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Radio Frequency Integrated Circuit Design - Webs

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284 <strong>Radio</strong> <strong>Frequency</strong> <strong>Integrated</strong> <strong>Circuit</strong> <strong>Design</strong><br />

H( j�) ≈ H( j�o ) + �� dH<br />

d�<br />

(8.77)<br />

Since the conditions of stable oscillation must be satisfied, H( j�o ) = 1.<br />

We let H1( j� o ) = H 1, where H 1 is a constant determined by circuit parameters.<br />

Now (8.76) can be rewritten using (8.77) as<br />

rule<br />

Nout(s)<br />

Nin(s) = H 1<br />

−�� dH<br />

d�<br />

Noise power is of interest here, so<br />

| Nout(s) Nin(s) | 2<br />

=<br />

|H1| 2<br />

(��) 2 | dH<br />

d� | 2<br />

(8.78)<br />

(8.79)<br />

This equation can now be rewritten using H(�) = |H | e j� and the product<br />

dH d |H |<br />

=<br />

d� d� e j� + |H | je j� d�<br />

d�<br />

noting that the two terms on the right are orthogonal:<br />

| dH<br />

d� | 2 d |H |<br />

= | d� | 2<br />

+ |H | 2 | d�<br />

d� | 2<br />

(8.80)<br />

(8.81)<br />

At resonance, the phase changes much faster than magnitude, and<br />

|H | ≈ 1 near resonance. Thus, the second term on the right is dominant, and<br />

this equation reduces to<br />

| dH<br />

d� | 2<br />

= | d�<br />

d� | 2<br />

Now substituting (8.82) back into (8.79),<br />

| Nout(s)<br />

Nin(s) | 2<br />

=<br />

|H 1| 2<br />

(��) 2 | d�<br />

d� | 2<br />

(8.82)<br />

(8.83)

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