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ineed an explaination

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khais

New Member
:D hey,
Iam realy facing aproblem in understanding the response of LC circuits when connected to one pulse (100ms width)..
the circuit consists of a 500uf capacitor in series with a 1mH coil..the output is taken from the coil.
when itried to sumilate the output igot sinwave keep decreasing it's peak to zero then start it's peak again from max to zero :!:
iwas expecting the circuit to oscillate for some time and not for good :!:
Is the problem with the software iam using :?: or there is something idon't know.
appreciate any feedback :D
 

Roff

Well-Known Member
I'm assuming you have some resistance in the circuit. Otherwise, it will oscillate forever.
Your tank circuit resonates at approximately 225 Hz. Each edge of your 100ms pulse excites the tank circuit and causes it to ring at 225 Hz, decaying exponentially due to the series resistance. In other words, it will ring when the leading edge of the pulse arrives, die out, and begin ringing again when the trailing edge arrives.
 

khais

New Member
Ron H said:
I'm assuming you have some resistance in the circuit. Otherwise, it will oscillate forever.
Your tank circuit resonates at approximately 225 Hz. Each edge of your 100ms pulse excites the tank circuit and causes it to ring at 225 Hz, decaying exponentially due to the series resistance. In other words, it will ring when the leading edge of the pulse arrives, die out, and begin ringing again when the trailing edge arrives.
Hi ron, :D
thanks for your feedback.the circuit doesn't have any resistance and the input is only one single (100ms)pulse....what idon't understand is how could the tank circuit resonate forever by asingle pluse :!:
ihave one more question :?: how did you know the osillation frequency?
thanks[/u]
 

Roff

Well-Known Member
Resonant frequency:
Fr=1/(2*pi*sqrt(L*C))
Where Fr=resonant frequency in Hertz, pi=3.1415926, sqrt means square root, L=inductance in Henries, C=capacitance in Farads.
An ideal LC circuit with no series resistance and lossless inductance and capacitance will oscillate forever once current is injected into the circuit. This only happens in simulations, because there are no lossless inductors or capacitors in the real world (at room temperature). Real-world LC oscillators overcome the losses by adding an amplifier with positive feedback around the LC circuit.
If your simulation showed a decaying waveform, then you either had some resistance in your pulse source, or your models had losses built into them.
 

khais

New Member
Ron H said:
Resonant frequency:
Fr=1/(2*pi*sqrt(L*C))
Where Fr=resonant frequency in Hertz, pi=3.1415926, sqrt means square root, L=inductance in Henries, C=capacitance in Farads.
An ideal LC circuit with no series resistance and lossless inductance and capacitance will oscillate forever once current is injected into the circuit. This only happens in simulations, because there are no lossless inductors or capacitors in the real world (at room temperature). Real-world LC oscillators overcome the losses by adding an amplifier with positive feedback around the LC circuit.
If your simulation showed a decaying waveform, then you either had some resistance in your pulse source, or your models had losses built into them.
Thanks again Ron,
what iknow resonant frequecy happens when xc=xL in the circuit and both
depend on the input freqency and the values of inductance,capacitance
xc=1/2*pi*f*c , xL=2*pi*f*L
without the right input frequency resonance won't be achieved .
My question again with asingle pulse(no matter what the width is)how could the circuit oscillate :!: (there is no frequency to meet the resonant formula)
sorry for giving you ahard time :oops:
 

Roff

Well-Known Member
Pulse waveforms are rich in harmonics. In the frequency domain, a single 100ms wide pulse has a small DC component, a null at 10 Hz (1/100ms), a peak at 15 Hz, another null at 20 Hz, etc. In other words, it has harmonics every 10 Hz, starting at 15 Hz and repeating every 10 Hz. The amplitude of the harmonics decreases with increasing frequency. The rate of this decrease is a function of the risetimes of the pulse. Faster risetimes result in a lower rate of harmonic amplitude decrease. Your pulse has significant energy content at the resonant frequency of your tank circuit.
 

khais

New Member
Ron H said:
Pulse waveforms are rich in harmonics. In the frequency domain, a single 100ms wide pulse has a small DC component, a null at 10 Hz (1/100ms), a peak at 15 Hz, another null at 20 Hz, etc. In other words, it has harmonics every 10 Hz, starting at 15 Hz and repeating every 10 Hz. The amplitude of the harmonics decreases with increasing frequency. The rate of this decrease is a function of the risetimes of the pulse. Faster risetimes result in a lower rate of harmonic amplitude decrease. Your pulse has significant energy content at the resonant frequency of your tank circuit.
:D Thanks alot Ron
your expaination is more than clear.
thanks again
 
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