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III. Gm-C Filtering - Epublications - Université de Limoges

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B.1.c Intercept Point Definition<br />

Now let’s consi<strong>de</strong>r the case where X1 = X2 = Vin. The average input power Pin and its<br />

corresponding level are given by [B.1]:<br />

L<br />

in<br />

⎛ Pin<br />

= 10 log⎜<br />

⎝1mW<br />

2<br />

⎞ ⎛ V ⎞ in<br />

⎟ = 10log⎜<br />

⎟<br />

⎜ ⎟<br />

. (B.17)<br />

⎠ ⎝ 2Rin1mW<br />

⎠<br />

Given the gain G of the system, the output level of the linear term is equal to:<br />

Now, given that:<br />

⎛ P ⎞ ⎛ GP ⎞<br />

L = 10log⎜<br />

⎟ = 10log⎜<br />

⎟ = L + G<br />

⎝1mW<br />

⎠ ⎝1mW<br />

⎠<br />

out1<br />

in<br />

out1<br />

in dB . (B.18)<br />

( k V )<br />

2<br />

2<br />

Vout<br />

2Rin<br />

1 in Rin<br />

2 Rin<br />

= . =<br />

= k1<br />

. (B.19)<br />

2<br />

2<br />

2RL<br />

Vin<br />

Vin<br />

RL<br />

RL<br />

G .<br />

The amplitu<strong>de</strong> of the third or<strong>de</strong>r intermodulation product leads to the following<br />

average output power:<br />

P<br />

out3<br />

⎛ 3 3 ⎞<br />

⎜ k3Vin<br />

⎟<br />

4<br />

=<br />

⎝ ⎠<br />

.<br />

2R<br />

Doing some calculations, one can obtain:<br />

L<br />

2<br />

⎛ ⎞<br />

⎜ ⎛ 3 3 ⎞<br />

⎜ k V ⎟<br />

3 in ⎟<br />

⎜ ⎝ 4 ⎠ ⎟<br />

= 10 log<br />

= 3L<br />

+ K<br />

⎜<br />

⎝<br />

⎟<br />

⎠<br />

KdB being a constant term.<br />

L<br />

2<br />

- 192 -<br />

(B.20)<br />

out3<br />

⎜<br />

in dB<br />

2R<br />

L . 1mW<br />

⎟<br />

, (B.21)<br />

Lout1 and Lout3 as a function of Lin are plotted on Figure 200. Because of compression<br />

and higher-or<strong>de</strong>r IM products, the intercept point (IP3) between the two curves is theoretical.<br />

However, the fictive intercept point can be projected on the abscissa (giving the input IP3<br />

called IIP3) or on the ordinates (leading to the output IP3 called OIP3).

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