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question re: resonant LC circuit

Ok, I admit this concerns a lab I did in electronics, however, the lab is complete and turned in and this queation was not a part of the lab. Using an oscilloscope to observe Vpk-pk across mystery components in a black box over a 1kHz to 1000kHz 10V input, we semi-log plotted the readings and indentified inductors, capacitors, resistors, series resonant, and parallel resonant components. Non-resonant components were further evaluated using frequency at V*Sin45° since a 1k Ohm resistor was in series with the mystery component(s). Further to this lab, I am attempting to identify the L and C ratings of the series and parallel resonant circuits. Using Xl =2PifL, Xc=1/2PifC, fr=1/2PiSqrt(LC), Xs=Xl-Xc, and Xp=XlXc/Xl+Xc, equations can be derived for series and parallel that solves C knowing resonant frequency, VSin45° frequency, and R of the series resistor. My question is whether this is practical or competely useless. I have found nothing in texts that directly addresses identification of unkown components in these filters. Q is also not known. The Rw of the inductor may be very important in these calcs and since the equations use the square of frequencies estimated from charts there may be too much error to make a reliable estimate of C or F. Thoughts? Clues?

Thanks, Chuck

Reply to
Chuck Jensen
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in article YC2ec.4422$ snipped-for-privacy@newsread2.news.pas.earthlink.net, Chuck Jensen at snipped-for-privacy@earthlink.net wrote on 4/10/04 8:06 PM:

I have some difficulty completely understanding you post. I does, however, look like you are describing a way of measuring the bandwidth of a resonant circuit. Realize that Sin45° = 1/sqrt(2). By definition, the bandwidth of a transfer function is the frequency difference between the half power transmission points. In voltage, that would be between the 1/sqrt(2) voltage points.

Bill

Reply to
Repeating Rifle

Yes, you are right, (pardon the confusion), it is the bandwidth of a series bandpass filter (LC series) and a series notch filter(LC parallel) where R(load) is 1k ohm and R(source) is 50 ohms from the generator. I thought I could rearrange equations to identify the unknown inductor and capacitor ratings, but I may be tricking myself.

bandpass example: C= [f(in)^2 - f(r)^2] / [2*Pi*f(r)^2*f(in)*(R(load)+R(source))] L= 1 / [4*Pi^2*f(r)^2*C] (this is a standard equation)

notch example: C= f(in) / [2*Pi*(f(r)^2+f(in)^2)*(R(load)+R(source))] L= 1 / [4*Pi^2*f(r)^2*C]

where f(in) = frequency at 1/sqrt(2) voltage (estimated from graph) f(r) = resonant frequency (estimated from graph) R(load) = series resistor 1k Ohm (given) R(series) = generator 50 Ohm output

I have found nothing talking about the problem of finding the values of L and C in a "blackbox" filter. In practical terms those values would be known. Chuck

Reply to
Chuck Jensen

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