinxlation

##### Advanced Member level 3
By digital communication, if you mean communication inside a wired digital system, its just DC. Any digital system is just a network of switches that switch on and off in a particular manner so as to do what ever function is required. But, any Dc signal switching that fast will not be a pure DC signal and will have plenty of transient components in it. If you look at the out put pin of even a simple and gate when it switches, with an oscilloscope, you'll not find an exactly clean transition there.

The output of fourier transform gives the frequency components of the wave on which its applied.
No, internally, they are a series of AC circuits charging and discharging the capacitors such as transisor gates and routing metals and keeps changing the direction of the current. For example, look at what a clock does to the nets on clock tree.

#### demetal

##### Full Member level 5
No, internally, they are a series of AC circuits charging and discharging the capacitors such as transisor gates and routing metals and keeps changing the direction of the current. For example, look at what a clock does to the nets on clock tree.

is the clock signal an AC......?

#### lostinxlation

##### Advanced Member level 3
A clock alternates the current direction periodically. AC stands for Alternate Current which defines what AC is.

#### ninju

##### Full Member level 3
No, internally, they are a series of AC circuits charging and discharging the capacitors such as transisor gates and routing metals and keeps changing the direction of the current. For example, look at what a clock does to the nets on clock tree.

In digital circuits, they are meant to be DC. Due to parasitic capacitances, there can be temporary overshoots and undershoots.I don't see how an 'AC' signal is used in any digital circuit. There can be AC generated accidentally, such as between interconnects and stuff.

In any cmos circuit, charging is through one path and discharging is through another.So, i don't see current flowing in two opposite directions.

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