ee4101 lectures 9 &amp; 10 -- 2010

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

This preview shows pages 1–8. Sign up to view the full content.

1 EE4101 RF Communications Part B Prof TS Yeo

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
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,. .

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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 )
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:

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
7 EE4101 RF Communications Part B ± RF Generation ± 2-port oscillator 2-port is potentially unstable Appropriate load termination Z T

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page1 / 25

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

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online