ee4101 lectures 9 & 10 -- 2010

ee4101 lectures 9 & 10 -- 2010 - EE4101 RF...

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1 EE4101 RF Communications Part B Prof TS Yeo
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2 REE4101 RF Communications Part B ± RF Generation RF Generation: ² Microwave oscillators convert DC power into RF power ² Solid-state oscillators use active devices (diodes or transistors) in conjunction with matching* passive circuits to produce sinusoidal steady-state RF signal ² Oscillators triggered by transients or noise at start-up ² Oscillators will then reach stable oscillation state ² Active devices must be non-linear and should present a negative resistance
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3 EE4101 RF Communications Part B ± RF Generation One-port oscillator: For oscillation to occur at the operating point: R L + R in = 0 and X L + X in = 0 Since R L > 0 => R in must be < 0 ZX R R ZZ o in in o in in o in o in in j j ±± ² ± ± ² * R R oL L L L j j ² ± ± ² * With R in = -R L , and X in = -X L , we have ī in ī L = 1 => | ī in || ī L | = 1 and / ī in + / ī L = 2n ʌ , n = 0, 1, 2,. .
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4 EE4101 RF Communications Part B ± RF Generation Process of oscillation: ² Z in (I, j Ȧ ) = R in (I, j Ȧ ) + jX in (I, j Ȧ ) ² Requires, at some Ȧ + ǻȦ ² R in (I, j Ȧ ) negative and that R in (I, j Ȧ ) decreases (less negative) with increasing I ² Select R L and bias current I b such that R in (I b , j Ȧ b ) + R L < 0 ² Noise will cause oscillation to build up at frequency ¹ ² I increases and hence R in (I, j ¹ ) less negative till I = I o , where ² R in (I o , j Ȧ o ) + R L = 0 and X in (I o , j Ȧ o ) + X L (j Ȧ o ) = 0 ² Note that Ȧ o in general is, not equal to Ȧ b , also X in (I o , j Ȧ o ) Į X in (I b , j Ȧ b )
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5 EE4101 RF Communications Part B ± RF Generation ± Example #1 A 1-pot oscillator uses a negative resistance diode with ī in = 1.25/40 o measured with a 50 ¡ reference at its desired operating point and at a frequency of 6 GHz. Design a load matching network for a 50 ȍ load impedance. Z in = Z o (1 + ī in )/(1 ± ī in ) = -44 + j123 ȍ , hence Z L = - Z in = 44 ± j 123 ȍ Shunt stub and series section can be used to convert 50 ȍ to Z L . Resulting RF circuit:
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6 EE4101 RF Communications Part B ± RF Generation For oscillator to be stable, perturbations at frequency away from Ȧ o must be damped out (or decayed) rapidly. Hence, a high-Q circuit is needed for oscillator stability. Cavity and dielectric resonators are often used for this purpose. Note: the difference between stability of device for amplifier design and stability of oscillator design
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7 EE4101 RF Communications Part B ± RF Generation ± 2-port oscillator 2-port is potentially unstable Appropriate load termination Z T
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ee4101 lectures 9 &amp;amp; 10 -- 2010 - EE4101 RF...

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