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Design >> RF Design >> The analysis of RLC in series is inconsistent with simulation results.
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Message started by xxgeneral on Feb 24th, 2012, 12:56am

Title: The analysis of RLC in series is inconsistent with simulation results.
Post by xxgeneral on Feb 24th, 2012, 12:56am
Hi everyone,

I simulated a simple circuit with RLC in series under Cadence Spectre. L and C resonate at 1MHz. A sinasoidal source with an amplitude of 600mV stimulates RLC at 1MHz. And, R=50Ohm, L=25.33mH, C=1pF.

My thought is, LC gets its minimum impedance at 1MHz. So in steady responses LC has a very low voltage, wheras R has a large one reaching 600mV.

However, the simulation result is reverse. That is, LC has a very large voltage, nearly 600mV. R has a small one, only 20mV.

I did another simulation with L=25.33uH and C=1nF. Then,  LC has a very low voltage, and R has a large one reaching 600mV.

This confuses me a lot.
Anyone can help me?
Thanks a lot!

Title: Re: The analysis of RLC in series is inconsistent with simulation results.
Post by raja.cedt on Feb 24th, 2012, 2:20am
hello,
be clear when you are saying some thing. Even though resonance frequency is same in both cases Quality factor is different, so the final voltage across the inductor and capacitor is equal for any circuit and it is equal to Q*Vin and voltage  across resister resister will be Vin.

case 1: (L=25.33uH and C=1nF): So here Q will be around 3.1, so volate across inductor and capacitor will be 3.1*600mv=1.8v, this is the value even i got through sim.
case2: (L=25.33mH, C=1pF): here Q=3183, so voltage across L and C wil be Q*vin.

But one thing may be you have to run sim for more time, because in the second case Q is very high so it takes time to reach steady state.

Thanks,
Raj.

Title: Re: The analysis of RLC in series is inconsistent with simulation results.
Post by loose-electron on Feb 24th, 2012, 2:02pm

raja.cedt wrote on Feb 24th, 2012, 2:20am:
hello,
be clear when you are saying some thing. Even though resonance frequency is same in both cases Quality factor is different, so the final voltage across the inductor and capacitor is equal for any circuit and it is equal to Q*Vin and voltage  across resister resister will be Vin.

case 1: (L=25.33uH and C=1nF): So here Q will be around 3.1, so volate across inductor and capacitor will be 3.1*600mv=1.8v, this is the value even i got through sim.
case2: (L=25.33mH, C=1pF): here Q=3183, so voltage across L and C wil be Q*vin.

But one thing may be you have to run sim for more time, because in the second case Q is very high so it takes time to reach steady state.

Thanks,
Raj.


Agreed!


and also is this AC or TRAN simulations?

Title: Re: The analysis of RLC in series is inconsistent with simulation results.
Post by xxgeneral on Feb 25th, 2012, 5:01am

raja.cedt wrote on Feb 24th, 2012, 2:20am:
hello,
be clear when you are saying some thing. Even though resonance frequency is same in both cases Quality factor is different, so the final voltage across the inductor and capacitor is equal for any circuit and it is equal to Q*Vin and voltage  across resister resister will be Vin.

case 1: (L=25.33uH and C=1nF): So here Q will be around 3.1, so volate across inductor and capacitor will be 3.1*600mv=1.8v, this is the value even i got through sim.
case2: (L=25.33mH, C=1pF): here Q=3183, so voltage across L and C wil be Q*vin.

But one thing may be you have to run sim for more time, because in the second case Q is very high so it takes time to reach steady state.

Thanks,
Raj.


Hi Raj.

Thanks for your help.

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