Please note these words carefully

the unloaded Q

That is what I am driving at in post 11.
Both the transistor and the load are in parallel with the tank.

 

It's like when I cringe when I see students reaching for a calculator to determine the single voltage step of an 8-bit DAC with a 2.56V reference.

I would expect a student (defined as one who is learning the craft) to work this out the first time, maybe several times before the braincell fires and they perceive the pattern and understand why manufacturers would offer 2.56V or 4.094V references.

The brightest ones see it for themselves. The rest may need to be shown the "why."

 

Wow. Some of you need to read your own posts. What a bunch of adult chidren.

I see no absolutely no value in this forum.

Good bye.

 

Wow. Some of you need to read your own posts. What a bunch of adult chidren.

That's rich. You come here whining about not being able to find someplace that will feed you formulas because you don't want to be bothered having to thing, and when people don't trip over themselves to stroke your ego you go stomping and pouting away.

I see no absolutely no value in this forum.

Not surprising, since you want everything handed to you on a silver platter and this forum is reluctant to do that, instead encouraging people to actually learn something. So it is fundamentally not a match for your tastes.

Good bye.

Good bye. If and when you want to learn something, feel free to come back.

 

I like that. Never heard of "calculator monkey" or "equation monkey" before. I have to add that to my vocabulary.

It's like when I cringe when I see students reaching for a calculator to determine the single voltage step of an 8-bit DAC with a 2.56V reference.

Yeah, it's a take-off of people that are code monkeys who -- to all intents and appearances -- use the Shakespearean method to producing code.

 

Wow. Some of you need to read your own posts. What a bunch of adult chidren.

I see no absolutely no value in this forum.

Good bye.

Nice knowing you.

 

We have a whole generation who have become computer cripples. Funny, the story on national news was how commercial pilots have no flying skills because of their dependence on automated controls.

Agreed. I've seen grads make some very stupid mistakes.

They forgot to teach them common sense.

 

Agreed. I've seen grads make some very stupid mistakes.

They forgot to teach them common sense.

Yes and no -- as with all things, it spans the gammut between teachers who, themselves, have never worked in the real world and therefore are very short on real-world practical common sense, to people that have a tremendous wealth of practical experience but have a really hard time passing that along because the students have little to no practical context on which to hang that knowledge. I'll be pointing out things in class and see a number of heads nodding as if they seem to be grasping the significant and yet those same students will show no awareness of it at all come the next assignment or exam. It's just words to them and so they can't hang onto it.

It's a huge and growing problem. At the same time that students coming into engineering programs increasingly have little or no hands-on experience with anything much beyond a game console, the schools are trying to find every way they can think of to eliminate the hands-on lab experiences due to the high cost associated with them and believe they can replace that with simulation-only "labs".

The good news is that there is some hope with the increasing availability of affordable (e.g., approx one textbook price) tools such as Digilent's Analog Discovery or National Instruments myDAQ tools. These at least hold the potential for giving students some meaningful hands-on experiences and have the added benefit of being tools that the student owns and can play with at their leisure. But the hurdles are still there. It's not enough to have the hardware or even some prepackages labs -- since the students are approaching it without much in the way of supervision and support, the labs must be extremely well put together and a lot of support material made available as well as some faculty/TA time dedicated to supporting things, otherwise the students will largely flounder.

 

There is no problem with ragwire's equation.
By definition, at resonance

Q = R/ωL

So R = Z =ωLQ since at resonance the impedance is resistive.

I`ve got the impression that the formula mentioned in post#1

Z = 2*pi*f*L*Q

should be applied as a FUNCTION for Z=Z(f).

Thus, it is sufficient to proove the correctness of that formula for one single frequency (resonance) only?
What about very large frequencies?

 

I`ve got the impression that the formula mentioned in post#1

Z = 2*pi*f*L*Q

should be applied as a FUNCTION for Z=Z(f).

Thus, it is sufficient to proove the correctness of that formula for one single frequency (resonance) only?
What about very large frequencies?

The fairly strong implication from the first couple of posts by the OP were that he was talking only about the impedance at resonance -- but it appears that that is one of the little caveats about his memorized formula that got lost in the intervening years.

This equation is simply a convoluted way of writing

Z=R at resonance

but doing it in terms of L, C, and Q parameters (specifically, leaving out explicit use of R).

It works neither above or nor below resonance.

 

The fairly strong implication from the first couple of posts by the OP were that he was talking only about the impedance at resonance --

I think, in his first post he spokes abot "impedance calculations for parallel resonant circuits".
That`s all.

 

Don't you think that what some soap box artist may or may not have meant is not worth the argument?

The actual method using Q can be quite useful and is worth discussing.

But Q varies with frequency and loading.

This is the formula in John F. Rider's V.T.V.M. book

I would guess that this was from the era when VTVMs had a high enough impedance to not disturb the circuit operating conditions - that is they did not load the tank. In fact there were many Q meters based on this.

In those circumstance the formula was valid.

 

I think, in his first post he spokes abot "impedance calculations for parallel resonant circuits".
That`s all.

But look at his second post (#3). He talks about "at the resonant frequency" and "a resonating parallel LC circuit".

 

Don't you think that what some soap box artist may or may not have meant is not worth the argument?

Yes - of course, you are right. On the other hand - from time to time - it is perhaps not only a waste of time to have a discussion why or under which circumstances a formula is correct.
For example, the question
* what is Q (are there different definitions?) , and
* if it really changes with frequency (as mentioned by you)
may be interersting to discuss.

 

* what is Q (are there different definitions?) , and
* if it really changes with frequency (as mentioned by you)
may be interersting to discuss.

Well Q is always resistance over reactance, but the trick is which resistance and which reactance.

Edit see post#50. This should be reactance over resistance. Sorry.


for instance see here for simple cases.

http://www.qsl.net/va3iul/Impedance_Matching/Impedance_Matching.pdf

Note also that I gave a reference to a more comprehensive source in post#11

 

OK. I'll bite. Anyone interested should go to: www.tubebooks.org/Books/rider_VTVM.pdf

Book page 238, pdf reader page 244 onward.

Now let me say this, (Italics will not turn off, by the way) I re read my original posting and I cannot for the life of me understand what has so severely offended some of you, not can I see what was unclear about my suggestion for a possible improvement.

I did not say to remove anything from the website, but it is obvious that some of you, somehow missed the context of my postings and were defensive. Sorry if I could have written it differently. I am surprised about the discussion of this simple, and I think, self explanatory equation.
Once again--it is a useful equation that fits in with all the other equations. I am surprised that there is even a discussion about what it means, or that many seem to have never have used it. Yet I have been spoken to in ways that, in person, would probably invite a punch in the mouth.Of course I was leaving the forum--who wouldn't?

Now I have been accused of not understanding this little equation by those who are arguing over its meaning and use. Who doesn't understand it, me or the accusers?

studiot. Read my posting. I never said it could be used to figure out loaded Q when the secondary of a tuned transformer is under load--not by itself. Get that through your head this time. Also, what part about having just joined this forum makes you think that I am new to studying the websiter or new to circuits.

WBahn: I feel that you are the one with the bruised ego becuase you never understood this equation, nor have heard of it, yet you accuse me of not understanding it and attacked me with all sorts of things--for no reason at all. I do not suffer fools gladly, and will not suffer you, either. Are you sure you are fit to teach?

 

studiot. Read my posting.

Well Well Well.

I did read you posting and I actually answered your original question (why) directly.

Your original question boils down to

Why do we not use this formula more today?

What's with the page of math on the impedance calculations for parallel resonant circuits?

Why not just use the old (apparently, lost knowledge?) formula of Z = 2*pi*f*L*Q?

I credited you with the knowledge of this but also observed that the formula was developed in an era when valves and valve circuitry were prevalent. Techniques appropriate to valve circuits are not necessarily appropriate to semiconductor circuits.

It is a pity that you have not followed your own advice, and read my posts properly, as you would have found much support and information you claimed was lacking in today's literature.

 

Techniques appropriate to valve circuits are not necessarily appropriate to semiconductor circuits.

Tubes or not is not relevent to finding the impedance in this way.

It is a pity that you have not followed your own advice, and read my posts properly, as you would have found much support and information you claimed was lacking in today's literature.

I do have some good, modern books. I refer to them a lot, but often find the one thing I am looking for missing. I am a hobbyist, not an EE. (I am an Engineer, but not an EE.) I was suggesting, for hobbyists like me, for a simpler approach to reference, but I see that considering any suggestions at all would be tantamount, in some minds, to failure.

And what's so hard about Z = ωLQ, anyway? If you have a loaded Q, put that into your goddam calculator. (Yes, I found the symbols table at last.)

 

Given a parallel resonant circuit consisting of R, L and C all in parallel (L and C considered to be ideal and having no parasitic resistance; figure 12.5 on page 240 of the Rider book), if Q is defined as Ragwire did in post #3:

L is in Henries. Just multiply the right side of the old XL equation by the Q of the circuit. Q here is the ohms of inductive reactance at the resonant frequency divided by its internal series resistance. (Assuming a negligible resistive loss of the capacitor) this is a good approximation, I believe, for finding the Z, in ohms, across a resonating parallel LC circuit for figuring load and therefore gain of amplifier.

This is the formula in John F. Rider's V.T.V.M. book, and was the way I was taught.
I guess that the authors of the websites I've been to do not know the simple form equations?

then the formula Z = 2*pi*f*L*Q is incorrect. The inductor in figure 12.5 of the Rider book has no internal series resistance. For this case, Q must be defined as studiot did in post #14:

There is no problem with ragwire's equation.
By definition, at resonance

Q = R/ωL

So R = Z =ωLQ since at resonance the impedance is resistive.

Hence my comment about the equivalent parallel resistance (impedence) of my tank circuit in my previous post.

However, the reason one finds a bunch of math associated with parallel resonant circuits is because more typically the losses in the circuit are primarily in the inductor, although for full generality some parasitic resistance can be attributed to both the inductor and the capacitor; see:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/parres.html

Also, for a parallel resonant circuit, there can be more than one definition of resonance.

One definition is that resonance is taken to be the frequency where the terminal impedance is a pure resistance (zero phase angle).

Another definition is the frequency where the impedance magnitude is a maximum.

If the circuit Q is high, these two frequencies are nearly the same, but for the low Q case they can be rather different.

For the case where the losses in the circuit are represented by a single resistor R in parallel with an ideal inductor and capacitor, and resonance is defined as the frequency where the impedance of the circuit is a pure real resistance (zero phase angle of the impedance):

For the general case where both the inductor has a equivalent series loss resistance RL and the capacitor has a equivalent series loss resistance RC with no separate resistor R in parallel with L and C, these two resonance frequencies are given by somewhat complicated expressions:

If the only loss in the circuit is the equivalent series resistance of the inductor, then the relevant expressions for the resonance frequency can be obtained by setting RC to zero in the above more general expressions (this is for the case where resonance is defined as zero phase angle of the impedance):

Notice that if Q is defined the way Ragwire did in post #3 (as the Q of the inductor by itself), then we have an expression for Z involving the inductor Q.

There's no escaping these complicated expressions if the losses are not just a single resistance in parallel, and one wants an accurate result in the low Q case.

But for the high Q case, and if one just wants a fairly good approximation AND if the circuit losses can be represented as a single parallel resistor with reasonable accuracy, then just use the simple expressions:

But if the losses are primarily in the inductor, then one needs to use the more complicated expression.

 

Also, for a parallel resonant circuit, there can be more than one definition of resonance.

One definition is that resonance is taken to be the frequency where the terminal impedance is a pure resistance (zero phase angle).

Another definition is the frequency where the impedance magnitude is a maximum.

If the circuit Q is high, these two frequencies are nearly the same, but for the low Q case they can be rather different.

I strongly support these statements.