Difference between frequency dependent phase shift and dc phase shift

[Manuv16589]

Hi,
While analysing oscillators i was completly undone by frequency dependent phase shift. Can somebody explain it with example. I think dc phase shift is signal inversion. Correct me if i am wrong.

 

[Nigel Goodwin]

I don't see how DC could have a 'phase shift' it's not applicable to DC.

 

[WTP Pepper]

DC phase shift is technically known as an inversion i.e. you are using an inverting amplifier so for an amplifier with a gain of say -2 will produce -2V for a +1V input. As the frequency increases, the phase will change from the minus value.

 

[Winterstone]

Hi,
While analysing oscillators i was completly undone by frequency dependent phase shift. Can somebody explain it with example. I think dc phase shift is signal inversion. Correct me if i am wrong.

I must confess that I never have heard about a "dc phase shift". When you look into the definition of "phase" you always will end up at the frequency domain.
However, I think this is not to important. It does not matter at all if you use the phrase "dc phase shift" to distinguish between a positive and negative dc value.
Keep in mind that a "real" phase shift always is generated by components having a frequency dependent impedance.
In particular for oscillators it is important that there is only one single frequency that produces a loop gain with a phase shift of zero degrees.

 

[Nigel Goodwin]

I must confess that I never have heard about a "dc phase shift".

Likewise, it's sounds a ludicrous suggestion - inversion doesn't bear the slightest resemblance to phase shift.

 

[WTP Pepper]

Likewise, it's sounds a ludicrous suggestion - inversion doesn't bear the slightest resemblance to phase shift.

It does once that DC starts moving for whatever reason and whatever rate.

 

[Nigel Goodwin]

It does once that DC starts moving for whatever reason and whatever rate.

If it's moving it's not DC any more

 

[Winterstone]

DC phase shift is technically known as an inversion i.e. you are using an inverting amplifier so for an amplifier with a gain of say -2 will produce -2V for a +1V input. As the frequency increases, the phase will change from the minus value.

The frequency of a dc signal?

 

[Mr RB]

A logic output making a squarewave freq is usually considered a "DC frequency" as compared to an AC sinewave that goes +/- referenced to ground.

 

[Winterstone]

A logic output making a squarewave freq is usually considered a "DC frequency" as compared to an AC sinewave that goes +/- referenced to ground.

Hello Mr RB,

thanks for your information - however, I have some problems with the term "DC frequency". Is it really "usually considered" as a synonym for the repetition rate of a squarewave? Please, can you give any reference?
According to my knowledge, the term "frequency" is given in Hz and applies to sinusoidal signals only.
All other periodic waves should be described using the term "repetition rate".
Regards

 

[Mr RB]

Hi Winterstone, sorry nothing official I think it's just a sign of being firmly in the microcontroller age. Analogue electronics (ie that generate a +/- AC sinewave) are only one way of making a "frequency" and becoming much less popular for making frequencies than microcontrollers and digital.

It is standard terminology in microcontroller terms to generate a "frequency" by turning a digital pin on/off, and the output would be a DC digital waveform either 0v or 5v.

If you wanted a reference almost any digital datasheet should contain references to "frequency" of events or digital clock waveforms or digital output signal etc.

As far as reserving the term frequency only for +/- AC sinewaves that seems pretty silly to me, "frequency" means "how many times something happens". It's just as applicable to a DC squarewave or even the frequency that a data packet is sent at etc. Where did you hear that frequency can only be used for +/- AC sinewaves?

 

[crutschow]

I agree that the term "frequency" is fine for digital signals, however I would not call it an AC frequency but rather a pulse frequency or a clock frequency. (I don't much care for the term DC frequency either. Seems like an oxymoron to me)

 

[Winterstone]

Hi Winterstone, sorry nothing official I think it's just a sign of being firmly in the microcontroller age. Analogue electronics (ie that generate a +/- AC sinewave) are only one way of making a "frequency" and becoming much less popular for making frequencies than microcontrollers and digital.
It is standard terminology in microcontroller terms to generate a "frequency" by turning a digital pin on/off, and the output would be a DC digital waveform either 0v or 5v.
If you wanted a reference almost any digital datasheet should contain references to "frequency" of events or digital clock waveforms or digital output signal etc.
As far as reserving the term frequency only for +/- AC sinewaves that seems pretty silly to me, "frequency" means "how many times something happens". It's just as applicable to a DC squarewave or even the frequency that a data packet is sent at etc. Where did you hear that frequency can only be used for +/- AC sinewaves?

Dear Mr. RB, thanks for the explanation. I am not involved in digital and MC electronics (I think, feel and design analog only) - this may be the reason for my doubts.
As another possible reason, I am from germany (the country Heinrich Hertz comes from) - perhaps we are here somewhat more "narrow" regarding this particular point.
Thanks and regards
winterstone

 

[unclejed613]

if you really want to get "technical", even square waves are made up of sine waves at the fundamental and various harmonics. changing the phase relationship or amplitude of the various harmonics changes the wave shape. changing the waveform to a logic level is simply the addition of a DC component. a logic level, can also be DC most of the time, and an AC component added when it changes...
i agree "DC phase" is an oxymoron... maybe the OP was talking about a shift in DC level at the output of certain types of oscillators?

 

[Mr RB]

Dear Mr. RB, thanks for the explanation. I am not involved in digital and MC electronics (I think, feel and design analog only) - this may be the reason for my doubts.
As another possible reason, I am from germany (the country Heinrich Hertz comes from) - perhaps we are here somewhat more "narrow" regarding this particular point.
Thanks and regards
winterstone

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.

 

[Winterstone]

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.

Mr RB, just for my understanding of your criticism: I assume your last sentences are related to the system you have described with the first two sentences, right?
But in general, I agree with you that - sometimes because of historical reasons - some daily used terms ("archaic" terms) are contradictory. As for example "AC voltage".

Regarding contradiction - in particular, I like the following sentence: In order to function properly a linear oscillator must contain a non-linear element.

 

[crutschow]

The term "AC" is broken in the sense that, in signal applications, it often refers to an alternating voltage or pulsating current rather than an alternating current. In that case it doesn't matter whether it is riding on a DC voltage or not. Perhaps the term AV would be more appropriate. And I consider the term DC sinewave a contradiction in terms since DC is commonly understood to be a voltage or current that is not time-varying (and we often add all kinds of filtering to minimize any time-varying part).

But since the term AC is commonly understood to be equivalent to AV, I will just stay with the term AC for all time-varying signals. As far as I see, that is not really confusing, unless you want to be pedantic about it (and I know a few on this forum are ).

 

[Mr RB]

Winterstone; Yes the last two sentences are a bit of humour regarding the more serious issue that "AC" as a term really is not correct for a lot of stuff (the majority?) that we use the term "AC" for.

Crutschow; You said "an alternating voltage ... it doesn't matter whether it is riding on a DC voltage or not."
I get that it is standard terminology to say there is "an AC sinewave riding on a DC voltage" but even "alternating voltage" in your example is not really correct is it? To me "alternating" means ig goes in alternate directions, ie + and -. Like the mains is an "alternating voltage".

But a 2v p/p sinewave riding on a 3v DC offset is not "alternating" at all, the sinewave is all + and there is no "alternation". It's varying DC voltage with the variance in the shape of a sinewave. So it's a DC sinewave.

These days the vast majority of the electronic items run on DC, even +/- power supplies are getting rare and mainly seen in power amps and some instrumentation. Apart from large scale mains power distribution we are really living in a DC world now as far as devices go. Computer devices make DC squarewaves, DC PWM waveforms, filter them and get DC waves of any shape including sinewaves etc. Even mains transformers are getting rare, the first thing all the new plugpacks etc do is convert the AC mains into DC then chop the DC and feed the DC PWM squarewave into the SMPS transformer.

 

[crutschow]

.........................
Crutschow; You said "an alternating voltage ... it doesn't matter whether it is riding on a DC voltage or not."
I get that it is standard terminology to say there is "an AC sinewave riding on a DC voltage" but even "alternating voltage" in your example is not really correct is it? To me "alternating" means ig goes in alternate directions, ie + and -. Like the mains is an "alternating voltage".

But a 2v p/p sinewave riding on a 3v DC offset is not "alternating" at all, the sinewave is all + and there is no "alternation". It's varying DC voltage with the variance in the shape of a sinewave. So it's a DC sinewave.

I'm know I'm probably beating a dead horse here but I'm a glutton for punishment.

In normal engineering practice the DC level of a time-varying signal is usually treated as a separate entity and ignored. Thus whether it is riding on 1mV or a million, the time-varying part is AC. The DC will do whatever the DC is there for (typically to provide power to something) and the DC is blocked to extract the AC signal for whatever its purpose is. (Running the time-varying signal through a DC block capacitor will always give equal area plus and minus excursions or AC). You are trying to complicate something that is not.

And if you want to keep using the term DC sinewave, go for it, but I don't think you'll have many followers for that oxymoron.

 

[Mr RB]

I understand everything you said and already knew what you were saying.

And as much as I'm being tongue in cheek in these posts, I don't see the term "DC sinewave" as such a problem at all. People get fussy about sinewaves but it's just a shape.

If there was a squarewave peaking from -1v to +1v that is most definitely an AC squarewave. But a squarewave going from +1v to +3v is easily considered a "DC squarewave".

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.

The shape of a wave does not specify whether something is AC or DC, what specifies AC is when the voltage *alternates* ie it goes + and then -. The term alternation comes from the very early AC days (Tesla, Westinghouse etc) when people had only known DC and now there was a current that "alternated" its direction.