TDA5250D2_07 Datasheet, PDF (41/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

Measured Magnitude of S11 of evalboard:
TDA5250 D2
Version 1.7
Application
S11_measured.pcx.
Figure 3-3 S11 measured
Above you can see the measured S11 of the evalboard. The –3dB-points are at 810MHz and
930MHz. So the 3dB-bandwidth is:
B = f − f = 930MHz − 810MHz = 120MHz
U
L
[3 – 1]
The_Qload₌ed_f
Q of
center
=^{the86re8s,3oMnaHntz}
circuit
= 7.2
is:
L _{B 120MHz}
[3 – 2]
The unloaded Q of the resonant circuit is equal to the Q of the inductor due to its losses.
Q =Q
≈ 36@868MHz
U
INDUCTOR
[3 – 3]
An approximation of the losses of the input matching network can be made with the formula:
Loss
=
−20 ∗
log_{}1 −
Q
L
Q
U

=
−20 ∗
log_1 −

7.2 _
36 _
=
2dB
[3 – 4]
The noise figure of the LNA-input-matching network is equal to its losses. The input matching
network is always a compromise of sensitivity and selectivity. The loaded Q should not get too high
because of 2 reasons:
more losses in the matching network and hence less sensitivity
Data Sheet
41
2007-02-26