Phase shifts with 'all pass' filters - understanding sought!

[HankMcSpank]

Following on from a thread created specifically to sort the maths aspect of calculating phase shifts ( Phase shift help (how's your maths?!) - many thanks to all who responded) I've started this thread towards understanding the phase shifts going on with all pass filters.

Now from my understanding, an all pass filter will....


1. Apply the same gain to all frequencies (unity gain)
2. Solely change the phase (the amount of phase shift is frequency dependent)
3. Phase shift the 'centre frequency' by 90° (the centre frequency can be determined with the formula 1/2TT x CR .....to see where C & R fit into the picture, please look at fig 2.4 here... http://sound.westhost.com/pcmm.htm#s22
4. The maximum 'usable' real world phase shift, amounts to approx 120° (source = http://www.maxim-ic.com/appnotes.cfm/an_pk/559 ...the text immediately under the third formula down - phase formula)

Ok, as can be seen there is a non-inverting & inverting variants of all pass filters. The phase shift for a given frequency will depend on the circuit used.

All good so far.

What's troubling me is some of the results the calculations are providing.

For example on the previous thread, using a resistance value of 68K & 10nF, yields a centre frequency of about 234Hz...where the phase will be shifted 90 degrees...*but* (& here's where I'm struggling), wrt to the inverting vs non-inverting .....the former gives -90° shift & the latter +90°?!!

For the non-inverting, does that +90° phase shift really mean the phase is lagging by 270°, ultimately meaning a 90° lead? (If so, then how does this marry with the aformention 120° useable maximum phase shift from an all pass?!)

Can anyone please explain real world +ve phase shifts versus -ve phase shifts wrt all pass circuits?

[lepomis]

From this graph you can see that the non-inverting all-pass filter (green) is phased forward and the inverting filter (red) is phased lagged. Both are 90 degrees out of phase with the signal (blue).

[HankMcSpank]

Thanks, but which graph?

and (pre-empting somewhat)...is the phase lead really just a long phase lag (if you get my drift). For example....for a sine wave, a 270° lag, could also be interpreted as a 90° phase lead?!

[lepomis]

For some reason I can not edit my last post. Its all wrong anyway Hopefully you can see this one. Green is "inverting".... red is "non-inverting" (this is only true at lower frequencies). You can see on the graph that the are both 90 degrees out of phase with the signal. Green is leading and red is lagging.

[lepomis]

It is easy to see the inverting vs. non-inverting in this one using 10hz instead of 234.05 hz.

[HankMcSpank]

Thanks lepomis....as it goes, I've no problem grasping what the lags/leads are, but more related to when is a 'lag' not a lag, but a 'lead'!!!

When it comes to pondering phase changes all pass, if the phase from either the inverting/non-inverting all pass circuit is lagging from 1° thru 180° vs the original ...I'm comfortable with that.

But where do the 'phase leads' come from? Are they simply *BIG* lags, but viewed as leads? (eg a 270° lag can be alternatively viewed as a 90° lead )- do you see where I'm coming from?

So, does an all pass purely lag the phase....?

(PS the reason you couldn't edit your post is, because this forum gives you a time 'window' to nip in there & correct typos etc - but then locks the post after a certain period of time...so just don't go posting stuff yu'd rather not have the internet populace viewing forever!)

[kenpeter]

Caps cause voltage to lag behind current.
Inductors cause current to lag behind voltage.

Time doesn't run backward, except maybe in
DSPs. But even that implies a recorded delay.

[lepomis]

I am pretty ignorant when it come to electronics, especially phase stuff, but with this circuit having only one capacitor.... I don't think you can get a big phase lag. Hopefully someone here can correct me if I am wrong... or at least can give you some more insight.

Originally Posted by HankMcSpank

Thanks lepomis....as it goes, I've no problem grasping what the lags/leads are, but more related to when is a 'lag' not a lag, but a 'lead'!!!

When it comes to pondering phase changes all pass, if the phase from either the inverting/non-inverting all pass circuit is lagging from 1° thru 180° vs the original ...I'm comfortable with that.

But where do the 'phase leads' come from? Are they simply *BIG* lags, but viewed as leads? (eg a 270° lag can be alternatively viewed as a 90° lead )- do you see where I'm coming from?

So, does an all pass purely lag the phase....?

(PS the reason you couldn't edit your post is, because this forum gives you a time 'window' to nip in there & correct typos etc - but then locks the post after a certain period of time...so just don't go posting stuff yu'd rather not have the internet populace viewing forever!)

[Key]

I don't know how to explain this better except that one does a 90 degree phase lag and one does a 90 degree phase lag and then inverts the *polarity* of the signal.

[HankMcSpank]

Originally Posted by kenpeter

Caps cause voltage to lag behind current.
Inductors cause current to lag behind voltage.

Yes, I know that - but we're comparing a 'signal voltage' against a 'signal voltage' here (ie input to circuit vs its output)

Originally Posted by Key

I don't know how to explain this better except that one does a 90 degree phase lag and one does a 90 degree phase lag and then inverts the *polarity* of the signal.

Bada bing! Doh... I can't believe i missed that *obvious* one!

With that in mind...I'm off to tap some examples into my spreadsheet!

Thanks!