Difference between frequency dependent phase shift and dc phase shift

[ericgibbs]

I am puzzled why we seem to want to hang another term like 'DC' in front of the words 'square wave'.

I would say by now most practising engineers know what a square wave is when use in context with waveform types.

It doesn't matter whether its superimposed upon a fixed voltage level or not, its still a square wave.

A sine wave signal doesn't have 'AC' in front of the word sinewave.

As Sigmund said, Sometimes a cigar is just a cigar, so lets keep the terminology simple.

E.

 

[crutschow]

...........................
It looks as though your point is based on the concept that DC is absolute and unchanging, so if DC cannot possibly vary any variance must be classified as "AC". But DC does vary and practically all modern electronics relies on varying DC. So if a battery DC voltage is slowly diminishing is that "AC"?

A logic chip can produce a DC squarewave from 0v to 5v. Would you prefer to be forced to say "The logic chip is producing a DC level of 2.5v with a superimposed 5v AC squarewave"? That's not even accurate as the logic chip cannot even produce a 2.5v DC level, it produces DC levels 0v and 5v only, and any waveform produced is a DC waveform. For the purpose of modern convenience and modern equpment we say "a 5v DC squarewave" and eveyone knows what that is.

.......................

Any change of a DC level has an Fourier AC frequency associated with it, even if it's in the microHertz region.

I don't understand why you say modern electronics "relies" on varying DC.

I have worked as a practicing engineer for 47 years and you are the only one I've known who used the term DC squarewave or DC sinewave so I don't think "everyone" knows what it is.

No, a square-wave from 0V to 5V is not a DC waveform. It is simply a squarewave. As Eric notes, placing an AC or DC term in front of the words sinewave or squarewave is not common or needed.

 

[unclejed613]

Thank you for your input too, I'm interested in hearing more.

I think part of the problem is the term "AC" is pretty much broken in modern usage.

For instance the AC mains is real alternating current as the current passes through the wire in alternating directions.

But for most of my work (<50v DC devices) almost any time I deal with an "AC" signal or "AC component" in a signal it is not AC at all, it is just a varying DC waveform. For instance a sinewave centred on 2v DC with peaks at 3v and 1v DC.

The term "AC signal" is completely wrong as the current does not alternate at all unless driving a reactive load. And "current" is not even right as what I have is a VOLTAGE sinewave, a DC voltage sinewave. Why should I call it an "AC sinewave"? It's very annoying terminology.

I'm for modernisation of broken archaic terms, so "DC sinewave" is fine as would be "DV sinewave" which may even be more accurate as it is a voltage sinewave not a current one.

actually, i would call it "AC with a DC offset", but i'm an amplifier designer and that's how i see it. the fact that it never changes polarity is irrelevant, because it's a simple matter of the use of a coupling cap to get the waveform to do so and eliminate the offset... in fact many amplifier circuits use this method for coupling the AC waveform to the next stage. other common ways of referring to what you describe is "DC with an AC component" or "DC with AC superimposed". more modern usage would actually replace the "AC" with "audio" or "RF", etc... and be more descriptive. when this is done with logic, this becomes "logic level pulses", and we're primarily looking for 1's and 0's, and maybe a clock rate and various delays in timing, but we're no longer calling it AC or DC, even though it has characteristics of both. in many cases the usage determines the terminology. here in the US, a capacitor is just a capacitor, unless it's used with ignition points in an engine, and then it becomes a "condenser" which is an archaic term for a capacitor. use the word "capacitor" and the old school gearhead will look at you as if you are from Mars. to him it is, and always will be a "condenser". to him a capacitor is the beer can shaped thing in a CDI (capacitive discharge ignition) box, and doesn't belong inside a distributor cap. so the terminology is archaic, but it has a meaning for a specific application. the same with "AC or DC", the original meaning is not really useful anymore, but the terms still exist. "conventional current" is still taught to simplify semiconductor theory, even though we all know (since Edison, who ironically was the first to wage a war between DC and AC) electrons flow from negative to positive.

 

[ChrisP58]

It seams to me that we are trying to force things into two categories that were defined long before any of the signals that have been mentioned.

When the terms DC and AC were created, electricity meant only one thing. *Power*. Used to run motors, heat a room, or light up the street.

DC was what came from an electro-chemical process, and AC came from an alternator. (note)

Audio had not yet been transmitted on wires. Even the telegraph hadn't been invented yet.

To me, AC and DC mean power rails. When I design a power supply I take AC from the wall socket, rectify and filter it into DC. Yes, there is some ripple voltage on the DC, but I don't really consider it AC. (Though I do use the AC coupling switch on my scope to look at it.) Then I turn it on and off real fast to get it to go through the transformer. Then it gets rectified again to make it DC.

I don't think of the string of PWM pulses from the controller chip to the gate of the mostet as either AC or DC. It is a signal. And signals all need to be defined in terms that apropriate to the signal.
- The magnitude on an AC signal with a DC offset should be difined in those two terms.
- A 5V square wave is called a squarewave, with a peak voltage of 5V, going to zero V between pulses.


Note: Even DC generators are AC at heart. They use a comutator on the armature contacts to flip the polarity whenever the current in the windings reverse, or rectifiers to do the same thing.

One thing I've always wondered. Does anyone know where the 'direct' in DC came from?

 

[unclejed613]

One thing I've always wondered. Does anyone know where the 'direct' in DC came from?

previously it was known as "galvanic current" referring to current from a "galvanic cell" (what we commonly refer to as a battery, but a "battery" is a collection of cells). the term "galvanic" from the name of Luigi Galvani. galvanic cells were made of copper and zinc plates in an electrolyte solution. i can't find any actual source for the word "Direct Current" but i suspect it may have been coined by Edison (or someone working with him). or it may be derived from "directional" meaning the current only goes in one direction.

 

[Mr RB]

...I don't understand why you say modern electronics "relies" on varying DC.
...

Because largely all the electronics runs from DC, and in many cases a single DC supply. In cases (say a small amp) with a single DC supply and DC coupling pretty much the whole thing is a DC amplifier with only DC waveforms present, apart from maybe AC to the speaker through the one AC coupling output cap (case LM386) as the speaker is the only component requiring actual AC!

I stopped using AC coupled dual rail etc opamps in the 1980's and I'm not even sure if I stock dual supply opamps these days, using single supply opamps for most applications and in most cases DC coupled with only DC waveforms and the scope on DC when checking waveforms (because then you see the waveform and the DC offset) etc. Entirely DC technology.

In the case of power electronics, in the old days we used AC transformers in plugpacks. But most modern plugpacks are essentially DC devices, they convert the mains to 170v DC, then that DC is chopped on and off (which is NOT AC as you and others have stated) and the output is DC pulses filtered to pure DC by a cap.

My point is the world is becoming dominated by DC technology, apart from long distance power distribution and audio amps (but which are now going DC too!).

...
I have worked as a practicing engineer for 47 years and you are the only one I've known who used the term DC squarewave or DC sinewave so I don't think "everyone" knows what it is.
...

That's entirely possible. But have you heard the term "DC ripple", "DC ripple voltage", "DC ripple waveform"? That's definitely a waveform, with a frequency.

...
No, a square-wave from 0V to 5V is not a DC waveform. It is simply a squarewave. As Eric notes, placing an AC or DC term in front of the words sinewave or squarewave is not common or needed.

I respect that it is not always (or generally?) necessary to specify if a waveform is AC or DC, but it can be useful to specify whether the waveforms is DC or AC.

My main point is not about being able to use the term "DC waveform" but the point that "AC" is a misnomer as many of the things we describe as "AC" do NOT alternate, so the "A" is busted, and in many cases we are talking about a voltage and specifically not a current so even the "C" is busted.

...
It seams to me that we are trying to force things into two categories that were defined long before any of the signals that have been mentioned.

When the terms DC and AC were created, electricity meant only one thing. *Power*. Used to run motors, heat a room, or light up the street.
...
To me, AC and DC mean power rails.
...

I agree absolutely, there is a large amount of obsolescence in the terms, especially the term AC which meant a power system where the current alternated in its DIRECTion (ie origin of the term direct current).

If you have a DC opamp DC coupled with a wave input +1v to +2v, and a wave output +4v to +8v there is no "alternating current". It is entirely a DC device and DC process. So using the term AC to descibe anything happening there is not correct, it's a tradition that we were taught which is becoming even less true as technology develops and more things are using less AC tech and more DC tech.

 

[ericgibbs]

That's entirely possible. But have you heard the term "DC ripple", "DC ripple voltage", "DC ripple waveform"? That's definitely a waveform, with a frequency.

hi RB,
I have never heard a practising engineer use any of the above terms, its more like noob web speak.

Its usually stated as an AC ripple on a DC voltage, which covers all the above misnomers.

Also as long as I can remember, electronics has been powered by DC, ie: batteries and rectified mains AC and the terminology is well understood.

E

 

[crutschow]

RB if you want to try to redefine the meaning of AC and DC differently from standard engineering practice, have fun, but the only ones likely to use your non-standard definitions are the amateurs. I assume you just enjoy arguing arcane issues, but if you seriously believe that a change in definition for AC and DC is needed then you are even further "Out there" than I realized.

 

[unclejed613]

In the case of power electronics, in the old days we used AC transformers in plugpacks. But most modern plugpacks are essentially DC devices, they convert the mains to 170v DC, then that DC is chopped on and off (which is NOT AC as you and others have stated) and the output is DC pulses filtered to pure DC by a cap.

My point is the world is becoming dominated by DC technology, apart from long distance power distribution and audio amps (but which are now going DC too!).

so i guess we go back and re-think the whole "War of the Currents" thing. i've been in the electronics field for over 40 years, and have seen some wild concepts on how things work. this is an "interesting" but inaccurate concept. yes, 120Vac gets rectified and then chopped at 20khz (or higher, as some SMPSs work at up to and beyond 1Mhz (but we can't call it khz or Mhz anymore because it's chopped DC?)). this is so we can vastly reduce the size and weight of the transformer. what's on the secondary side of the transformer? AC? high frequency AC, but AC none the less, which needs to be rectified to produce your "pure DC". the thing here is that AC and DC coexist in the real world in various combinations DC can have an AC component (as in ripple), or AC can have a DC component (as in an audio amplifier with offset), or they can be present in equal magnitudes (as in logic levels or SMPS pulses on the primary side of a wall wart).

as for the "that's the way the market and technology is going", that doesn't hold true either. for good sounding audio, the class D amp isn't going to supplant class AB (fed by op amps with bipolar supplies, and the amps themselves being powered by bipolar supplies, because speakers are AC devices) any time soon. sure, the "DC" amplifiers have made a splash, but Big Amps and Big Iron are still flourishing in the audio market today, not because of any high tech whistles and bells, but because they sound good. you just can't get a "digital" amplifier to match (let alone surpass) the sound of a well designed analog amplifier. Denon, Pioneer, Yamaha, Onkyo, and many others are still producing receivers with class AB amps, and selling them in huge quantities, and many of them still have a "pure direct" mode that uses an analog-only signal chain (with bipolar op amps).

i think you are over-complicating a simple concept, and in the process making into an unworkable theory. if you want to re-ignite the "War of the Currents", and claim Edison's position as a technologically sound one, then go right ahead. but it would be advantageous if you re-examine why Edison's vision of how DC power distribution was such a boondoggle. there isn't enough copper in the world to supply house current to every home and business (in the US alone) if DC were to be used for power distribution in our modern society, and putting a power station on every square mile would be a logistic nightmare.

 

[Mr RB]

...
Also as long as I can remember, electronics has been powered by DC, ie: batteries and rectified mains AC and the terminology is well understood.
...

That's perfectly true but does not address the main point I made, that we (with me included as I did AC and DC theory in the classroom almost as long ago as you) use the term "AC" to describe a wavefom that *does not alternate* and we use the term to describe something that is a voltage and not a current!

I've seen some strong criticism from engineering people here when someone in relaxed fashion uses the term "power" instead of "energy" as they are different things, but you seem to be completely supportive of using the term "current" to describe voltage? Or "alternating" to describe a varying DC voltage? Exactly what part alternates?

RB if you want to try to redefine the meaning of AC and DC differently from standard engineering practice, have fun, but the only ones likely to use your non-standard definitions are the amateurs. I assume you just enjoy arguing arcane issues, but if you seriously believe that a change in definition for AC and DC is needed then you are even further "Out there" than I realized.

Haha you KNOW I like arguing the unconventional side of some grey areas especially if the forum is slow.

I'm not arguing a change in definition of AC, I think it is clearly defined as current that alternates. It's a commonly used term that we are comfortable (maybe too comfortable?) with.

Maybe what I'm arguing with is that we can also be comfortable with terminology "DC waveform" or "frequency of the DC waveform". You can argue that is technically wrong, and I respect that as DC generally implies no variance so there is something of an oxymoron. But it is no more wrong that saying a varying DC voltage is "alternating current" which is also an oxymoron.

So i guess we go back and re-think the whole "War of the Currents" thing. i've been in the electronics field for over 40 years, and have seen some wild concepts on how things work. this is an "interesting" but inaccurate concept. yes, 120Vac gets rectified and then chopped at 20khz (or higher, as some SMPSs work at up to and beyond 1Mhz (but we can't call it khz or Mhz anymore because it's chopped DC?)).
...

Thank you for backing up my case. You have given a fine example of the problem. Others in the thread have suggested that a chopped DC etc is not a "frequency" and/or not be described as "Hz". If a SMPS chops 170v DC are we not allowed to say 20kHz? I'm sure Hertz would have been happy for his unit to describe "cycles per second" of waveforms other than just sinewaves.

My point is that DC technology is becoming the standard now even in some of the last bastions of AC like voltage transformation and amplifiers. And we are working with it more and more in situations where the DC is "chopped" or has some specific "waveform" and specific frequencies. This is the modern technology. I have worked with electrical engineers who said "condenser".

 

[RichTheDude]

I think what Mr. RB is trying to say is that technology is moving away from linear and moving towards switched techniques, such as switched mode PSUs rather than transformers, class D amplifiers rather than class a, b, and ab amplifiers.

 

[Mr RB]

I have never heard a practising engineer use any of the above terms, its more like noob web speak.

I was genuinely surprised at that so just did a google search for "DC ripple voltage". The first 2 pages of search provided;
------------------
How to Calculate DC Ripple Voltage | eHow.com
How to Calculate DC Ripple Voltage. In electronics there are two types of current:
alternating current and direct current. Rectifiers are used to change from an ...

Technical Specifications
480 (F.V.:540V). End-Cell Voltage, Vdc/Cell, Selectable from 1,65 to 1,90 (for
VRLA/Wet Cells). DC ripple voltage in float & Const V Ch.mode, %, <1 (RMS
value) ...

Rectifier Filtering
The pulsating DC voltage of a rectifier can be made into a pure DC voltage by ....
filter filtering rectifier capacitor diodes pulsating dc ripple voltage rr0002 ...
www.wisc-online.com/objects/index_tj.asp?objID=SSE4303 - 24k

Simple Control Scheme for DC Output Ripple Voltage Suppression
The DC ripple voltage produces the associated ripple current in the DC-link,
which may flow through into battery, when battery is connected to the DC output.
It is ...
ieeexplore.ieee.org/iel5/9792/30877/01433413.pdf?arnumber=1433413

PULSE MULTIPLICATION IN FORCED-COMMUTATED CURRENT ...
that includes any number of reinjection converters. These are connected to the dc
ripple voltage via separate secondary windings of the reinjection transformer.
works.bepress.com/cgi/viewcontent.cgi?article=1002&context=lasantha...

M10-AD350T
DC Ripple Voltage: 30mV .Input Voltage: 110~127VAC±10%/220~240VAC±10%
.Dimensions: 150(W)X110(H)X215(D)mm .Weight: 4.5kg. M10-AD350T-5 ...
www.mcpsh.com/M10-AD350T.html - 21k -

A Current Reinjection Scheme That Adds Self-Commutation and ...
where the dc-ripple voltage is used as the commutating voltage ... includes
transformer (T2), excited by the dc ripple voltage and a thyristor pair. – . A.
Modified ...
eprints.qut.edu.au/16930/1/16930.pdf -

Reference data for engineers: radio, electronics, computer, and ...
Polyphase power produces less dc ripple voltage, less input harmonic current,
and reduced dc voltage regulation. A power transformer allows for voltage

---------------------
Which includes a couple of datasheets and a university paper. Welcome to the future.

 

[crutschow]

I think what Mr. RB is trying to say is that technology is moving away from linear and moving towards switched techniques, such as switched mode PSUs rather than transformers, class D amplifiers rather than class a, b, and ab amplifiers.

I have no argument with that. But RB has a long row to hoe if he actually thinks he can change the common and accepted usage of the terms DC and AC in the engineering world. I think it'll be a cold day in a warm place before the term DC waveform or its variants becomes accepted by any significant number of engineers to signify a time varying signal, DC offset or not. And right now I'm nice and toasty.

 

[unclejed613]

part of any engineering job is to "communicate concepts and ideas effectively" and you can't do that if you use terminology that's obscure or confusing. the common terminology is, as i pointed out earlier "DC with an AC component", or "AC with a DC component", etc... "chopped DC" is acceptable, but something like "DC waveform" is an oxymoron unless something like ripple or noise is present... but then it would revert to "DC with ripple", which is clearer language.

 

[RichTheDude]

I kept quiet on the nomenclature but I agree with you crutschow .

 

[Mr RB]

I have no argument with that. But RB has a long row to hoe if he actually thinks he can change the common and accepted usage of the terms DC and AC in the engineering world.
...

I don't expect anyone to change based on the simple (and tongue in cheek) points I've put forward.

But in the aim of communication if I'm describing a PIC output squarewave 0v to 5v (especially if communicating to a beginner) then don't be surprised if I go out on a limb to use radical heretic terms like "producing a 5v DC squarewave out of pin4" or even risk the wrath of Hertz by saying "a squarewave at 10kHz".

I'm just not going to say "the PIC is producing is a 2.5v DC signal with 5v AC squarewave overlaid operating at a repetition rate of 10 thousand times a second".

Maybe I was hoping someone would be brave enough and put their hand up and say "Hey, why do we have to say "alternating" if the signal is all DC??".

 

[crutschow]

I don't expect anyone to change based on the simple (and tongue in cheek) points I've put forward.

But in the aim of communication if I'm describing a PIC output squarewave 0v to 5v (especially if communicating to a beginner) then don't be surprised if I go out on a limb to use radical heretic terms like "producing a 5v DC squarewave out of pin4" or even risk the wrath of Hertz by saying "a squarewave at 10kHz".

I'm just not going to say "the PIC is producing is a 2.5v DC signal with 5v AC squarewave overlaid operating at a repetition rate of 10 thousand times a second".

Maybe I was hoping someone would be brave enough and put their hand up and say "Hey, why do we have to say "alternating" if the signal is all DC??".

So with tongue firmly in cheek I will continue....

I think few besides yourself will understand what you mean by a "5v DC squarewave". And the PIC is not producing a 2.5v DC signal so that is certainly not accurate. And if you don't want to use the term alternating or AC for this type of signal then don't. I don't. I would just say "the PIC output is a 0-5V 10kHz squarewave with a 50% duty-cycle" (I think Heinrich Hertz would be ok with that since Hertz is cycles per second of a any periodic phenomenon, not just AC ). Is that so hard?

 

[Mr RB]

That's cool. We'll just outlaw the term "DC" altogether anytime there's a waveform present.

 

[crutschow]

That's cool. We'll just outlaw the term "DC" altogether anytime there's a waveform present.

Unless it's followed by the term "offset".