AC or DC signal?

Thread Starter

k1ng 1337

Joined Sep 11, 2020
85
Hi what are the conventional definition for these common waveforms and their polarities?

- sine
- square
- triangle
- sawtooth
- pulse

On many schematics as well as datasheets I see an oscillator's waveform drawn without the horizontal x axis leaving me to wonder if it's an AC or DC signal. While playing with dual supply op amp oscillator circuits my scope reveals interesting data ($50 scope) although I have no definitive data about if what I'm seeing is really AC or DC.

On a side note.. I harvested the passive element of an active piezoelectric buzzer to confirm an AC signal and I made the following observations with AC then DC square wave across the buzzer

1) AC produced an audible tone as expected around 440hz A

2) DC produced a very faint hissing noise, this leads me to believe there is really an AC voltage across the element

One theory describes a square wave as a sine wave with an infinite number of odd harmonics which would seem to make sense in regards to #2

So to bring this all together, how is a signal type identified and what are the conventions for each type?

Thanks :)
 

bertus

Joined Apr 5, 2008
21,364
Hello,

Any signal that is passing 0 Volts is AC.
Any signal that is only positive or negative is DC.
There can be a signal added to a DC signal in such a way that the signal still does not pass the 0 Volts line.

Bertus
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
85
Hello,

Any signal that is passing 0 Volts is AC.
Any signal that is only positive or negative is DC.
There can be a signal added to a DC signal in such a way that the signal still does not pass the 0 Volts line.

Bertus
The problem is my voltmeter shows non zero values for some signals on both it's AC and DC setting, and depending on the AC or DC setting of my scope, it adjusts the zero line leaving me to conclude:

1) I'm not measuring correctly
2) junk testing tools
3) signal has an AC and DC component
 

MrChips

Joined Oct 2, 2009
23,515
The terms DC and AC can be very confusing.
There are two common usage of the terms which are very different and one needs to know which case applies.

In the first usage, one labels the voltage from a battery as DC and the voltage from the electric utility as AC. With DC, one assumes that current flows in one direction and never reverses direction. In AC, the direction of the current will reverse at times.

In the second usage, all signals are DC + AC when viewed from a frequency perspective. For example an AC signal (using the definition from above) might have an offset voltage, i.e. the signal is not symmetrical about the 0V reference. Conversely, a battery or DC supply might have some ripple on the DC output voltage.

The frequency spectrum of any and all signals extend from 0Hz to ∞.

One can examine the frequency spectrum in terms of frequency components from 0Hz to ∞.
The component at 0Hz is considered DC.
All other components are considered AC.

This is the context that is used when analyzing signals on an oscilloscope. When the input channel is set to DC input, all DC + AC components can be viewed. When the input channel is set to AC, the 0Hz component has been removed via a high-pass filter and only the >0Hz components can be observed.

As an example, a DC power supply might have 10mV ripple superimposed on 10V output.
One calls the PSU a DC power supply. From a frequency spectrum perspective the output of the PSU is DC + AC.
In order to examine the waveform on an oscilloscope, one sets the channel input to DC when one ones to measure the DC output voltage. In order to measure the ripple voltage, the channel input must be set to AC.

Without knowing how a DMM is designed, the DMM is a poor choice for measuring DC + AC waveforms. In other words, don't fault the instrument.
 

Audioguru again

Joined Oct 21, 2019
3,173
A piezo buzzer is a piezo transducer (speaker) with an oscillator circuit inside it which is powered from DC.
A piezo transducer does not pass DC because it acts like a capacitor.

A voltmeter and oscilloscope have a high input resistance then its red probe and wire is an antenna that picks up interference.
 

MisterBill2

Joined Jan 23, 2018
8,741
A voltage measured on your meter can easily have both AC and DC components. If the variations are in a repeating form then there is AC, if the voltage is not changing then it is DC, And iif the average value does not fall on the line on the scope when there is no input, then there is both AC and DC. ALL three conditions are quite common.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
85
So how then how do engineers go about identifying signal characteristics starting from scratch? If every instrument introduced into the circuit adds some noise how can we be sure of what we are dealing with?

For example, is signal X AC or DC?

Based on theory I could think of of two ways to confirm AC, a transformer and a piezoelectric material
 

wayneh

Joined Sep 9, 2010
17,152
So how then how do engineers go about identifying signal characteristics starting from scratch? If every instrument introduced into the circuit adds some noise how can we be sure of what we are dealing with?

For example, is signal X AC or DC?

Based on theory I could think of of two ways to confirm AC, a transformer and a piezoelectric material
An engineer knows better than to try to categorize every signal into one of only two buckets. There are many varieties and gradations, and whether you use the terms AC or DC often depends more on the context and application than on the signal itself.

Precise communication requires precise language and confining yourself to the simple terms DC or AC limits yourself.
 

MrChips

Joined Oct 2, 2009
23,515
So how then how do engineers go about identifying signal characteristics starting from scratch? If every instrument introduced into the circuit adds some noise how can we be sure of what we are dealing with?

For example, is signal X AC or DC?

Based on theory I could think of of two ways to confirm AC, a transformer and a piezoelectric material
We avoid using the terms DC and AC when describing a complex signal. We characterize the signal by its frequency components.

Mathematically we can model a signal as,

v(t) = a0 + a1.sin(ω1.t + φ1) + a2.sin(ω2.t + φ2) + ....
 

MisterBill2

Joined Jan 23, 2018
8,741
The other advantage that engineers have in knowing about a signal or a voltage is based on the whole of the engineering education. One does not know what a house looks like based on the appearance of one brick, and likewise, after one has completed their education, one does not often deal with voltages found in free space, devoid of context. A voltage devoid of a circuit, either actual or intended, is a math concept and seldom needs consideration. In the real world it tends to be clearer, as a voltage is found in a circuit, and that circuit usually has a great effect on what the voltage is.
And as for noise, that is usually defined as a random component of a voltage, and thus it is fairly obvious.

A very common signal combining DC, AC, and noise is found in the output of mains powered supplies, comprising a llarge DC component, some AC ripple voltage, and a small bit of noise voltage that has passed through from the mains, or has been added by noisy resistors and rectifiers. The value never passes through zero voltage, but maintains some desired amplitude away from zero.
 
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