TE4201-Communication Electronics 1 3. Tuned Circuits LC circuits LC circuitsLC circuits Bandwidth and Selectivity Bandwidth and SelectivityBandwidth and Selectivity
TE4201-Communication Electronics 2 LC Circuits The Coil and Quality Factor Q The Series Resonant Z has minimum value for Series Resonant at resonant frequency 0 ZSZS f Z S = r f0f0f0f0 f0f0 ZSZS
TE4201-Communication Electronics 3 The Parallel circuit of the coil
TE4201-Communication Electronics 4 The Parallel Resonant f Z p = R p ZpZpZpZp f0f0f0f0 Z has maximum value at resonant frequency 0 for Parallel Resonant ZpZp
TE4201-Communication Electronics 5 Output voltage e 0 across the parallel tuned (resonant) circuit is proportional to the Z p at resonant frequency 0. Therefore same response curve is used for e 0 f0f0 fHfH fLfL e0e e 0 From LC bandpass filter analysis output voltage e 0 at resonant frequency 0 will fall to 0.707e 0 at lower cutoff f L as well as at higher cutoff f H. The difference of the two frequencies is called Bandwidth (BW). Bandwidth and Selectivity
TE4201-Communication Electronics 6 0 dB -3 dB f Q is larger Effect of Q and Bandwidth A 0 is the gain at resonant frequency f A 0 is the gain at cutoff frequencies L and H Then BW = f H - f L f0f0 A0A A 0 fHfH fLfL BW A0A A 0 fHfH fLfL BW Q is smaller Then: BW will be larger for low Q circuits and BW will be smaller for high Q circuits
TE4201-Communication Electronics 7 Q is larger Q is smaller fXfXfXfX f0f0f0f0 AXAXAXAX A0A0A0A0 Rejection of f x from f 0 is given by A0A0A0A0 AXAXAXAX Effect of Q and Frequency Rejection Rejection of f x (dB) = 20log(A o /A x ) If A 0 = e 0 / e in (at resonant frequency 0 ) and if A x = e 0x / e in (at any other frequency x ) and by using the selectivity curve, the gains A 0 and A x of the tuned circuit can be compared by the tuned circuit equation as follows: From above equation,if Q is large rejection at f x given by A 0 /A x is better Therefore if better channel (frequency) rejection is required, large value of Q is used in RF amplifiers