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Example from textbook looks wrong. - RF Cafe Forums

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prolfe
Post subject: Example from textbook looks wrong. Posted: Tue Jun 02, 2009 6:17 am

Lieutenant

Joined: Tue Jun 02, 2009 6:06 am
Posts: 3
This example (see attachment if I can work out how to attach a file) from text book seems wrong. Or am I missing something?

This page is from the book RF Circuit design by Chris Bowick. Page 74.

The workings seem valid up until at the end where they simplify the circuits. (Arrow pointing right)

I can see how A anb b obviously simplify.

But how does C and D simply +j101 // -j231 into +j107 ???

Is there some kind of magic formula, or is the book wrong?

I could not actually work out how to attach. Please look at the image of the page at www.vmsys.com/problem1.jpg
[img]www.vmsys.com/problem1.jpg
[/img]


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Kirt Blattenberger
Post subject: Re: Example from textbook looks wrong.Posted: Tue Jun 02, 2009 9:51 am

Site Admin


Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Here's the image (you forgot the http:// part of the URL):



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RF Cafe Progenitor & Webmaster


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prolfe
Post subject: Re: Example from textbook looks wrong.Posted: Tue Jun 02, 2009 9:59 am

Lieutenant

Joined: Tue Jun 02, 2009 6:06 am
Posts: 3
Thanks for putting the picture up. I got all excited thinking you had solved the problem!


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Kirt Blattenberger
Post subject: Re: Example from textbook looks wrong.Posted: Tue Jun 02, 2009 10:12 am

Site Admin


Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings prolfe:

I'll take a stab at the answer for you, too. First, I think you have a typo in asking how

"But how does C and D simply +j101 // -j231 into +j107 ?"

The answer given by the author is 179, not 107.

That particular example refers to figure D, since it has +j101 (an inductor) in parallel with -j231 (a capacitor).

Here's how to transform to 179:

+j101 || -j231 = [(+j101)*(-j231)]/[+j101 + -j231)]
= 23331/-j130 = j179

The "j" indicates positive imaginary reactance, which is an inductor, so the author gives the value of 179 (ohms) next to the inductor.

Working the circuit "C" example yields the reciprocal of the "D" circuit, or -j179, which is a capacitor of reactance 179 ohms.

Does that make sense to you, or did I miss something?

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prolfe
Post subject: Re: Example from textbook looks wrong.Posted: Tue Jun 02, 2009 10:27 am

Lieutenant

Joined: Tue Jun 02, 2009 6:06 am
Posts: 3
yes yes.. you are somewhat a gun.

the + * - thinggy.

Year 9 maths is somewhat coming back to me now!

I was thinking it was something more complicated than that.

I had also asked a mate who studied such things at UNI less than 10 years ago! He did not pick this up either.

Thanks for your answer.

Regards,

Paul
and yes... my dislexix typing skills have surfaced again.







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